[HN Gopher] The scientific "unit" we call the decibel
___________________________________________________________________
The scientific "unit" we call the decibel
Author : Ariarule
Score : 568 points
Date : 2025-05-22 04:24 UTC (18 hours ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| smat wrote:
| Great post, the fusion of scale with unit is a mess.
|
| When using it as a factor, for example when describing
| attenuation or amplification it is fine and can be used similar
| to percent. Though the author is right - it would be even more
| elegant to use scientific notation like 1e-4 in this case.
|
| For using it as a unit it would really help to have a common
| notation for the reference quantity (e.g. 1mW).
|
| But I guess there is no way to change it now that they are
| established since decades in the way the author describes.
| rocqua wrote:
| Scientific notation would be worse, because you can't add them
| to get the combined gain. That's where dB's really shine.
| rebolek wrote:
| While the author is technically right, I must argue that in the
| area of sound work, decibels make sense. Zero is base level, -3db
| is half loudness, +3dB is double. There may be a better way to
| describe loudness, but decibel is good enough.
| readthenotes1 wrote:
| So on my amplifier, 0db is the loudest and -50db, where I
| usually listen is what?
|
| -47db is definitely not twice as loud as -50db, of course.
| hgo wrote:
| I'm not confident in what I'm saying here, so please correct
| me if I'm wrong as I'd like to learn:
|
| Human hearing isn't linear in terms of loudness. So a 3db
| increase in loudness sounds like "an increase", but the
| pressure is actually double. Hence, it makes sense to use db
| to describe loudness even in the context of perceived
| loudness to human-hearing.
|
| This is similar to brightness. In photography, "stops" are
| used to measure brightness. One stop brighter is technically
| twice the light, but to the human eye, it just looks
| "somewhat brighter", as human brightness appreciation is
| logarithmic, just like "stops" and "db".
| bloppe wrote:
| I mean, it should be, unless your amplifier is taking its own
| liberties with the definition of dB.
| rebolek wrote:
| Again, technically, it is. But ear isn't scientific device,
| neither is your amplifier. What I was describing is more than
| an agreement than some precise measurement. 3dB is more or
| less double the volume but different frequencies have
| different responses so I really wouldn't want to have some
| "perfect" way of measuring loudness as it would be so
| needlessly complicated that it would be useless.
| timewizard wrote:
| Human hearing perception response is logarithmic. This is
| part of the reason we use the unit. Our senses naturally work
| in that domain.
| coldtea wrote:
| > Human hearing perception response is logarithmic.
|
| It is, but -50db to -47db (+3db) is not double perceived
| loudness. It's double power. About +6 or even +10db would
| be double perceived loudness.
| hashhar wrote:
| Going from -50dB to -44dB is a much louder change than
| going from -6dB to 0dB.
|
| Human hearing is logarithmic. The dB is measuring ratio
| of sound pressure level and it's accurate that +/-3dB is
| almost doubling/halving of the SPL.
| davrosthedalek wrote:
| That doesn't make sense. -50dB to -44dB and -6dB to 0dB
| is the same change in power, as a factor. If human
| hearing is logarithmic, the same factor produces the same
| increase in loudness.
| masklinn wrote:
| 3dB is "twice as loud" in that it's twice (or half) as much
| _power_.
|
| Part of the reason why the bel is used is that human
| perception is closer to logarithmic than linear (weber-
| fechner law), so a logarithmic scale is a better
| approximation of "loudness" than a linear one.
| coldtea wrote:
| -47db is double the power. Perceived double loudness would be
| ~ -40db
| relaxing wrote:
| You should have a legitimate question as to what dB's the amp
| is referring to. What brand and model is it?
|
| First, is it actually specified as dB's? Most amps I've seen
| display volume on an arbitrary scale, no units specified.
|
| Second, the amp has no way to know dB sound pressure, as that
| depends on the rest of signal chain.
|
| So is the displayed figure referring to dBu, dB mV, something
| else, or something totally bunk?
| davrosthedalek wrote:
| If 0 dB is its maximum, then -50dB is 50 dB down in power
| from its maximum (so 10^-5). No m V or other post-fix
| needed.
| relaxing wrote:
| That's true but not that helpful for determining what
| your ears actually hear.
| creeble wrote:
| For music, they hear about 10dB is half/twice as loud.
| djaychela wrote:
| As someone else alluded to, 3dB is a doubling in power, not
| perceived loudness. 10dB will be perceived as a doubling in
| loudness. This is the original unit (the Bel, rather than
| deciBel) which was I believe derived by testing on human
| subjects to measure this.
|
| TBH I don't agree with a lot of the article - yes, dB on its
| own only indicates a ratio, but certainly in the field I work
| with this is known, and there are qualifiers (dBA, dbFS dbU)
| which tie the ratio to a known value so you're talking about
| an absolute, known quantity - even the dBa which is mentioned
| as if it comes out of nowhere is something which most audio
| engineers know about and use regularly because it's important
| to know the difference betweent he signal present and
| perception of it by the listener.
| svara wrote:
| For practical day to day use in very narrow context, yes.
|
| But, what you're saying is only approximately correct (the
| factor is a bit less than 2) and there are many related fields,
| even in areas that would be relevant to the physics of sound
| reproduction, in which the same notation "dB" means something
| different.
| ajuc wrote:
| > -3db is half loudness, +3dB is double
|
| It isn't tho. It's close but not exactly. And there's nothing
| about -3 behing half that makes sense except for familiarity
| (and it's not even wide-spread familiarity - most people
| wouldn't know how much louder +3dB is).
|
| It's just an unnecesarilly confusing definition that stuck for
| historic reasons.
| danadam wrote:
| > It isn't tho. It's close but not exactly.
|
| It isn't tho :-). It's not close to double loudness. It's
| double power, which is 1.41 higher sound pressure, which is
| only slightly louder.
| kazinator wrote:
| But, no! -3dB is half power, not half loudness. -3dB down is
| just slightly less loud.
| globular-toast wrote:
| Exactly. There's no way +3dB is perceptibly twice as loud.
| That's easy to test for yourself. +10dB is roughly twice as
| loud.
| creeble wrote:
| The BBC used to teach +6dB for twice as loud for music
| sources, but they've always been a bit conservative.
| Sharlin wrote:
| > -3db is half loudness
|
| Sure, perfect sense.
| verzali wrote:
| 10*log10(0.5) = -3
|
| That's the math of it.
| rocqua wrote:
| That's great, but only if 'Base level' is clear to everyone
| involved. Besides the base level, you also need to define what
| exactly you are measuring. Including the frequency weighting.
|
| This makes a spec sheet that says "this machine produces X dB
| of sound" effectively useless.
| davrosthedalek wrote:
| Any reasonable spec sheet would not say that. I say
| reasonable, because in the audio world, people pay extra
| money for oxygen-free copper cables.
| ziofill wrote:
| The nice thing about dBs is that they are logarithms: successive
| applications of gain/attenuation are easily computed by just
| adding up the dBs. (but yes, I agree that there should be more
| consistency in their definition)
| xnx wrote:
| Weird that the author makes no mention of Sones:
| https://en.wikipedia.org/wiki/Sone
| itchingsphynx wrote:
| Nor phons!
|
| "The phon is a logarithmic unit of loudness level for tones and
| complex sounds."
|
| https://en.wikipedia.org/wiki/Phon
| svara wrote:
| A pet peeve I share! An expanded version of this article should
| become the article on decibels on Wikipedia.
|
| I've read that article many times over my life and for the first
| couple times came back thinking I was too dim to understand.
|
| Transparently leading it with "Here's something ridiculously
| overcomplicated that makes no sense whatsoever..." wouldn't fit
| Wikipedia's serious voice but actually be pedagogically very
| helpful.
| esperent wrote:
| There's often a Criticism of... section in Wikipedia pages.
|
| Maybe this blog post could work as a source, although it would
| be better to find something more established.
| timewizard wrote:
| > That said, I don't know how to pronounce "3e5"
|
| "Three to the exponent of five." Or "Three Exponent Five." Or
| "Three Exp Five."
|
| > Seeing this, some madman decided that 1 bel should always
| describe a 10x increase in power, even if it's applied to another
| base unit. This means that if you're talking about watts, +1 bel
| is an increase of 10x; but if you're talking about volts, it's an
| increase of [?]10x
|
| This is power vs. amplitude. This is the specific reason the dB
| is so useful in these systems.
|
| > the value is meaningless unless we know the base unit and the
| reference point
|
| No you just need to know if you have a power or a root-power
| quantity. Which should generally be obvious.
|
| https://en.wikipedia.org/wiki/Power,_root-power,_and_field_q...
| ggm wrote:
| > _No you just need to know if you have a power or a root-power
| quantity. Which should generally be obvious._
|
| There has to be a Yogi Berra witticism about obvious things.
| Suffice to say fools like me work unadvisedly in spaces where
| this kind of axiom isn't obvious, because we're simpletons.
| bigiain wrote:
| > > That said, I don't know how to pronounce "3e5"
|
| > "Three to the exponent of five." Or "Three Exponent Five." Or
| "Three Exp Five."
|
| Somehow, you need to distinguish between 3^5 (=243), 3 x e^5
| (=~445.24), and 3 x 10^5 (=300,000).
|
| I'd pronounce "3e5" and "three times ten to the 5" in most
| cases.
| timewizard wrote:
| > 3^5 (=243)
|
| Three to the power of five.
|
| > 3 x e^5
|
| Three times the fifth power of e. Or Three times e's fifth
| power.
|
| > 3 x 10^5 (=300,000).
|
| Three to the exponent of five.
|
| A calculator user once suggested "decapower." I think
| exponent and "EXP" are comfortable and easy to say and are
| ingrained to most old school calculator users. Which is also
| why I think "e's fifth power" can be a more natural sequence.
| bigiain wrote:
| > Three to the exponent of five.
|
| I think that's still ambiguous. The base of the exponent is
| implied, and there are decent arguments or fields in which
| the assumed base of 10 isn't universal. In your "Three
| times e's fifth power." version, you needed so specify e. I
| think for accuracy it's also needed to specify 10 in that
| "Three to the exponent of five." case.
|
| (Having said that, there are certainly cases where the 10
| as the base of the exponent will be clear/unambiguous from
| the context. )
| ggm wrote:
| Do a deep dive into audio vu Meters and how they got calibrated.
| Without being 100% sure, it's basically a totally subjective
| model, where back in the 1920s the BBC and some US company
| decided to assert "like us" and two models persist which have
| been retconned into some BIPM acceptable ground truth but it
| basically was "test it against the one we made which works"
|
| The hysteresis in the coil-magnet meter response turned out to be
| a feature, not a bug.
| formerly_proven wrote:
| Decibels make sense and it's usually only laymen who use only
| ,,dB". I'd be surprised if you'd find ,,-45 dB" as the
| specification for a microphone. Here's two random examples:
|
| Sensitivity at 1 kHz into 1 kohm: 23 mV/Pa [?] -32.5 dBV +- 1 dB
|
| Sensitivity: -56 dBV/Pa (1.85 mV)
| hock_ads_ad_hoc wrote:
| The author seems confused. Decibels aren't units. They're a way
| to express a ratio between a reference value and some other
| value.
|
| They're used where they are useful.
| coolcase wrote:
| They are units too. When you have regulations relating to sound
| they will say how many dB the limit is.
| formerly_proven wrote:
| I've never seen dB being used like that in anything remotely
| official.
| coolcase wrote:
| https://www.osha.gov/noise
|
| Maybe the reference is implied though?
| formerly_proven wrote:
| That's hilarious, though they do seem to get it right on
| the other, more in-depth pages.
| throwaway290 wrote:
| Yep, the NIOSH meter app they recommend uses dB(A).
| radiowave wrote:
| Though that's still incomplete, because (more of the
| context stuff that you just have to know already) the "A"
| here refers to the frequency-weighting scheme used in the
| measurement, and not to the reference level (which is
| SPL).
|
| It should probably be given as: dB(A) SPL, or dB SPL
| (A-weighted).
| formerly_proven wrote:
| Though that's still incomplete, because (more of the
| context stuff that you just have to know already) the
| "SPL" here refers to the reference level in air (20 uPa)
| and not water or oil (1 uPa).
|
| It should probably be given as: dB(A) SPL (1=20 uPa).
|
| Though that's still incomplete, because (more of the
| context stuff that you just have to know already) the 20
| uPa only applies at the standard temperature and
| pressure.
|
| It should probably be given as: dB(A) SPL (1=20 uPa,
| T=293 K, P=101.3 kPa, in standard air).
|
| Though that's still incomplete, because (more of the
| context stuff that you just have to know already) the (A)
| here is actually the A-weighting curve specified in IEC
| 61672:2003.
|
| It should probably be given as: dB A-weighted (IEC
| 61672:2003) SPL (1=20 uPa, T=293 K, P=101.3 kPa, in
| standard air).
|
| Though that's still incomplete, because (more of the
| context stuff that you just have to know already) ...
|
| ...
|
| The point of communication is to transport information,
| not pointless pedantry (except for a small subset of the
| population). Nobody is confused as to what 85 dBA refers
| to.
| radiowave wrote:
| Granted, I should have been clearer about my intended
| point, which is just the hazard of assuming that the
| letter(s) after the dB tell you the reference, because
| sometimes they don't.
| neepi wrote:
| No. dB is not a unit. If it's quoted as a unit, then the
| person using it doesn't know what they are doing. dBx where x
| is some reference value is a unit and that means dB relative
| to the reference unit. So dBm/dBv etc are units. dB is not.
|
| Mathematically, dB is the ratio between two unit values and
| for example if you divide metres by metres you cancel the
| units.
| qmmmur wrote:
| Exactly. If someone says that "this plane is about 130dB"
| what they mean is in reference to 0dB given 0dB is the
| threshold of human hearing. It seems most people are
| confused by dB because they don't understand it provides a
| logarithmically scaled unit compared to a reference which
| happens to be quite useful for something like audio where
| our perceptual models are stacked with these types of
| curves.
| jononor wrote:
| No they will specify something like dB SPL, A-weighted with
| Fast time integration. Sometimes referred to as dB(A) SPL.
| And measured with a particular method, which specifies things
| like distance from sources, environmental conditions, etc.
| rocqua wrote:
| dBs are so often used as units. That's 80% of the complaint in
| the article.
|
| The fact that dBs aren't units, but are used like units, is the
| point being made.
| atoav wrote:
| But dB is not a unit, it is a multiplier. dB on its own is
| unitless and if we say "X is reduced by 6 dB" you know that the
| value of X is half of what it was before (x0.5) if something is
| amplified by 6 dB it is double of what it was before (x2.0)1
|
| Note that the unit only starts to play a role when you reference
| your dB value to some absolute maximum, e.g.: dBV
| which is referenced to 1V RMS dBu which is
| referenced to 0.775V RMS (1mW into a typical audio system
| impedance of 600 Ohms) dBFS which is referenced to
| a digital audio maximum level (0dBFS) beyond which your numeric
| range would clip (meaning all practical values will be negative)
| dBSPL which is refrenced to the Sound Pressure Level that is at
| the lower edge of hearing (0 dBSPL), this is what people mean
| when they say the engine of a starting airplane is 120dB loud
|
| Now dB is extremely useful in all fields where your values span
| extremely big ranges, like in audio engineering, where the ratio
| between high and low values can easily have a ratio of 1:10
| Millions. So unless you want people to count zeroes behind the
| comma, dB is the way to go.
|
| When we think about the connection between analog and digital
| audio dB is useful because despite you having volts on the one
| side and bits on the other side a 6dB change on one side
| translates to a 6dB change on the other, the reference has just
| changed. If we had no dB we would have to do conversions
| constantly.
|
| Going from multiplier x to dB: 20xlog10(x)
|
| Going from dB to multiplier x: 10^(x/20)
|
| If you use dB to describe the power of a signal that is slightly
| different (you use 10 instead of 20 as multiplier/divisor)
|
| But you can see, dB is just a way to describe a unreferenced size
| change in a uniform way or to describe a referenced ratio. And
| then it would be good to know what that reference is. So if
| someone says a thing has 40dB you they forgot to tell you the
| unit.
|
| 1 this is true for the amplitude of a signal and differs when we
| talk about the power of a signal, where 3dB is a
| doubling/halving.
| rocqua wrote:
| > But dB is not a unit, it is a multiplier. dB on its own is
| unitless.
|
| The point of the article is exactly that this _should_ be the
| case. But it ends up not being the case. Mostly because people
| use dB with reference to some assumed baseline. But also
| because a 20db change could be a 10x change conpared to
| baseline, or a 100x change compared to baseline, depending on
| what unit you are measuring in.
| mikewarot wrote:
| 20 dB is always the same, actually. If you multiply voltage
| (and thus current as well) by 10:1 (20 db) the power is
| multiplied by 100:1 (also 20 dB)
| rcxdude wrote:
| That's the point that's confusing. Especially in contexts
| where it's not obvious whether it's power-like or
| amplitude-like. It should be 40dB of power gain or 20dB of
| amplitude gain, with the context made explicit.
| atoav wrote:
| Yes but my conclusion would not be that _Decibels are
| ridiculous_ , but that "People don't understand Decibels".
|
| Decibels are okay. They are useful. They work. The problem is
| that people use referenced decibel values without adding
| anything that would allow us to understand which reference
| was used.
|
| Maybe one could have come up with a better numeric way of
| doing the same thing (I am missing a proposal for this in the
| blog post), but then you'd have the XKCD-yet-another-standard
| problem. Everything uses dB for ages now, so dB it is or you
| need to convert between one and another all the time.
|
| As an audio engineer I have no issue with dB as a unit. It is
| much better than using raw amplitude numbers.
| roelschroeven wrote:
| > If you use dB to describe the power of a signal that is
| slightly different (you use 10 instead of 20 as
| multiplier/divisor)
|
| This is the part I don't get. This is the part where "dB is
| just a multiplier" falls short. It's there to "so that the
| related power and root-power levels change by the same value in
| linear systems" (that's how Wikipedia formulates it). Why is
| that even something you would want? Isn't it much more logical,
| intuitive, consistent and useful to reflect the fact of power
| being proportional to the square of the signal in it having
| double the dB value?
| thrdbndndn wrote:
| > this is in the same tradition that prompted us to name the
| "wat" in honor of James Watt.
|
| The unit is Watt, not Wat.
| ahofmann wrote:
| I'm sorry, that you missed the hilarious joke!
|
| For reference: https://www.destroyallsoftware.com/talks/wat
| thrdbndndn wrote:
| Ahh, I get the joke now. Good one :)
|
| As for the reason, if I have to guess, is because "decibel"
| looks better than "decibell".
|
| (Keep in mind decibel was actually renamed from the previous
| unit called "Transmission Unit" and was meant to be used as
| the main unit even at the beginning. "bel" was simply
| derived/implied from it, not the other way around).
| kazinator wrote:
| Whoosh ...
| reichstein wrote:
| Your irony detector may need calibration.
|
| Or mine does.
| kristjank wrote:
| This seems exceedingly ignorant of the work decibels do in
| telecommunications, RF and fibre engineering. The voltage vs
| power relationship is something that exists and is a core memory
| of beginner blunders in the field, but it boils down to a simple
| 10 vs 20 division operation. Besides that, the decibel simplifies
| a lot of multiplying very small and very big numbers to summing
| of two-digit numbers that you can do in your head, and still
| preserve a big degree of accuracy.
|
| Whining about it makes me really doubt that the OP has any
| practical experience about the things they're talking about.
| neepi wrote:
| I concur. See my other heavily downvoted post. It's a hobby
| blogger speaking authoritatively about something they have
| little experience with. This is a curse on any field. It
| probably looks legitimate to people who have no experience in
| the field who feel like they are learning something.
| fouronnes3 wrote:
| However you must concede that the perspective of an outsider
| can be refreshingly eye opening sometimes, even to experts.
| Especially when it reveals the arcane practices that make a
| field difficult to learn - for example here the point about
| the base being too implicit is very valid. The article is
| perhaps ignorant of a few things, but its criticism shouldn't
| be dismissed outright IMO. Accepting constructive criticism
| from someone less experienced at something is a good exercise
| in humility, and can often help you improve. It's far from a
| "curse in any field".
| neepi wrote:
| That would assume that the entire world hasn't already
| tried this a thousand times over.
|
| Measurements are standardised communication tools. If you
| start changing the definitions, things fall out of the sky
| and on your head.
| baxuz wrote:
| This mindset is why the US is still using barley seeds as
| a standardized unit of measure.
| yxhuvud wrote:
| No, changing definitions was exactly how we ended up with
| SI units. That was a very good (and necessary) thing and
| definitely not the sky falling down.
| viraptor wrote:
| Glad we never changed them and still drink in hogshead,
| use the original foot reference, and never resolved what
| base is a megabyte. ;)
| dns_snek wrote:
| You haven't addressed any points the author has made, we
| don't need to have "experience" in a field to know that
| ambiguous units are ridiculous and bad.
|
| When I'm buying a piece of string for my garden, I don't need
| to find an agricultural textbook to know whether "10 meters"
| in agriculture is the same as "10 meters" in engineering, and
| whether the definition of "meters" depends on whether the
| string is made out of cotton or polyester. The same is not
| true for "decibels". People seem to assume that we're too
| stupid to understand logarithms, we're not.
| Ekaros wrote:
| Well 10 millimetres of rain is different that 10
| millimetres thickness for say branch...
| kqr wrote:
| At first I was going to contradict you but then I
| realised 10 mm of rain is actually a measure of volume or
| mass, corresponding to 10 kg, right?
|
| And of course, people use it mainly as a rate, i.e. 10 kg
| per hour, or per four hours, or six hours, or 24 hours.
|
| And it gets worse when we start talking about snow, the
| density of which can vary a lot!
| Ekaros wrote:
| Well depending on area you are counting it on. So yes per
| square meter it would be 10 kg. Your rain gauge might for
| example not have 10mm spaced markings...
| margalabargala wrote:
| Could you elaborate on this?
|
| If 10mm rain falls, and you have an open collection
| container, then the depth of the water in that container
| would be more, less, or the same as the thickness of the
| branch?
| camtarn wrote:
| ...as long as the container has straight sides. The area
| of the opening needs to be the same as the area of the
| container all the way down, otherwise the two areas don't
| cancel out.
| margalabargala wrote:
| Sure you can do that, and now you're measuring something
| different. The volume of an exotic container is measured
| in different units than the depth of rain falling from
| the sky evenly.
| camtarn wrote:
| I have no idea what you mean here. Exotic containers? I'm
| talking about something like a measuring cylinder or a
| straight-sided glass or mug.
|
| This StackOverflow answer probably does the subject more
| justice than I can: https://earthscience.stackexchange.co
| m/questions/14587/what-...
|
| but I'll try and explain it succinctly:
|
| What you're measuring is the volume of rain falling per
| unit area in a given time (usually 24 hours). If you're
| collecting rain in a vessel, you divide the volume
| collected by the area of the vessel's mouth to get the
| volume collected per unit area. And in order to measure
| the volume in the container, you measure the height of
| the water and multiply by the area of the vessel's
| bottom.
|
| If the bottom and mouth have the same area, those cancel
| out and you can just specify the rain height _regardless
| of the size of the container_. That is, if you have 1ml
| of rain falling per square mm over 24 hours, it will
| produce the same height of water if you set out a big
| container or a small container, as long as the containers
| have straight walls.
|
| If they don't have the same area, then you can't use the
| handy mm unit.
| margalabargala wrote:
| I couldn't have explained it better myself. Glad you
| understand!
|
| Aside from the last bit. The rainfall per unit area is
| not relevant to the shape of a particular container. 5mm
| of rain falls regardless of your container shape. Whether
| that 5mm of rain falling, also means that the water in
| your container is 5mm deep, is a function of your
| container shape.
|
| It's not a unit problem. If you're trying to measure
| rainfall in a conical vessel, you can do that, and the
| conversion from collected volume to fallen rain will
| still yield the same 5mm out of the sky.
| foxglacier wrote:
| It's like telling a chemist that moles are a useless
| complication. They'll swear up and down how important they
| but have no reasons except that everyone in the field uses
| them so they must be important. Sometimes conventions live on
| past the original need they met and experts are blind to that
| because they're only experts in using the convention, not
| evaluating conventions. Moles might have made sense when we
| didn't even know atoms existed and perhaps dB are similar,
| though I wouldn't know about that one.
| tzs wrote:
| > Moles might have made sense when we didn't even know
| atoms existed and perhaps dB are similar, though I wouldn't
| know about that one.
|
| Moles make sense _because_ atoms exist.
| atoav wrote:
| As someone who teaches this concept to.. art students I have to
| say that this complaint sounds a lot like the typical
| misconceptions and confusions a beginner would have.
|
| Yes, dB is a weird and unintuitive concept and it takes a
| moment to understand it, but it is also extremely useful once
| you get it. The fact that people don't write out the reference
| values does not help either, people will bounce out that audio
| mix at -20dB when in fact they mean -20dBFS which is referenced
| to the digital maximum (Full Scale) value. Above 0dBFS you clip
| the waveform.
|
| People leaving out the reference part is the mean reason for
| the confusion IMO.
| im3w1l wrote:
| So one thing I always wondered about with audio mixing is
| that clipping should be random? Like if the sine waves
| somehow all line up the momentary amplitude could be much
| greater than the typical amplitude.
|
| And this would mean that there isn't a fixed volume above
| which you get clipping and below which you don't get
| clipping. Playing tricks with phases could prevent/cause
| clipping.
|
| Is -20 db then simply a rule of thumb for preventing
| occasional clipping?
| mark-r wrote:
| You're talking about the difference between Average
| amplitude and Peak amplitude. Yes, the peaks will be higher
| than the average, but not by a constant amount unless
| you're working with a pure constant sine wave. It will
| depend on the sound you're working with. The phase is part
| of that sound so generally you can't mess with it.
|
| -20 db would just be a starting point, you could adjust up
| or down from there.
| rocqua wrote:
| Decibels in gain, those are fine. Though they use a silly base.
| dBm makes a decent amount of sense, given the RF background.
| The fact that decibels work differently for voltage and power
| is very weird, but understandable in isolation.
|
| But audio decibels are horribly underspecified. And any other
| use of a decibel as a dimensionful unit is horrible. I think
| the RF people know, and that's why they use dBm. Any system
| that uses decibels as dimensional units needs to make their
| baseline clear.
|
| I recently saw a fan advertising a low decibel noise "at 3
| meters". And it's nice that they advertise (part of) the
| baseline, but it sweeps a ~10db difference in pressure under
| the rug, comparee to the standard 1m reference.
| mikewarot wrote:
| Decibels aren't units... they are ratios. The ratio could be
| gain, or loss, or compared to the noise floor, or the signal
| of interest, or a standard unit, such as Watts, milliWatts,
| or microVolts into 50 ohms.
|
| >The fact that decibels work differently for voltage and
| power is very weird, but understandable in isolation.
|
| If you have a given load, increasing the voltage by a ratio
| of 10:1 (20 dB) is exactly the same as increasing the power
| by a ratio of 100:1 (20 dB) (because increasing the voltage
| ALSO increases the current, and the power is the product of
| the two)
| IsTom wrote:
| > Decibels aren't units... they are ratios.
|
| Until they are a ratio to a specific arcane reference level
| as mentioned in the article.
| grues-dinner wrote:
| It's not that arcane, it's referenced to a sound pressure
| level of 1 pascal. Which, yes, is still arbitrary in that
| it all ends up back at the arbitrary values of things
| like metres, seconds, kilograms, etc.
|
| Volts per pascal might make sense in some contexts (like
| your input buffer power supply), but log volts per pascal
| makes sense in others, particularly for audio
| applications where you stack gains and attenuations onto
| an already-logarithmic domain.
| wl wrote:
| dB SPL is referenced to 20 uPa, not 1 Pa. You might be
| confused by the fact that the 94 dB SPL (1 Pa) is the
| default for microphone calibrators and specifying
| microphone sensitivity.
| grues-dinner wrote:
| I was referring to the latter, which is why volts are
| involved at all. But I suppose if they'd chosen to follow
| the SPL zero point for the voltage-pressure scale as well
| as the absolute pressure scale that's would be equally
| arbitrary in its own way, even if more consistent in
| another.
| margalabargala wrote:
| > If you have a given load, increasing the voltage by a
| ratio of 10:1 (20 dB) is exactly the same as increasing the
| power by a ratio of 100:1 (20 dB) (because increasing the
| voltage ALSO increases the current, and the power is the
| product of the two)
|
| It's not that we don't understand this. We _do_ understand
| this, and simply think it 's ludicrous that the same
| nominal "unit" is used to refer to both, rather than
| calling the voltage one, say, "hemidecibels". Because we're
| not talking about power always, we're talking about, as you
| say, ratios.
| ajuc wrote:
| Would you defend using pound for force and mass because
| "it's often the same"?
| holowoodman wrote:
| Decibels are completely ridiculous. They are only useful if you
| are a cable monkey just adding and subtracting
| amplification/dampening factors. As soon as you need to do any
| kind of non-trivial conversion or computation, dB* is more of a
| hindrance to understanding and simplicity, because you will
| always need to look up in some strange table of dB weirdness.
| Integrate over a spectral power density in dB/Hz? You better
| convert that to real metric first... Need to solder in a
| capacitor/resistor/coil as a filter? Better convert to real
| metric first, because only some pre-made filters are specified
| in dB (and there are quite a few weird conventions, so you
| better convert to real metric and check first...).
| lifthrasiir wrote:
| I do see needs for something like decibels. It is a useful
| way to communicate an inherently logarithmic quantity. The
| real and IMHO only mistake is that it was described as a
| typical unit, while it should have been a unit _constructor_
| and specially treated in the SI unlike others. (Not in the
| SI, but the [milli]meter in mmHg etc. and `p` in pH---really
| p[H+] [1]---are other popular examples of unit constructor.)
|
| [1] To be clear, I'm aware that pH and p[H+] are technically
| different. But that's orthogonal here.
| MegaDeKay wrote:
| I'd say the engineers I knew building satellites at companies
| like SS/L and Boeing deserved a better title than than "cable
| monkeys".
| svara wrote:
| > makes me really doubt that the OP has any practical
| experience about the things they're talking about.
|
| Maybe not, but you can get used to many odd things given enough
| experience.
|
| I totally share the authors view. I don't usually have trouble
| grasping the definition of a unit, but dBs are just hilariously
| overloaded.
|
| The same symbol can literally mean one of two dimensionless
| numbers, or one of who knows how many physical units.
|
| That's not normal, something as basic as units is usually very
| cleanly defined in physics.
|
| Someone in this comment section said it's not a problem because
| there's usually going to be a suffix that is unambiguous. If
| that were actually the case, you wouldn't see these types of
| complaints.
| rusk wrote:
| dB for sound in particular aligns with human experience. The
| (10 -) 1-10 on a volume knob typically aligns with a
| logarithmic scale because we hear differences in loudness at
| an order of magnitude.
|
| A linear volume knob would be frustratingly useless as you
| would have to crank it many many many times the higher up you
| want to go. Presumably hundreds of times. A traditional pot
| couldn't do that of course but maybe you could satisfy your
| curiosity with a rotary encoder?
| IsTom wrote:
| You could have log Watts or something, it doesn't have to
| be dB to be logarithmic.
| rusk wrote:
| That would be dishonest. You don't adjust input power -
| you adjust attenuation
|
| EDIT if you did let's say approximate power, or measure
| and present the consumed power (as some systems do) you
| would still be in a situation about how to present this
| data. Do you present your users with a simple 1-10
| (logarithmic) or a 10 digit display which sweeps over
| vast ranges of uninteresting values.
|
| If you opted for a more compact scientific notation ...
| well guess what that's also logarithmic but in two parts
| LOL
| KeplerBoy wrote:
| We have a unit for that. It's dBm and very easy to grasp.
| 0 dBm is 1 mW, every 10 dBm is an order of magnitude more
| (10 dBm = 10 mW).
|
| dB is only confusing if people omit which quantities they
| are relating. If it's clear like in the case of dBm which
| relate to 1 mW, it's an awesome tool.
| jfengel wrote:
| Unfortunately, people omit the quantities all the time.
| Domain experts assume it when talking to each other, and
| non-experts repeat it without knowing that it refers to
| anything at all. (I still don't really know what it means
| for a sound to have "decibels".)
| jononor wrote:
| When referring to sound in the physical world, "decibel"
| mean dB SPL (sound pressure level). Which is defined as
| the ratio to the smallest perceivable sound pressure
| level. Unfortunately that is still a bit underspecified,
| it may be measured with a frequency weighting like A
| weighting. And then there is the integration time or
| other temporal aggregation, but that is separate from
| decibel/log.
|
| When in analog audio, it usually means dbV, relative to a
| reference voltage.
|
| And in digital audio, usually dBFS - relative to the
| maximum amplitude that can be represented.
| foxglacier wrote:
| You can't take the log of a quantity with units like
| watts. It would be log of some ratio of powers, and then
| it doesn't matter what unit of power you use because they
| cancel out. Instead, it matters what the denominator in
| the ratio is so we're back at needing something confusing
| like dB :(
| taneq wrote:
| Ever have a cheap set of external speakers that got super
| loud in the first quarter turn of the volume knob but were
| pretty much the same loudness after that? Yeah, linear pot
| for the volume knob.
|
| No need for an encoder and software, though, logarithmic
| pots are readily available for precisely this reason. :)
| rusk wrote:
| No I've never had one of those LOL
|
| Pots do log and lin scales but they only have a limited
| angular range.
| mattmanser wrote:
| I've actually noticed this two days ago with some
| bluetooth headphones and my phone.
|
| The volume control on my android phone was acting just
| like this when my headphones were connected. When
| changing the volume with the phone only a small section
| of the bottom quarter of the volume control actually made
| a difference, but the volume controls on the headphone
| themselves were acting "normally".
|
| Usually the phone volume is fine, it only screws up on
| bluetooth devices (my speakers + my headphones). I have
| to use the volume control on the device itself to have
| any good control.
|
| This explains the weird behaviour, the phone volume
| changes are being sent linearly, but the
| headphone/speaker settings are correct and being set
| logarithmically.
|
| i.e. somewhere a developer working on the bluetooth
| integration didn't understand the difference, screwed up
| and never tested it. That it's happening to both my
| Edifier speakers and my cheapo headphones probably means
| it's on the stock Android end (it's a pixel phone).
| Severian wrote:
| I've had the same issues as you, and here are some things
| I've done or tried as a remedy.
|
| Try going into Android "Developer options" and enable the
| option "Disable Absolute Volume". Some devices cannot
| handle the way Android maps the "master" volume of the
| system to Bluetooth. With the option enabled you will
| have a separate slider to adjust the Bluetooth volume,
| and the volume buttons will instead only control the
| "Media" volume.
|
| An alternate thing to do is under the same Developer
| Options is instead of disabling Absolute control is to
| change the Bluetooth AVRCP version to at least v1.5. v1.5
| AVRCP introduces the Absolute Volume control
| functionality.
|
| But, it could also be what you may have are Bluetooth
| devices that do not support Absolute Volume, or lack
| AVRCP v1.5 compatibility. If none of this works, I
| suggest purchasing the "Precise Volume 2.0 + Equalizer"
| app. I use this as it gives you more fine-grained control
| over the number of steps in the volume slider (for
| example, I now have 100 steps). It also allows you to
| calibrate the number of steps to a specific device, so
| you can literally change how many steps from quiet to
| loud. It's worth all of the $10 it costs, and has other
| nice quality of life features as well.
| godsinhisheaven wrote:
| This doesn't make any sense to me. Isn't this completely
| backwards? Wouldn't this behavior be expected from a
| logarithmic knob, and not a linear knob? I know what a
| logarithmic curve looks like, it rises quickly and then
| it tapers off, exactly the behavior you describe. But
| then you attribute that to a lineae knob! The parent
| comment confuses the hell out of me too, I am just really
| not putting 2 and 2 together here.
| hansvm wrote:
| "logarithmic" here refers to the number on the scale
| being logarithmic in the sound pressure level. Restated,
| power is exponential in the knob value, which roughly
| matches human perception of a linear increase. An actual
| linear function is far too slow.
| godsinhisheaven wrote:
| Got it, so the sound pressure is logarithmic, but the
| sound power is exponential, and you can control both at
| once with one knob, and they, align, quite well I guess.
| brazzy wrote:
| The point is that sound _perception_ is logarithmic. You
| perceive a 10 times stronger air vibration as twice as
| loud. So if you have a knob that increases the power that
| produces the vibrations linearly, you hear a logarithmic
| increase.
|
| You need a knob that increases power _exponentially_ to
| hear a linear increase in loudness.
| msandford wrote:
| You're missing a critical piece of information. Human
| hearing (and vision) are logarithmic sensors.
|
| Ears can register sounds from maybe 20-30 dB upwards of
| 120ish which isn't a factor of 4-6 in terms of power but
| rather a factor of 120-30=90 decibels or 9 bels or 10^9
| or one billion.
|
| Because your ears have absolutely enormous range you need
| the potentiometer (pot) to have a logarithmic taper to
| it. The amplifier has an essentially fixed amount of
| amplification so that's a fixed sound dB output. Your
| ears can hear a vast range. A linear pot essentially
| locks the entire output into the same 10 decibels as the
| amplifier maximum output through its linearity. Once
| you've turned it to 10% of the range it has precisely 10
| decibels worth of range left. If you want to turn the
| volume down by 40 decibels you have to do that within the
| 0-10% part of the pot's range.
|
| A logarithmic pot will give you maybe 40-60 decibels
| worth of adjustment by dividing things up differently.
| Every 20% of the range increases the output not by 20%
| but by a factor of 10 let's say. That gives you a pot
| with a range of 50 decibels which is enough that it
| roughly matches the absolutely miraculous range of the
| ear.
| nomercy400 wrote:
| Does that also work for showers, with mixing hot and cold
| water. I feel that a 1% change in the knob/balance goes
| from too cold to too hot.
| detourdog wrote:
| Nothing to do with decibels but the thermostat on the
| water heater might be set too high. If one lowers the
| overall temperature the ratio of cold to hot will
| balance.
| rusk wrote:
| You are bounded by a minimum floor for the hot water.
| Below a certain point you can get legionnaires.
|
| To me it seems like one of two things: external pressure
| between hot and cold is mismatched so a small change to
| one side overwhelms the weaker flow.
|
| Alternatively it might just be a broken or poor quality
| mixer that isn't providing the appropriate 'nuance' of
| control, and that may indeed be expressed as some sort of
| non-linear relationship.
| detourdog wrote:
| The thermostat could still be set too high. I think you
| might be overstating the dangers of legionnaires.
|
| It also a simple thing to check and would be the first
| step in my troubleshooting routine for this complaint.
|
| Here is a link describing the dangers.
|
| https://www.heatgeek.com/hot-water-temperature-scalding-
| and-...
| GuB-42 wrote:
| > Below a certain point you can get legionnaires.
|
| I know you mean legionnaires' disease, but the idea of a
| bunch of soldiers getting to your house because you
| turned your boiler too low made me chuckle. Good thing
| the US have the third amendment to protect against this.
| detourdog wrote:
| In this day and age I believe anything is possible.
| BlueTemplar wrote:
| "What have the Romans ever done for us ?
|
| ... hot water baths ..."
| ajuc wrote:
| Logarithmic scale aligns with human experience.
|
| OP isn't criticizing logarithmic scale in general but dB in
| particular.
|
| If dB in particular aligned well with human experience -
| volume knobs would be labeled with dB values instead of
| 1-11.
| rectang wrote:
| A properly designed volume control affects the sound
| power output logarithmically, but is labeled linearly
| (with 1-11 or 0-1 or something like that) to reflect how
| humans perceive the effect.
|
| People in the field get this wrong all the time -- for
| instance, the volume control on ChromeOS appears to be a
| linear multiplier, yielding a control with huge perceived
| steps in the output between 0 and 3, and negligible
| perceived change in the output between 7 and 10.
|
| I suspect that the confusing design of the dB contributes
| towards how often such mistakes get made.
| losvedir wrote:
| Nothing in the article and nobody in the comments takes
| issue with the fact that it's logarithmic. It's everything
| _else_ that 's the problem. (It's a ratio where the base
| value is situation dependent, and the base of the logarithm
| varies.)
| rusk wrote:
| But the base value is well defined, qualitatively ("a
| quiet room"), which is fine for what we are talking about
| which is "experienced loudness". Once you go past log(3)
| it really doesn't matter what the noise was at log(0).
| sanderjd wrote:
| I'm sorry, but did you read the article? This is not the
| complaint.
|
| If decibels were used _only_ to measure sound relative to
| "experienced loudness" there would be no complaint.
|
| The complaint is that it is used in many other ways,
| often without distinguishing what the base unit is.
| rusk wrote:
| The issue is that it's not "well defined" that it's
| somehow subjective. But the whole point with a
| logarithmic scale is the small values don't matter.
| sanderjd wrote:
| This isn't what the article is about!
| sgarland wrote:
| Sure, but a. IME, volume controls with 1-10 are nowhere
| near log b. many manufacturers don't even do this. A lot of
| car manufacturers seem to use 0-40 for the stereo volume,
| which seems completely arbitrary. I'm assuming they decided
| that's a good balance between granularity and annoyance,
| but c'mon... couldn't have at least capped it at 50?
| Halfway to 100 feels vaguely more intuitive.
|
| This also doesn't even begin to touch on frequency response
| curves.
| TheOtherHobbes wrote:
| This like arguing that aspect ratios are stupid and wrong
| because sometimes they apply to a screen which is really big
| and sometimes it's really small and sometimes they apply to a
| physical print or a photo or a billboard or a vintage TV and
| sometimes it's a jpg or a PNG.
|
| Aspect ratio is a ratio. It can be a ratio between pixel
| counts, or between print dimensions, or physical display
| dimensions.
|
| All of which are useful in their own way, none of which are
| directly comparable, all of which are understandable in
| context.
|
| dB is the same. It's a ratio split into convenient steps -
| more convenient than Bels would be - that compares two
| quantities. The quantities can be measured in different
| units. The units are implied by the context.
|
| The only mild confusion is the relationship between voltage
| and power ratios. But that's a minor wrinkle, not a
| showstopping intellectual challenge.
| sandblast wrote:
| Ratios are numbers. They are literally just fractions. No
| one argues that the numbers don't make sense because you
| can have different units. But that's what units are for -
| to know what does the preceding number refer to. Why have a
| unit that doesn't give you full information?
| more-nitor wrote:
| this
|
| if a TV seller went bonkers and only said "it's 10:16",
| can you guess the actual size of that TV?
| jhbadger wrote:
| But that's not what is being expressed. You might as well
| complain that the ratio doesn't give you information on
| the price, weight, or power usage of the TV.
| gwd wrote:
| > Aspect ratio is a ratio.
|
| Right, but in this case they only give you one of the two
| numbers. Imagine being told that your TV had an aspect
| ration of ":16", and you just have to magically know what
| the other number means in the context. And sometimes ":16"
| actually means ":4", because quadratic mumble mumble, and
| sometimes the number is scaled according to some other "how
| big it seems to humans" factor; all of which you also just
| have to know in context.
| megous wrote:
| Ratio is just a single number. 4:3 can be expressed as
| 1.3333~ and it just says how much bigger one number is
| compared to another.
|
| RE: Yes, I was able to read and understand the article. I
| also have 8 years of EE. Ratio is still a single number
| in the end. You can have an actual size of a monitor 1600
| x 1200 and the ratio of sides is 1600/1200, which is just
| a single number. You can express it in multiple ways. You
| still need at least one size + understanding of what the
| aspect ratio is used to describe in a particular
| situation (units (mm, px, ...), ...) to be able to
| calculate the complete dimensions of a monitor screen.
|
| Same issue with % or ppm, or whatever.
|
| You always need a defintition to understand what the
| numbers are abstracting in any particular situation.
| gwd wrote:
| Sure, and 10db means "10x more power". But:
|
| 1. 10x more power than what? It changes, and you Just
| Have to Know.
|
| 2. It's 10x more power; so if you're measuring power,
| like pascals, then 10db means 10x more pascals. But if
| you're measuring something like voltage, then it's not
| 10x more voltage, it's something else.
|
| 3. And if you're talking about sound, you may be talking
| about objective power; or you might be talking about how
| much more powerful it _seems to humans_.
| ants_a wrote:
| So my display aspect ratio is 2.5dB. Or is it 5dB because
| it's not measuring power?
| dagw wrote:
| _Imagine being told that your TV had an aspect ration of
| ":16"_
|
| We kind of have that with people talking about a screen
| or image being "2k" and then expect you to infer what the
| actual resolution and aspect ratio is from context.
| moefh wrote:
| I think the difference is that if you write a blog post
| complaining about the silliness of these labels, you
| don't get people telling you that no, you don't get it,
| it's totally fine, these are just aspect ratios.
| oasisbob wrote:
| I remember a recent post about the ambiguous nature of a
| pixel, and extended to aspect rations, that garnered VERY
| similar responses to what you described.
| NikolaNovak wrote:
| Yes, and many of us find it silly :-)
| vrc wrote:
| 4:3 only makes sense to you because you know which is
| length and width a priori. I for example, always have to
| recheck that. So if it was written as 1.33 or 4/3 it
| makes the same difference to me, and is similar in that
| way to dB
| pwdisswordfishz wrote:
| Why, you're right: aspect ratios _are_ stupid because
| people often don 't bother distinguishing between SAR, PAR
| and DAR, leading to lots of confusion. And sometimes even
| mistake the ratio of pixel densities for the pixel aspect
| ratio:
|
| https://trac.ffmpeg.org/ticket/11279
|
| https://trac.ffmpeg.org/ticket/11284
| sgarland wrote:
| Nothing better than spending hours encoding something,
| only to discover you got one of those wrong.
| svara wrote:
| Respectfully, I'm not sure you fully understand how dB is
| used. The analogy to aspect ratios only works for one of
| multiple uses of decibel.
|
| dB SPL and dB(A) are not ratios, they're absolute. You can
| derive them from a ratio and a reference level, but the
| former can be expressed in Pascal and the latter relates to
| Pascals after applying a perceptual correction function.
|
| Similarly, dBm can be expressed as an absolute potential in
| Volt.
|
| And then you've got the cases where it really just is a
| ratio (one of two possibilities).
|
| You'll see all of these called "decibels".
|
| You see why people are irritated?
| wisty wrote:
| Yep you have dB for a ratio, and dB SPL as a physical
| unit. Just as the article says.
|
| And yeah, the issue is when people forget to use physical
| units, like if they say it's 30 degrees outside amd not
| saying C F or K or latitude.
|
| The historical context is a bit meaningless as well since
| the main application for the OG dB is 101 classes.
| hgomersall wrote:
| Stop saying "one of two possibilities". It isn't. A dB is
| a power ratio. The fact that you can describe that in
| terms of voltage ratio is a simple reflection of the fact
| that power ratios can be described as a voltage ratio
| squared. When talking about voltage ratios directly from
| dB you're just short circuiting the necessary square
| root.
| sgarland wrote:
| > The units are implied by the context.
|
| This is the absurd part. There do exist other ratios that
| masquerade as units, e.g. specific gravity, and its meaning
| also changes depending on what you're using it for -
| liquids are compared to pure water at 4 C, gases are
| compared to air at 20 C. As the parent comment points out,
| you can get used to things with experience. That doesn't
| make them any less absurd. Look at Fahrenheit, for example.
| I'm American, and I still think it's absurd, but because
| I'm extremely used to it, it feels natural.
| Aachen wrote:
| Coulomb? In a discussion about units, leaving out the
| degree word or symbol made me initially try to parse it
| as that you must mean it literally, though context
| quickly makes clear that isn't the case ^^'
| sgarland wrote:
| Fair point. I did briefly think about finding the Unicode
| symbol on my phone. My bad.
| tzs wrote:
| > Look at Fahrenheit, for example. I'm American, and I
| still think it's absurd, but because I'm extremely used
| to it, it feels natural.
|
| What do you find absurd about Fahrenheit?
|
| If it is that its 0 point is not absolute 0 then I think
| you can make a good case, at least for scientific work.
| It's a bit harder to make the case for absolute 0 being
| the 0 point a scale for ordinary day to day use since all
| temperatures most people deal with will be 3 digit
| numbers (and making you degree large won't help because
| people will still need 3 digit number--they just won't be
| integers any more).
|
| If you find it absurd compared to Celsius then I think it
| is hard to make a convincing case. They are both scales
| with a 0 point way above absolute 0, differing only on
| where they put their 0 point and the size of the degree.
| (They originally differed on direction, with Celsius
| putting 0 at the boiling point of water and setting the
| degree size so that water froze at 100, but Celsius soon
| came to his sense and flipped so the numbers went up as
| it got hotter).
|
| Fahrenheit set 0 at the coldest temperature he could make
| in his lab and tried to set 100 at body temperature.
| Celsius (once he got the direction fixed) set 0 at water
| freezing and 100 at water boiling.
|
| That gives Fahrenheit a smaller degree and puts the range
| of temperatures most people deal with most of the time
| above 0.
|
| Celsius made it easier to memorize two temperatures that
| are very significant in many human activities, namely the
| freezing point of water and the boiling point of water
| (although the latter is probably less
| important...generally most people only deal with boiling
| water when they are trying to boil water and don't need
| to care about the temperature. It's not like freezing
| which can happen naturally and so people often need to
| monitor temperature to find out if there is danger of
| freezing).
|
| But that 0 point in Celsius means that a lot of people
| have to regularly deal with negative temperature which is
| a little annoying.
|
| The metric system chose Celsius, but I've not been able
| to find any compelling technical reason for that. A
| metric system with Fahrenheit would have fine too.
|
| Note that unlike mass, length, area, and volume units
| pre-metric systems generally only had one temperature
| unit. There was nothing in temperature like miles, yards,
| inches, feet, furlongs, etc. for length and gallons,
| pints, cups, etc. for volume. A system that went with one
| single length unit (the meter) and one single volume unit
| (the liter) and then derived larger and smaller units
| from those using consistent ratios and prefixes that were
| the same across different types of units was a massive
| simplification.
|
| I asked an LLM why metric went with Celsius and got a lot
| of circular reasons. For example it cited that various
| thermodynamic forumals would not work with F degrees
| because the Boltzman constant is defined in the SI system
| using K. But the Boltzman constant is defined that way
| because SI uses the metric system. In an F based metric
| system the Boltzman constant would be defined in R and
| everything would work fine.
|
| The non-circular reasons it suggested were also not
| satisfactory. One was that C was more common than F in
| Europe at the time the metric system was created, which
| technically does answer the question I asked but then
| raises the question of why C became more common pre-
| metric.
|
| It also suggested that having water freeze at 0 and boil
| at 100 fits in better with a decimal system which doesn't
| really make a lot of sense.
|
| As for why C became more popular than F pre-metric it
| suggests that the 0 and 100 points were easier to
| reproduce. Fahrenheit's choice of body temperature for
| the 100 point was definitely a mistake as it is too fuzzy
| (it was even dumber than metric's initial choice for the
| meter as 1/10000000th of the distance from the distance
| from the North Pole to the equator along the meridian
| passing through Paris).
|
| Freezing and boiling of water do take some care to use
| (you need to control pressure and contaminants) but are
| going to be more consistent that body temperature.
|
| But there is no reason I can see that the fuzziness in
| Fahrenheit's 100 point couldn't have been fixed by simply
| changing the defining points from 0 and 100 to water
| freezes at 32 and boils at 212. Yes, it is not as easy to
| memorize as 0 and 100 but does let us have a scale where
| most temperatures dealt with by most people most of the
| time are 2 or 3 digit positive integers.
| BlueTemplar wrote:
| It's likely as simple as Celsius befriending/visiting
| France, while Fahrenheit - England...
| sgarland wrote:
| For the same reason that every other Imperial measurement
| is absurd - they're completely arbitrary. 1 inch is 3
| barleycorns, which can have very different sizes. 12
| inches to a foot, because a human foot is a decent
| measurement, I guess? 3 feet to a yard, 5280 feet to a
| mile... these make sense _for their time,_ but we are no
| longer in that time.
|
| I understand the argument for Fahrenheit having better
| granularity with whole numbers in the human range of the
| scale. I don't think that justifies everything else about
| it, especially considering the rest of the world somehow
| manages with Celsius.
| NobodyNada wrote:
| Consider the signal chain of a live audio system, from a
| singer to a PA.
|
| The singer produces sound pressure waves into a mic. These
| pressure waves are measured in dB SPL, and the microphone's
| specifications will tell you its sensitivity, maximum sound
| pressure level, frequency response, on- and off-axis
| response, and so on. An engineer can look at all these specs
| to choose a mic based on the environment, how much background
| noise there is and where it's coming from, the quality and
| character of the singer's voice, etc.
|
| The microphone will produce a voltage, which is measured in
| dBV or dBu. This voltage travels to a preamplifier, which
| boosts the signal to a nominal level by applying a gain (in
| dB, with no units since it's just a dimensionless ratio
| between input level and output level) and converts it to a
| digital signal -- measured in dBFS ("decibels full-scale",
| signal level relative to the maximum level that can fit in
| the bit-width of the digital signal).
|
| Note that all the different signals going into the preamps
| have _wildly_ different voltage levels -- a kick drum
| produces much more sound energy than a singer. And the ratio
| between SPL and voltage is not fixed; different mics have
| different sensitivities, and an active guitar amp will
| generate a signal many orders of magnitude stronger than a
| passive microphone. For some instruments like synthesizers,
| "SPL" isn't even a concept that makes sense because the sound
| is produced entirely electronically, rather than by capturing
| a mechanical wave. So, the preamps are configured to
| normalize all the incoming signals to one nominal level for
| processing.
|
| After the preamp, the signal goes through a digital signal
| processing chain. Most of these processing steps will affect
| the level of the signal, and the amount is measured in dB
| (without units, since this is a dimensionless ratio between
| input level and output level. Remember, we're dealing with a
| fully digital waveform, so there is not even a physical
| measure of "loudness" or "signal strength" that can apply to
| the signal at this point.)
|
| The signals from all the different sound sources are mixed
| together, and the mixer's per-channel volume faders are
| marked in decibels -- usually from +10 to -infinity, with 0
| corresponding to "unity gain" (which means the output signal
| should have the same intensity as the input signal). Again,
| no units because we aren't measuring a physical quantity,
| just how much the mixer should change the intensity of the
| signal.
|
| Finally, the signal is converted back to an analog voltage
| (measured in dBu) and sent to a power amplifier. The power
| amplifier applies an adjustable gain, and outputs a signal
| measured in watts (dBm) for sending to the PA system. You
| might then walk around the room with an SPL meter to ensure
| the sound is at a safe level; the value you measure will
| depend on the frequency and directional response of the
| speakers, as well as how far away from the speaker you're
| measuring.
|
| Throughout this process we had signals in the form of sound
| pressure, voltage, digital samples, and wattage, at power
| levels ranging from microwatts to kilowatts. Even a simple
| physical quantity like sound pressure for a single signal is
| not straightforward -- how far are you measuring it from, and
| are you talking about SPL before the microphone or after the
| speaker?
|
| The fact that a single unit is used for all of these
| different purposes is a feature, not a bug. If my preamp is
| close to clipping and I turn it down by 3 dB, how much do I
| need to turn up the gain at my compressor to compensate? Easy
| -- 3 dB. If we used more "appropriate" physical units at each
| step of the signal chain, it would be impossible to determine
| this number quickly or in your head; and would involve taking
| into account even more factors I haven't mentioned (like
| input impedances). The fact that decibels are scaled
| differently depending on whether you're working with voltage
| or power means that 1 dB corresponds to the same change in
| loudness everywhere without having to remember whether it's 1
| dB of voltage or power.
| jeroenhd wrote:
| The same can be said of any unit. "If you just learn to work
| with it regardless of its weirdness, it's completely normal"
| doesn't mean the standard isn't ridiculous.
|
| I don't think anyone has stated that a logarithmic scale is
| bad. The type of logarithmic scale changing depending on the
| field it's being used in, and the non-standard notation (dBm
| being used instead of dBmW for instance), is just inconsistent
| for no reason.
|
| For a unit of immense scale, I rarely see it used outside of
| the -100 to 100 range, though. That puts its daily use square
| in the middle of SIs giga/nano range. I'm sure the formulae are
| a bit easier by not having to include exponents, but I don't
| see a practical reason why dB's normal use can't have been
| covered by normal prefixes.
|
| What sets the dB* aside from other American units is that this
| one is very close to following standard units. If it weren't
| for the deci- prefix and the usage of standard units like Watts
| and Volts ("Bell-horsepowers"), the inconsistencies in
| practical use would probably have been expected, making
| learning about the weird intricacies of each field a lot less
| infuriating.
| em3rgent0rdr wrote:
| > I don't see a practical reason why dB's normal use can't
| have been covered by normal prefixes
|
| Because instead of numbers going like 1, 10, 100, 1k, 10k,
| 100k, 1M, 10M, 100M, 1G, and so on when using prefixes, we
| get a much more smoother numbers of 0 dB, 10 dB, 20 dB, 30
| dB, 40 dB, 50 dB, 60 dB, 70 dB, 80 dB, 90 dB. You can see the
| the number for the dB get bigger, while when using prefixes
| the numbers get bigger two times in a row and then go back to
| smaller. With dB you usually just see a number from 0 to
| around +/- 100 or so. You can plot dB nicely as an axis of a
| chart and then see the slope of a curve in so many dB per
| decade.
| tuetuopay wrote:
| > You can see the the number for the dB get bigger, while
| when using prefixes the numbers get bigger two times in a
| row and then go back to smaller
|
| Interesting, I don't have any issues with that, and I see
| the numbers getting bigger and bigger no problemo. Perhaps
| it's an issue of metric/imperial, as I grew up in a metric
| country: I have a mental visual model of decades, while dB
| feels linear. The opposite is likely true e.g. in the US.
|
| > You can plot dB nicely as an axis of a chart
|
| Nothing prevents you from putting the decimal scale on a
| chart. As a matter of fact, many engineering fields do
| precisely that. One example that comes to mind are
| components datasheets: a lot is in log scales, but
| explicitly so, by putting the 1-10-100 numbers with naught-
| k-M-G. It's explicitly logarithmic.
| em3rgent0rdr wrote:
| Sure, log scales are done quite often with SI prefixes
| for absolute (non-ratio) data that doesn't have a
| standard reference (which is the case for component
| datasheets). But dB can be more convenient when you want
| to present logarithmically-varying ratios, particularly
| for gain or attenuation (which are relative to unity) or
| when you have a commonly-accepted reference of your
| absolute data that is a useful standard to compare
| against. This gives your 0-point typically at the top or
| bottom of the axis a standard meaning across all graphs
| in a particular domain, and then datapoints typically
| have about a couple significant digits above the "." of
| the dB datapoint that you generally are interested in.
|
| I grew up in a metric country too, but still it is much
| easier to speak of 0 to 100 or so of decibels with
| decades of power being in increments of 10 rather than
| having to say 10, 100, 1k, 10k, 100k, 1M, 10M, 100M, 1G,
| 10G, etc of gain or 100m, 10m, 1m, 100u, 10u, 1u, 100n,
| 10n, 1n, 100p, etc of attenuation. Especially when gains
| or attenuations would be multiplied, then decibel makes
| it really easy to just add and or subtract in decibels.
| For an example, a signal with 10M of gain (or that is 10M
| times some reference) that gets passed through 100m
| attenuation would result in a 1M signal (which takes my
| brain some fiddling with those letters and numbers), but
| in decibel we are just dealing with simple addition &
| subtraction: 70 dB minus 10 dB equals 60 dB.
| viraptor wrote:
| Logarithmic axes exist outside of dB and are just fine.
| They work for any unit too, so log(bytes) for example is
| ok. We don't specifically need dB for it.
| MegaDeKay wrote:
| > I don't see a practical reason why dB's normal use can't
| have been covered by normal prefixes
|
| Because you can easily add dB values that are on greatly
| different scales in your head, especially when the values
| aren't exactly powers of ten. If I have 86 dBW of Effective
| Isotropic Radiated Power and -162 dB of free space loss to
| some distance, the power flux density at that distance is -76
| dBW/m^2.
| neuroticnews25 wrote:
| I think the "whining" is just a stylistic choice and an excuse
| to talk about the things he finds worth noting. I didn't
| perceive the tone of the article as negative.
| ajuc wrote:
| You can use logarithmic scale without making it a pseudo-unit
| that changes the factors depending on the application.
|
| Additionally - attacking people you don't know for ignorance
| because they have different opinions is very narrowminded.
| baxuz wrote:
| You haven't addressed any of the points the author made. You
| only show habituation and status quo bias.
| iLoveOncall wrote:
| Doesn't OP addres that multiple times in the article? He
| constantly reminds that the unit means vastly different things
| based on the domain, and focuses on acoustics after that, which
| is what every laymen would think about given the mention of
| decibels.
| metalman wrote:
| sound IS funny, db does work, and for people building the
| things that produce or record sound. Useing db is just another
| unit used in engineering. Now if someone wants to get into the
| (very facinating), psychoacoustic side of sound, where another
| unit has been tried, "phons", prounounced fonz ;) , then it
| becomes clear that determining sound pressure levels is a
| tricky thing, as at the low end of perceptable sound the real
| world power of the actual sound is miniscule when converted
| into heat or mechanical power, and at the othete end a
| amp/speaker set up that was 100% efficient would only need 5 or
| 10 watts of electricity to power a stadium concert. we work
| with what we have, in a past lifeI got a degree in sound
| engineering, but barely practiced it, found myself way more
| into enjoying the products that have been built to be used, but
| am always greatfull to the people who make it possible, who on
| occasion are entertained by the sometimes exotic ways that I
| abuse the equipment, as the good ones take pride in having
| thought of and planed for such things, but do like confermation
| from the field. STC ratings anyone?
| jvanderbot wrote:
| The fact that the numbers on the scale align with intuition at
| a few points does not mean that the scale is a good one. dBs
| are jargon encoded with false rigor. No other "unit" has
| flexible meaning based on the background of the speaker, but
| jargon does.
| verzali wrote:
| Yeah that was my thought as well. Decibels make it much easier
| to do link budgets as you can just add or subtract instead of
| multiplying or dividing.
| lxgr wrote:
| That's an excellent application of Decibels (or really any
| other logarithmic unit). TFA's gripes are with the many not
| so great ones.
| emilfihlman wrote:
| You are completely missing the point, which is that decibels
| are opaque and carry hidden field specific meaning that is not
| possible to know from the "unit".
|
| This is not a criticism of how useful they are in calculations.
| rcxdude wrote:
| I work in a related field (lots of signal processing, but not
| necessarily RF-y or audio-y), and it's a constant source of
| confusion. I actively avoid using them in technical
| communication because they _will_ be misinterpreted by someone,
| and when we have customers who use them they usually don 't
| actually know enough context to disambiguate them.
| mapt wrote:
| Having a logarithmic scale is a very different feature than
| having this one symbol to express all logarithmic scales with
| contextual meanings that sometimes need to be incorporated as
| two or three separate variations in a conversation.
|
| Why wouldn't this work better broken out as half a dozen
| different units, with objective zero points and mathematical
| convertibility?
| sanderjd wrote:
| "It's weird and blunder prone but useful" is not actually a
| counterargument to the author, who is not arguing that it is
| not a useful thing.
| viraptor wrote:
| > and is a core memory of beginner blunders in the field
|
| Are you into creating artificial issues to make understanding
| things harder? If not... what's your point?
|
| > doubt that the OP has any practical experience
|
| One quick search would save you from writing a very silly
| thing.
| jeremyscanvic wrote:
| Another surprising place decibels pop up, pretty far from
| loudness related things, is in image comparison metrics. Peak
| signal-to-noise ratio is mean squared error normalized by a
| certain peak intensity and it is generally expressed in dB, i.e.
| as 10 log10(normalized mse).
| https://en.m.wikipedia.org/wiki/Peak_signal-to-noise_ratio
|
| Edit: typo
| rocqua wrote:
| Decibels make a lot of sense for ratios in any kind of signal
| processing.
|
| They are not that suited to sound, but sound is generally hard
| to quantify.
| fouronnes3 wrote:
| When I worked on a radar project, my fellow radar engineers (I'm
| software) used dB a lot. A lot of them would actually agree with
| the article, but historical sometimes wins even when you're aware
| of its shortcomings. Aren't we the same in software anyway? The
| email protocol, terminal escape sequences, the UX of git command
| line, etc... Each of those could have an "X is ridiculous" blog
| post (and I would enjoy every single one).
|
| One upside of dB not touched in the article is that it changes
| multiplication into addition. So you can do math of gains and
| attenuations in your head a bit more conveniently. Why this would
| be useful in the age of computers is confusing, but on some radio
| projects both gains and losses are actually enormous exponents
| when expressed linearly, so I sort of see why you would switch to
| logs (aka decibels). Kinda like how you switch to adding logs
| instead of multiplying a lot of small floats for numerical
| computing.
| lxgr wrote:
| > A lot of them would actually agree with the article, but
| historical sometimes wins
|
| Indeed - as evidenced by some parts of the world still using
| non-metric units in daily life or even engineering :)
| perching_aix wrote:
| As a European if somebody tells me the diagonal size of a
| display in cm or meter, I'm simply not able to "grasp" it. I
| need to whip out the calculator and divide by 2.54, turning
| it into inches. It's just how monitors are measured in my
| head at this point.
| lxgr wrote:
| When the Euro was introduced, I've heard people say things
| like "older generations will _never_ adapt to the new
| currency, they 'll always refer to the old denominations in
| their head" - which turned out to be completely untrue.
|
| Adapting to new units is very possible, but it needs a
| concerted effort. Absent a good inherent reason or anybody
| capable of artificially creating one, it won't happen on
| its own.
| djhn wrote:
| Did it? I feel like 60+ year olds (current age) mostly
| still convert (bonus points for not accounting for
| inflation)
| perching_aix wrote:
| Wasn't arguing otherwise. If anything, I was illustrating
| so: I used inch so much, that despite it not being the
| native unit of length measurement over here, for the
| specific purpose I used it for it managed to stick
| incredibly hard.
| nfriedly wrote:
| Using the diagonal to measure the size of a display is also
| a bit weird because it doesn't compare well across aspect
| ratios. A 34 inch 21:9 ultrawide is actually smaller (in
| terms of area) than a 32 inch 16:9 display.
| lifthrasiir wrote:
| The only thing you should know is that any use of bel and thus
| decibel should ideally have the reference level _suffixed_
| (usually in parentheses or subscript), not implied. The absolute
| sound pressure level is dB(SPL). The human-perceived loudness
| level is dB(A) and similar. The RMS voltage expressed in power is
| dB(u) (formerly dB(v), not same as capital dB(V)). And so on. And
| then each different instance of dB unit is simply distinct, only
| connected by the fact that it represents some ratio in the
| logarithmic fashion. Treat any new dB unit you haven 't seen as
| an alien.
| hashhar wrote:
| This is exactly it. The people who get confused by decibels are
| treating it a unit in it's own right when it's really just a
| ratio of some unit.
| margalabargala wrote:
| Disagree.
|
| The people who get confused by decibels, are exposed to
| _other_ people treating it like it 's a unit in its own
| right.
|
| I agree that what the parent described, should be done. If it
| _was_ what was done, this article wouldn 't exist.
| lifthrasiir wrote:
| As I've said in the other comment, I believe this should be
| ultimately addressed by the SI.
| margalabargala wrote:
| I would agree.
|
| Right now what we've got is basically "millis", and you
| just have to know whether the speaker is talking about
| length or mass. I like your proposal.
| lifthrasiir wrote:
| I actually want those suffixes mandatory, because there
| may be multiple plausible suffixes for each use. For
| example the loudness might be dB(A), dB(B), dB(C), dB(D)
| depending on the exact curve or even dB(SPL) if the sound
| pressure level is used as a proxy. So it is much more
| confusable than, say, "millis" when suffixes are implied.
| davrosthedalek wrote:
| There is a legitimate use of dB without a reference
| point. An attenuator attenuates by -20dB, not by -20dBm.
| marcosdumay wrote:
| This is right and all... But this usage still leads to
| confusion about what you are measuring your filter by.
|
| There are filters we measure on power, there are filters
| we measure on signal amplitude, and "signal amplitude"
| can be ambiguous on some contexts too. There should be a
| way to specify this one better.
| davrosthedalek wrote:
| Well, dB is fully specified in that regard. It's always
| power. You can calculate the voltage gain from it under
| certain assumptions, and under normal assumptions you get
| that factor 2. But a -20dB attenuator will always reduce
| the power by a factor of 100.
| Merrill wrote:
| There is also antenna gain in decibels.
| itcrowd wrote:
| dBi
| agos wrote:
| people are often confused by decibels because the necessary
| disambiguation is more often than not absent (see: spec
| sheets of some kind of appliance talking about noise)
| jancsika wrote:
| > The human-perceived loudness level is dB(A) and similar.
|
| But db(A) doesn't really measure that for anything but sounds
| that could cause hearing damage, or test tones. You've
| essentially taken a newcomer's problem of underspecification
| and carried it into the given domain.
|
| I feel like it'd be better to say dB(A) measures _flaunkis_ ,
| which is defined by the human frequency response. Then the
| newcomer's next question will be something like, "how do I use
| flaunkis to compute the loudness of a music recording?" And
| that's the right question to ask, because the answer is: it's
| complicated. :)
| davrosthedalek wrote:
| That is of course not true. dB without reference is perfectly
| fine to use for gain and attenuation. dBm or any of the
| variants would be flat out wrong.
| mitthrowaway2 wrote:
| Even then, you'll get different results if it's 10 dB of
| voltage gain or 10 dB of power gain. You need to know what
| the actual units are.
| mcnamaratw wrote:
| No, dB are always power gain.
| nyeah wrote:
| ...unless we're talking about a unitless ratio, like "this has
| 10dB less power than that". Which happens a lot.
| lifthrasiir wrote:
| But we used to use the same unit for the absolute measure and
| the relative measure, like degrees Celcius/Fahrenheit. (Okay,
| % vs. %p is different but is probably an exception.) I see no
| particular reason to avoid suffixes in such situations.
| nyeah wrote:
| It's correct to use dB without a suffix to indicate a pure
| ratio. (It's ok if some people don't see why that's
| correct. That's acceptable.)
|
| Nobody (I hope) says that 100mW is "10 dBm" more than 10mW.
| That would be wrong. Readers would be confused. Most people
| would cross their fingers and correct it to "10 dB", but
| who knows what it might mean?
| yuvadam wrote:
| Decibels aren't actually that ridiculous if you just accept them
| as plain logarithmic ratios, between your signal and the noise
| floor or some other signal. (Or between anything else, really.)
|
| 3dB is roughly double, 10dB is 10x, but only sounds about twice
| as loud because our ears are weird.
| kazinator wrote:
| > _This means that if you're talking about watts, +1 bel is an
| increase of 10x; but if you're talking about volts, it's an
| increase of [?]10x. This is nuts: it's akin to saying that the
| milli- prefix should have different meanings depending on whether
| we're talking about meters or liters._
|
| Well no, because even if you are focusing on a signal measured in
| volts, the bel continues to be related to power and not voltage.
| As soon as you mention bels or decibels, you're talking about the
| power aspect of the signal.
|
| If volume were measured in meters, which were understood to be
| the length of one edge of a cube whose volume is being given,
| then one millimeter (1/1000th of distance) would have to be
| interpreted as one billionth (1/1,000,000,000) of the volume.
|
| When you use voltage to convey the amplitude of a signal, it's
| like giving an area in meters, where it is understood that 100x
| more meters is 10,000x the area.
|
| There could exist a logarithmic scale in which +3 units
| represents a doubling of voltage. We just wouldn't be able to
| call those units decibels.
| dj3l4l wrote:
| The Bel is a unitless quantity. Yes, by convention, in certain
| fields, it applies to the logarithm of the ratio of powers. But
| in other fields (for example, quantifying a change in the
| degree of evidence for a hypothesis, as in Bayesian probability
| theory) it is applied to a ratio of different quantities (in
| the Bayesian case, a ratio of probabilities). There is no
| reason why dB can't be used for any unit, and its meaning is
| incomplete until the denominator of the ratio within the
| logarithm is known.
|
| This is the gripe that is being conveyed in this article.
| Mathematically, the Bel is unitless. It is only by additional
| context that one can understand the value of the denominator in
| the logarithm.
| paipa wrote:
| The denominator isn't the issue. The context-dependent base
| of the logarithm is, which makes 1 Bel = 10x for some things
| and 1 Bel = 3.16x for others.
|
| I've never heard of decibels used in probability theory. Did
| they adopt it with the same baked-in bastardizations? Please
| tell me +10dB(stdev) = +10dB(variance) isn't a thing.
| dj3l4l wrote:
| The problem stated in the article is that the unitless
| quantity of 1 Bel is effectively applied only to power
| ratios. It is of course true that one can transfer a
| scaling of powers into the base of the logarithm when we
| are trying to figure out what effective scaling of voltages
| corresponds to 1 Bel of power scaling, but it is ultimately
| more meaningful to state that a Bel is "only a scaling of
| power", which is a statement about the units of the two
| variables in the logarithm (but ultimately, once we know
| the denominator that belongs to the definition, the
| numerator is also known, so we only need to know the
| reference denominator).
|
| In Bayesian probability theory, there is a quantity known
| as the "evidence". It is defined as e(D|H) = 10 * log_10
| (O(D|H)), where O(D|H) is the odds of some data, D, given
| the hypothesis, H.
|
| The odds are the ratio of the probability of the data given
| that the hypothesis H is true, over the probability of the
| data given that the hypothesis is false, or: O(D|H) =
| P(D|H)/P(D|NOT(H)).
|
| Taking the logarithm of the odds allows us to add up terms
| instead of multiplying the probability ratios when we are
| dividing D into subsets; so we can construct systems that
| reason through additive increases or decreases in evidence,
| as new data "arrives" in some sequence.
|
| The advantage of representing the evidence in dB is that we
| often deal with changes to odds that are difficult to
| represent in decimal, such as the difference between 1000:1
| (probability of 0.999, or an evidence of 30dB) and 10000:1
| (probability of 0.9999, or evidence of 40dB).
|
| This use of evidence has been around at least since the
| 60s. For example, you can find it in Chapter 4 of
| "Probability Theory - The Logic of Science" by E.T. Jaynes.
| paipa wrote:
| The puzzling thing is why they chose to adopt a term used
| to describe "only a scaling of power" to talk about log10
| values. If 1 Bel simply meant 10x, I'd get it, but Bel
| has baggage and also means sqrt(10)x.
|
| Is odds a power-like or amplitude-like quantity? If you
| can't tell, dB isn't the most fortunate choice. It's not
| like mathematicians need fake units to talk about
| unitless ratios and their logarithms.
| dj3l4l wrote:
| The useage of dB in the case of probability is exactly as
| the 1 Bel -> 10x meaning, but that is the technical
| meaning, without further context of the unit in the
| logarithm. There is no need to conceptualise this further
| as involving power or amplitude (which does not apply in
| probability).
|
| I think that the unit having been popularised in the
| telecommunications industry just meant that every other
| instance of a log_10 ratio in physics lead to a
| realisation that it was a Bel. For Bayesian odds, this
| was probably because even the development of Bayesian
| probability was largely advanced by physicists (E.T.
| Jaynes being a famous example), who also were trained,
| and often worked, in signal processing of some kind or
| another. But I doubt they would have thought about this
| "power-ratios-only" adherence that is more the conception
| of telecommunications engineers, as opposed to
| physicists.
| kazinator wrote:
| (Funny you should mention that because while writing my
| grandparent comment above I vaguely ruminated about some
| kind of example involving standard deviation and variance,
| due to those being linked by squaring.)
|
| Even if there isn't a +10dB(stddev), logarithmic graphs are
| a thing, in many disciplines. You just refer to the axes as
| "log <whatever>". Any time you are dealing with data which
| has a wide dynamic range, especially with scale-invariant
| patterns.
|
| Back in the realm of electronics and signal processing, we
| commonly apply logarithm to the frequency domain, for Bode
| plots and whatnot. I've not heard of a word being assigned
| to the log f axis; it's just log f.
| BlueTemplar wrote:
| One parallel comment mentions 'octaves' being used for
| that.
| kazinator wrote:
| Mathematically, the Bel is unitless because it is a function
| applied to a ratio of measurements. Whatever units those
| measurements have disappear due to the cancelation.
|
| If that measurement is linked to another one in a nonlinear
| way, that nonlinearity doesn't disappear: the fact that if
| the unitless fraction in which certain units canceled out is
| 2, then the corresponding unitless fraction obtained via
| different units is 4.
|
| Just because the units disappeared in a fraction doesn't mean
| they are not relevant.
| leoedin wrote:
| I worked in RF (radar) for a while and the dB/dBm is an
| incredibly useful tool there. It makes reasoning about amplifier
| chains and insertion loss so much more straightforward. It also
| means you can talk about transmitters and receivers in a
| comparable unit - in reality the signals are many many orders of
| magnitude apart.
| kazinator wrote:
| The confusing thing about decibels is not the Watts versus Volts
| thing.
|
| It is the following.
|
| If you mix two identical signals (same shape and amplitude) which
| are in phase, you double the voltage, and so quadruple the power,
| which is +6 dB.
|
| But if you mix two unrelated signals which are about the same in
| amplitude, their power levels merely add, doubling the power: +3
| dB.
| cycomanic wrote:
| That has nothing to do with decibels at all, that's the
| fundamental physics (or mathematics if you want) of adding
| waves, i.e. interference.
| kazinator wrote:
| Sure, but people who are confused by the voltage vs power
| thing (also fundamental physics) will likely be quadruply
| confused by this.
| mousethatroared wrote:
| If they're identical and in phase, the addition is obviously a
| multiplication by 2 -> 6dB.
|
| If they're unrelated the signals can also cancel out. So not
| 6bB. If not 6 dB then what is it? An integral that has already
| been solved for us :D
| calmbonsai wrote:
| If you're specifying decibels in written form you _always_
| include the basis or you 're simply being incomplete with your
| units. I don't understand the complaint there.
|
| In casual conversation, the context implies the basis.
|
| Dealing with decibels is also another shorthand to know the
| domain has a wide enough value gamut such that logarithmic values
| (where addition is multiplication) makes sense. See also, the
| Richter scale.
| Aachen wrote:
| > If you're specifying decibels in written form you always
| include the basis or you're simply being incomplete with your
| units. I don't understand the complaint there.
|
| It seems that a lot of people who are specifying decibels in
| written form aren't aware of what you just said
|
| Did you ever see someone use p as a unit and expect anyone to
| understand it means pico? Because I've seen that with dB. In
| this very thread. You really don't have to look far for the
| tendency for dB to be used as incomplete unit
|
| > See also, the Richter scale
|
| Looking at the article and trying to substitute Richter, I
| can't imagine what you'd plug in the different positions:
|
| - it's not a derivative of another relative unit (no
| decirichter)
|
| - it was not originally meant for something else (dB was
| originally meant for power, leading to difference in meaning of
| dB in dB(V) vs. dB(W), if I'm understanding the article
| correctly)
|
| - it's not trying to indicate the magnitude of the actual unit
| (you don't specify Richter-Volts or something, just Richter, so
| you can't forget to specify the actual unit)
|
| - there aren't different meanings depending on the field you're
| in (a -45dB microphone is mentioned in the article as referring
| to Volts whereas an acoustic 10dB loudness difference is
| apparently in Pascals; Richters are always Richters)
|
| The article criticises a lot of dB's aspects but I'm not sure
| the exponentialness is one of them
| viraptor wrote:
| > I don't understand the complaint there.
|
| The problem is that almost nobody does. I can't remember the
| last time outside of HN comments here that I've seen dB(A) or
| similar in real world.
| BenjiWiebe wrote:
| Exactly! I have seen temperatures specified as degrees, without
| specifying Fahrenheit or Celsius. It's a bit like that.
|
| My exposure to dB is mainly the RF field, and it's simple. dB
| is gain/loss (power ratio, just like the dictionary says!) and
| dBm is relative to a milliwatt. In software defined radio,
| you've got dBFS - relative to maximum representable. And
| they're all labeled correctly.
| badlibrarian wrote:
| Horsepower is a unit of measurement. There's hp and bhp and hpE
| and...
|
| "Other names for the metric horsepower are the Italian cavallo
| vapore (cv), Dutch paardenkracht (pk), the French cheval-vapeur
| (ch), the Spanish caballo de vapor and Portuguese cavalo-vapor
| (cv), the Russian loshadinaia sila (l. s.), the Swedish hastkraft
| (hk), the Finnish hevosvoima (hv), the Estonian hobujoud (hj),
| the Norwegian and Danish hestekraft (hk), the Hungarian loero
| (LE), the Czech konska sila and Slovak konska sila (k or ks), the
| Serbo-Croatian konjska snaga (KS), the Bulgarian konska sila, the
| Macedonian konjska sila (KC), the Polish kon mechaniczny (KM)
| (lit. 'mechanical horse'), Slovenian konjska moc (KM), the
| Ukrainian kins'ka sila (k. s.), the Romanian cal-putere (CP), and
| the German Pferdestarke (PS)." [1]
|
| Decibel is not a unit of measurement. Decibels are a relative
| measurement. It tells you how much louder or powerful something
| is relative to something else. And frankly far less ridiculous
| than horsepower, which has a hilarious Wiki article if you read
| it with a critical mindset.
|
| Deriving some of the constants without Googling is a fun exercise
| to verify that you're not as smart as you think you are.
| "Hydraulic horsepower = pressure (pounds per square inch) * flow
| rate (gallons per minute) / 1714"
|
| [1] https://en.wikipedia.org/wiki/Horsepower
| thaumasiotes wrote:
| > "Other names for the metric horsepower are [...]"
|
| I'm not clear on what point you think you're making. Is it
| interesting that the same thing might have different names in
| different languages?
|
| That subsection of the article, by the way, is obviously lying:
|
| > The various units used to indicate this definition (PS, KM,
| cv, hk, pk, k, ks and ch) all translate to _horse power_ in
| English.
|
| cv and ch translate to _steam horse_ , which isn't hard to see
| even if you only speak English. What does "vapor" mean to you?
|
| The opening of the article does suggest some problems, though
| not problems that wouldn't apply to the word "ounce". But you
| seem to have pulled an extended quote describing a completely
| expected state of affairs, while ignoring this:
|
| > There are many different standards and types of horsepower.
| Two common definitions used today are the imperial horsepower
| as in "hp" or "bhp" which is about 745.7 watts, and the metric
| horsepower as in "cv" or "PS" which is approximately 735.5
| watts. The electric horsepower "hpE" is exactly 746 watts,
| while the boiler horsepower is 9809.5 or 9811 watts, depending
| on the exact year.
|
| I don't get it.
| badlibrarian wrote:
| That a unit of power has a dozen metric abbreviations and
| converts to watts at different values depending on what it is
| measuring (and when) is precisely the point.
|
| Decibels aren't ridicluous or a unit of measure. Horsepower,
| however...
| thaumasiotes wrote:
| > That a unit of power has a dozen metric abbreviations and
| converts to watts at different values depending on what it
| is measuring (and when) is precisely the point.
|
| How is that point illustrated by an extended digression on
| the words for the unit in various different languages?
| marcosdumay wrote:
| Yes, those are the reasons why nobody uses horsepower in an
| engineering context.
| bob1029 wrote:
| I think dB is a good way to represent certain user interfaces.
| I've always preferred to operate my audio equipment using a
| logarithmic scale. Changing the volume is much more intuitive to
| me this way than with some linear 0-100 mapping.
| severusdd wrote:
| While I thoroughly enjoyed reading this piece of internet-rant,
| I've to argue that dB is still probably the best we have on this!
|
| In RF engineering, expressing signal levels in dBm or gains in dB
| means you can add values instead of multiplying, which definitely
| appeared like a huge convenience for my college assignments! A
| filter with -3 dB loss and an amplifier with +20 dB gain? Just
| add. You can also use this short notation to represent a variety
| of things, such as power, gain, attenuation, SPL, etc.
|
| I guess, engineers don't use dB because they're masochists
| (though many of them surely are). They use it because in the
| messy world of signals, it works. And because nobody knows
| anything that might work better!
| modeless wrote:
| There's nothing wrong with using a logarithmic system, that's
| not the complaint here. The complaint is that using decibels
| instead of bels is weird, and also that it's a scale and not a
| unit but people use it as if it was a unit without specifying a
| reference point, and also that the scale changes for different
| base units. Crazy how many people here are missing the point.
| blackguardx wrote:
| dBm is fully defined. Its reference is 1 mW into 50 ohms. I
| agree that using dB as a unit and not as a comparison (10 dB
| more power) is confusing, though.
| em3rgent0rdr wrote:
| > For some reason, the bel -- again, what started as a sensible
| 10x increment -- was soon deemed too big to use. I don't quite
| know why: in other aspects of life, decimal notation suits us
| just fine.
|
| Decimal notation can be a tad cumbersome to write and speak.
| Meanwhile, decibel usage commonly results in nice simple numbers
| that range between 0 and 100, with the fractional digits often
| being too insignificant to say out loud. For instance, the
| dynamic range of 16-bit audio (which is generally all the range
| that our ears care about) is 96 dB, while volume increments
| smaller than 1 dB aren't really noticeable, so decibel makes it
| easy to communicate volume levels without saying "point" or
| writing a "." or breaking out exponential notation. Even in
| fields other than audio the common ranges also conveniently will
| be around 1 dB for being on the verge of significance to around
| 100 dB or 200 dB for the upper range. (Also the whole power vs
| root-power caveat is simply something users of dB have to be
| cognizant of because we need to stick with one or the other to
| make consistent comparisons, and at the end of the day physical
| things hapen with power.) So while decibels may seem ridiculous,
| they actually are quite convenient for dealing with
| logarithmically-varying numbers in convient range from 1 dB to
| around 100 dB or so in many engineering fields.
| em3rgent0rdr wrote:
| Its like why we use percent from 0% to 100% instead of speaking
| of ratios from 0 point something to 1.
| cousin_it wrote:
| Yeah. And don't get me started on the folks who think
| "6dB/octave" is a reasonable thing to say. Which is, apparently,
| everyone who works with audio filters except me.
| jpc0 wrote:
| How else would you describe what occurs other than 6dB/octave?
| badmintonbaseba wrote:
| Notice how both dB and octave are ratios, but for some reason
| for frequency ratios we don't use bels or decibels. An octave
| is a ratio of 2, a bel is a ratio of 10.
| cousin_it wrote:
| For a 6dB/octave lowpass filter, I would say power gain is
| inversely proportional to frequency squared (1/f^2). There's
| precedent for this notation, for example pink noise is known
| as both 3dB/octave and as 1/f.
| taneq wrote:
| You use a unit that maps well between the thing you want to
| measure and the thing you want to know about it. If you do this
| enough with the same until you give it a name. I don't get the
| author's complaint.
| mjaseem wrote:
| When you want to measure a related but different quantity,
| don't use the same name as the first one because that leads to
| confusion. This rule is not followed by Decibel and that is
| complaint.
| undebuggable wrote:
| If decibels in acoustics confuse you, try learning decibels in
| optical communication. That's a witchcraft.
| varjag wrote:
| Decibels are not anywhere near as ridiculous as Marketing
| Kilobytes introduced in the 1990s. Pot, meet kettle.
| nyanpasu64 wrote:
| I have a vague report that "root-power" or voltage-like
| quantities (20 dB per order of magnitude) are signed or vector-
| like, while power-like quantities are unsigned scalars?
| nabla9 wrote:
| Decibel is ridiculously overloaded concept.
|
| It can be used to express and calculate relative change in power,
| amplitude ratios, and absolute change. All of these are different
| units and should always use different notation, but sometimes
| it's skipped.
| stephen_g wrote:
| But a lot of them can be combined - like you can take dBm of a
| microwave signal going into a filter, subtract the dB figure of
| loss through a filter, add the dB figure of gain through an
| amplifier, then add the dBi of antenna gain, and get a
| meaningful number (equivalent isotopic radiated power or EIRP,
| in dBm) out the end! It's quite amazing!
| amai wrote:
| Fahrenheit on the other hand makes sense...
| pajko wrote:
| Loudness is not measured in dB, but in phon
| (https://en.wikipedia.org/wiki/Phon) or sone
| (https://en.wikipedia.org/wiki/Sone).
| dubcanada wrote:
| Tell that to everyone on the planet who uses dB
| mattclarkdotnet wrote:
| Or dB(SPL) as a ratio of the phon.
| cesaref wrote:
| Generally speaking, the db scale is very useful for many
| practical situations, and this is overlooked in this critique.
|
| As have been pointed out, it's just a power ratio on a
| logarithmic scale, but this has many benefits, the main one being
| that chaining gain/attenuation in a system is just a case of
| adding the db values together. 'We're loosing 4db in this cable,
| and the gain through this amp is 6db, so the output is 2db hotter
| than the input'. Talk to any sound engineer and you'll do this
| sort of thing successfully without necessarily understanding the
| science, so that's a massive win.
| remram wrote:
| I didn't get that reading the article. The author acknowledge
| that ratios are useful, but that there are specific problems in
| how we use this unit and how we picked and express the
| reference scales.
|
| > On the face of it, the idea makes sense.
|
| Your specific example is a pure ratio so there's no problem
| with it (there is no reference). Apart from the fact that I
| have to guess whether you are measuring volts or watts through
| your cables, of course...
| BenjiWiebe wrote:
| So what I learned today, via another comment and confirmed
| with a dictionary, is that decibels (unspecified reference)
| is NOT pure ratio - it's a _power_ ratio. So it will be
| Watts, not Volts.
| bitdivision wrote:
| But that's why it's so confusing. Did they purposefully
| leave off the reference to indicate Watts, or did they mean
| volts (as I'd guess audio engineers typically do when
| talking about amplifiers).
| Aachen wrote:
| > this has many benefits, the main one being that chaining
| gain/attenuation in a system is just a case of adding the db
| values together. 'We're loosing 4db in this cable, and the gain
| through this amp is 6db, so the output is 2db hotter than the
| input'.
|
| I'm not a sound engineer, so to check my understanding: would
| this not be the case for any other scale indicator?
|
| If you have a cable that loses 4m and you're sending 6m into
| it, you'd not get 2m out?
|
| (The m being milli here, as in millivolts or whatever unit
| would be useful here -- leaving it unspecified to keep the
| comparison to dB as close as possible)
| BenjiWiebe wrote:
| It's a ratio, not a fixed amount. The amount you lose depends
| on the amount you send.
| bitdivision wrote:
| No, because db is logarithmic. That's the key property that
| allows you to do addition instead of multiplication. Remember
| these are ratios here as well.
|
| So the original example in linear units would be: We're
| reducing the signal by 63% in the this cable, and the gain
| through this amp is 199.5%, so the output is...
|
| 0.63 * 1.995 = 1.26
|
| 126% which is 20log10(1.26) db or around 2db.
|
| -4 + 6 = 2 is a little easier to do.
| layer8 wrote:
| Tangential question: Are there any sound meters that can actually
| measure down to 0 dB? The best I've seen specify 20 dB, which to
| me is still very audible.
|
| (Note that, as the article mentions, 0 dB doesn't mean "zero
| sound pressure", but just "threshold of hearing".)
| danadam wrote:
| Some laboratory-grade equipment probably.
|
| > 0 dB doesn't mean "zero sound pressure"
|
| From the decibel definition, zero of anything is -[?] in
| dB_suffix scale.
| layer8 wrote:
| > Some laboratory-grade equipment probably.
|
| I wasn't able to find any listings, that's why I'm asking.
| 867-5309 wrote:
| still doesn't answer what 0.3 Sone is in dBA
| dogman1050 wrote:
| His first equation is wrong. It should be P=V^2/R, not P=V^2R.
| halayli wrote:
| this is one of the best videos to understand what a dB is and
| when it is used, and how to interpret them. what's unique about
| this unit is it relies a lot on the context.
|
| https://www.youtube.com/watch?v=1mulRI-EZ80
| numpad0 wrote:
| I just can't understand why "[specialized domain] uses these
| stupid wrong units, it should be [unit that no educated smart
| SMEs use]" types don't do their researches to understand why
| those are used at all. This type of weird non-SI units appear
| when means of measurement and unit is related to one another and
| has downstream dependencies.
|
| Just look at aviation. An airplane's: - speed is
| measured in knots, or minute of angle of latitude per hour, which
| is measured by ratio of static and dynamic pressure as a proxy.
| - vertical speed or rate of climb is measured in feet per minute,
| which is a leaky pressure gauge, probably all designed in inches.
| - altitude is measured in feet, through pressure, which scale is
| corrected by local barometric pressures advertised on radio, with
| the fallback default of 29.92 inHg. When they say "1000ft"
| vertical separation, it's more like 1 inHg or 30 hPa of
| separation. - engine power is often measured in "N1 RPM %"
| in jet engines, which obviously has nothing to do with anything.
| It's an rev/minutes figure of a windmilling shaft in an engine.
| Sometimes it's EPR or Engine Pressure Ratio or pressure ratio
| between intake and exhaust. They could install a force sensor on
| the engine mount but they don't. - tire pressure is psi or
| pound per square inch, screw tightening torques MAY be N-m, ft-
| lbs, or in-lbs, even within a same machine.
|
| Sure, you can design a battery charging circuits in Joules, fly
| an airplane with a GPS speedometer, analog audio-radio circuitry
| in millivolts. _Absolutely no one does._ I think that cognitive
| dissonance should trigger curiosity circuitry, not rant mode.
|
| I mean, just type in "use of decibels[dB] considered harmful" at
| the box at chatgpt.com. It'll generate basically this article
| with an armchair version of the top comment here as the
| conclusion.
| anilakar wrote:
| Wait until you hear about unit conversions in aeronautical
| mental math. 1 NM across ground corresponds to 1000 or 100 feet
| of altitude in many conversion formulas. For small angles,
| sines and tangents are approximated by degrees of azimuth x
| distance in NM = 60.
| tim333 wrote:
| The author gives the downsides but not the upside of why it is
| like that.
|
| It's basically so it describes sound levels on an understandable
| scale with 0db being just audible and 100dB being very loud.
|
| It also corresponds to the energy carried by the sound - 0 dB is
| 1 pW per square meter so it is kind of a scientific unit. It's
| probably easier to have a measure that is understandable by the
| public and let engineers do conversion calculations for signal
| levels in networks than the other way around.
| jeroenhd wrote:
| I assume you mean dB(A) here, because the sound level at
| dB(SPL) depends on the reference pressure chosen (20 mPa for
| db(A) with a calibrated microphone) and the constants for dB(A)
| are applicable to a large part of human sound perception.
| Though, if you use human perception as a reference, you'll also
| need to take dB(B) and dB(C) into account, depending on the
| sound you're playing and where you're playing that sound.
|
| You can't use "dB" standalone. You need to specify what kind of
| dB you're talking about, because every field has different dBs
| that measure different units or are adjusted to different
| constants.
|
| 10dB is like 10k. 10dB is a tiny number if you're talking about
| volume, but blows your audio equipment if we're talking dB(u)
| or dB(v) (not to be confused with dB(V). In the same sense, 10k
| is a low number if it's the price for a new electric car, or an
| election changing amount of voters when talking about a
| population.
| tim333 wrote:
| Ah - it's complicated I guess. I was looking at this stuff ht
| tps://en.wikipedia.org/wiki/Sound_intensity#Sound_intensit...
| cb321 wrote:
| Abbreviation confusability is relative to { in fact one might say
| _measured by_ ;-) - number? entropy? etc. } the listener
| /reader's knowledge/exposure, much as sound levels need a
| reference distance.
|
| I have heard "bare K" refer to a great many different things, not
| just kilobits (transmission) or kilobytes (storage) or kilograms
| (drug trade) or kilometers (foot races) and on & on, but pages or
| items or etc.
|
| The fundamental problem is that some humans like to abbreviate
| while others get caught and annoyed by the necessary ambiguity of
| such abbreviation. Sometimes this can be the very same human in
| different contexts. ;-)
|
| In fact, there even seems to be some effect where "in the know
| people" _do this intentionally_ - like kids with their slang - as
| a token of in-group membership. And yes, this membership is at
| direct odds with broader communication, by definition
| /construction. To me this article seems to be just complaining
| about "how people are". So it goes!
|
| This is the primary complaint. The secondary one about voltage
| and power and the ambiguity of the prefix itself was addressed in
| another comment (https://news.ycombinator.com/item?id=44059611).
| lambdaone wrote:
| It's a bit like hating numbers, and saying "What do you mean by
| 'three'; what is it - three volts, three amps, three metres?
| Clearly 'three' is meaningless, and we should stop using it and
| all the other numbers besides."
|
| decibels are simply a dimensionless ratio, used as a multiplier
| for some known value of some known quantity.
|
| In every context where decibels are used, either the unit they
| qualify is explicitly specified, or the unit is implicity known
| from the context. For instance, in the case of loudness of noise
| to human ears in air, the unit can be taken to be dBA (in all but
| rare cases which will be specified) measured with an appropriate
| A-weighted sensor, relative to the standard reference power
| level.
|
| And similar (but different) principles apply to every other thing
| measured in dB; either theres an implicit convention, or the 0 dB
| point and measurement basis are specified.
|
| People who assume that everyone is an idiot but themselves are
| rarely correct.
|
| I look forward to the author discovering about (for example) the
| measurement of light, or colorimetry, and the many and various
| subtleties involved. The apparent excessive complexity is
| _necessary_ , not invented to create confusion.
| weinzierl wrote:
| _" decibels are simply a dimensionless ratio, used as a
| multiplier for some known value of some known quantity."_
|
| Except they are not. 1 dB can sometimes mean a ratio of ~ 1.26
| and other times it can mean a ratio of ~ 1.12.
|
| _" In every context where decibels are used, either the unit
| they qualify is explicitly specified, or the unit is implicity
| known from the context."_
|
| Maybe in university, but certainly not in the real world.
| 20k wrote:
| I have a feeling that a lot of people here haven't read the
| article. There's lots of "Its _just_ a dimensionless ratio "
| comments
| hgomersall wrote:
| It's a power ratio. 1dB always means a power ratio of 1.26.
| That might mean a voltage or current ratio, say, of 1.12, but
| that is because the relationship between voltage ratios and
| power ratios is a simple square.
| klodolph wrote:
| Exactly. I've never seen it otherwise.
| weinzierl wrote:
| I understand that and it just reinforces my point. What 1
| dB essentially means is highly dependent on a most of the
| time silent context.
| hgomersall wrote:
| No, it always means a power ratio. That sometimes can
| imply something else, but that's on you to work that out.
| lxgr wrote:
| Yes, but that, i.e. putting the burden of disambiguating
| some meaning from context on the receiver instead of the
| sender, is just bad communication.
| hgomersall wrote:
| What? Blaming a bad communicator for bad communication is
| fine. Blaming their words because they used them badly is
| not.
| lxgr wrote:
| What about blaming a common but confusing usage of some
| words pervasive in some fields?
| twelvechairs wrote:
| The other side of the authors discussion is the use of
| 'decibels' to describe 'loudness'. The big difficulty there
| is that 'loudness' is a sense perception that varies
| between people and in different contexts. The article
| touches on this 'weighted to mimic human hearing...' but
| doesnt mention the systems to do this - DB(A) and others,
| none of which achieve scientific perfection.
|
| Our senses are all like this - for the same reason we have
| dozens of systems to describe color. And why perfume and
| wine makers can never agree descriptions.
| lxgr wrote:
| That's maybe what it "is", but not the only thing people
| use it for in many fields. Very often, it's a ratio between
| a measured quantity and some (implied) base unit of the
| same quantity.
| hgomersall wrote:
| In which case they're misusing it, which is hardly a
| problem with the notation.
| lxgr wrote:
| As I read it, the criticism of TFA is directed at how
| people actually use the notation, not how it ought to be
| used in an ideal universe.
| hgomersall wrote:
| But people misuse words and notation all the time without
| anyone arguing they're ridiculous. I use dBs on a daily
| basis with plenty of other people and never have any
| trouble. Indeed, we'd struggle if we had to use something
| else.
| lxgr wrote:
| > But people misuse words and notation all the time
| without anyone arguing they're ridiculous.
|
| That only indicates that you haven't found the many angry
| blog posts yet, not that they don't exist :)
| arghwhat wrote:
| The problem isn't really the ratio, but the use in
| arbitrary contexts that require a lot of pre-existing
| knowledge. The reference value is sometimes a rather
| arbitrary value in an arbitrary unit, neither of which is
| communicated by the "dB" unit suffix.
|
| The SI way to write `10 dBm` is to write `10 dB (1mW)`,
| clearly communicating both the power level and the
| reference point and unit. This ensures that you do not have
| to just memorize a bunch of decibell suffixes and their
| magical reference values.
| tpoacher wrote:
| This sounds more like a problem with the 'real' world then.
| lambdaone wrote:
| In the "real world", techically competent people understand
| decibels just fine.
|
| But this is not arcane knowledge only known by a priesthood.
| Since you are clearly confused about what a decibel is and
| how and why it is used, you can read
|
| https://en.wikipedia.org/wiki/Decibel
|
| and all will be revealed.
| sanderjd wrote:
| > In every context where decibels are used, either the unit
| they qualify is explicitly specified, or the unit is implicity
| known from the context.
|
| The author's whole point is that this is not true.
|
| To adapt your analogy, it's not like being mad at the number
| three, it's like being mad about people not attaching any units
| to the number three, arguing that it's clear in context. It
| isn't!
| klodolph wrote:
| Sure, but the author is wrong.
|
| "How old is your son?" "He's 3."
|
| Clear in context. People write things like dB SPL
| (A-weighted) in spec sheets because spec sheets benefit from
| being unambiguous. Most of the time it's really clear, like
| you're talking about insertion loss or amplifier gain and
| there's only one reasonable way to interpret it.
| viraptor wrote:
| That only works if you're already familiar with the context
| and system and assume other people are too and don't care
| about anyone new to that area. (Good luck coming to the
| audio equipment datasheet with no experience and figuring
| out what the dB means in each case) "He's 3" works because
| of the previous question and because everyone had
| experience of talking about age.
|
| dB for anyone not already knowing the answer is like going
| to another planet and hearing "he's 3". Of course it's on a
| logarithmic scale, offset to -5 as starting point, counting
| the skin shedding events - clear in context and you
| should've known that.
| klodolph wrote:
| Maybe I just live on the planet, but I don't have this
| problem with dB and to me, it sounds like you're the
| alien. Maybe you could elaborate, or give a motivating
| example?
|
| I just don't remember encountering the problem you're
| describing, and it's unfamiliar to me. There's something
| about your experience that I don't understand, but I
| don't know what it is.
| sanderjd wrote:
| The explanation and motivating examples are in the
| article.
| klodolph wrote:
| The article only had bad examples in it, I was hoping for
| perspective from someone that made sense.
|
| The voltage / power example doesn't make sense. It's
| always power or voltage squared, which are equivalent
| when the load is resistive.
| sanderjd wrote:
| The articles examples were fine. You (and lots of others
| here) are being obtuse.
| viraptor wrote:
| Moving from EE to audio to radio is enough to go through
| a few iterations of "people just write dB but mean
| completely different things". I got used to it, but that
| doesn't stop me from saying it's a bad idea and we should
| improve things for the next person.
| klodolph wrote:
| Audio, the only gotcha I've seen is that -10 is -10 dBu
| and +4 is +4 dBV. That one is sloppy.
|
| But this comment doesn't illustrate your point, and I
| still don't really understand where you're seeing this.
| sanderjd wrote:
| Yeah exactly. Lots of people here can't seem to
| distinguish between "thing that I'm used to" and "thing
| that is good practice".
|
| But these are totally different. I'm used to and thus
| comfortable with lots of things that are nonetheless
| terrible!
| 00N8 wrote:
| I often see pop sci articles saying something like '400
| dB would represent a sound strong enough to tear the
| world apart', or 'military sonar is X dB -- strong enough
| to liquefy your organs at Y distance'. It's rarely clear
| to me which of these usages of 'dB' are directly
| comparable. I _think_ the dB measurement for sonar is a
| different scale /unit than the one for hearing damage
| thresholds in air, but I couldn't figure out how to
| convert between the two last time I spent a few minutes
| trying to look it up, so in my opinion it can be fairly
| confusing.
| formerly_proven wrote:
| Can you show one of those audio equipment datasheets
| where it just says "<number> dB" a bunch of times and
| it's really unclear and confusing?
| drob518 wrote:
| Then your complaint is with the dropping of units, not
| dB.
| sanderjd wrote:
| Yes. The point is that for some reason dB seems
| particularly susceptible to people dropping the units.
|
| For instance, I've heard loudness of sounds described in
| decibels for my whole life, and first saw the actual
| units people are describing when I read this article and
| thread today.
| drob518 wrote:
| That's because you're a casual observer. If you're an
| audio engineer, recording things, designing microphones,
| amps, or speakers, then you'd know it. Trust me. I'm a
| digital electrical engineer (computer engineering,
| basically). I thought that dBs were weird, too. My dad
| worked in microwave communications systems for his career
| and dBs are perfectly natural for him. Ditto my daughter
| who is an audio engineer. Dropping units when you're
| working in a particular field is quite common, as who
| wants to be needlessly wordy when it's redundant and
| everyone in the industry understands it? IMO, this
| article is just the author raging about his own
| ignorance.
| viraptor wrote:
| There are two paths: "it was weird but then I got used to
| it, you're just ignorant" or "it was weird, I got used to
| it, but we should improve the situation". I know which
| side I want to be on. Even if it takes decades like the
| data SI prefixes.
| drob518 wrote:
| You're discounting familiarity as being stupid. The real
| path is "it was weird but once I spent some time with it,
| it made perfect sense."
| JadeNB wrote:
| > You're discounting familiarity as being stupid. The
| real path is "it was weird but once I spent some time
| with it, it made perfect sense."
|
| I don't think either of your parent's paths say that:
|
| > There are two paths: "it was weird but then I got used
| to it, you're just ignorant" or "it was weird, I got used
| to it, but we should improve the situation". I know which
| side I want to be on. Even if it takes decades like the
| data SI prefixes.
|
| I believe that they're saying that, yes, experts get used
| to it, after which it makes complete sense (as would any
| arbitrary but consistent convention, once you got used to
| it), but, in any living field, there will constantly be
| non-experts looking to become experts. If there is a way
| to make the process easier for them while not introducing
| any lack of precision that would hamper experts, then why
| not?
| sanderjd wrote:
| No, it doesn't make perfect sense. It's a bad practice to
| leave off units. You just got used to it and seem to have
| developed pride in being in the in-crowd of people
| comfortable with an unclear jargon, and that has now
| blinded you to a bad status quo. Many such cases!
| klodolph wrote:
| What's to improve? I think the situation works well for
| people who work in the fields that use dB.
| sanderjd wrote:
| The improvement would be to specify the units you're
| working with, as is well known best practice in all
| science and engineering disciplines.
| sanderjd wrote:
| That's the whole point. You're failing to communicate
| clearly. You think it's fine because you're used to it.
| But it's bad. That you are used to and comfortable with
| something does not imply that it is not bad.
| viraptor wrote:
| Yes, that's what the whole article is about...
| drob518 wrote:
| But it's not. He's raging against dB.
| viraptor wrote:
| Yes. He's complaining against dB with no reference, not
| against dB(A) for example. (Apart from the naming of some
| of them being silly)
| davrosthedalek wrote:
| But dB without reference makes sense in many many
| occasions. Either because the reference is implicit (not
| ideal, but we have many implicit assumptions in
| communication), or because it's genuinely a ratio.
| Attenuation, gain.
|
| If you every find an "official" written document that
| uses dB not as attenuation/gain and is not specifying the
| reference (at least in a footnote), it's written either
| by idiots or for idiots, or both.
| sanderjd wrote:
| No it doesn't. It's always bad for the actual unit to be
| implicit.
| klodolph wrote:
| dB(A) is a weighting. It's not a reference and it's not
| units. I think some of the confusion here comes from
| people not actually understanding units.
|
| A-weighting describes how different frequencies are
| summed up. It's like saying "RMS". RMS is not units,
| A-weighting is not units. You can apply A weighting to
| voltage, digital signals, or audio. They all have
| different units but can all be A-weighted.
|
| You could invent a new unit for A-weighted audio, but you
| would need several.
| RichardLake wrote:
| That depends, the dropped unit could be either a year or a
| month.
| klodolph wrote:
| It's understood in context, that's the point.
| pzo wrote:
| no it's not - it's assumed by maybe taking the most
| likely unit (year). But if the conversation is in
| hospital with your kid having emergency I guess doctor
| would appreciate to know if they will have to do surgery
| on 3 months child or 3 years kid.
| icehawk wrote:
| A 3 month old kid looks very different to a 3 year old
| kid.
| pzo wrote:
| and thats the exact point - you assume doctor see the kid
| instead of you calling doctor or doctor is getting
| briefed by emergency stuff.
| thowawatp302 wrote:
| Exactly! It makes sense in context.
| marcosdumay wrote:
| As long as you construct a strawmen strict enough that
| can be no ambiguity, and refuses to acknowledge any
| context where it's not enough, yeah, it always make sense
| in context.
| observationist wrote:
| If the doctor has trouble figuring out the difference
| between 3 months and years, there are bigger problems
| than specificity.
|
| There are places specificity is necessary, and there are
| places the implicit assumptions people make are specific,
| and only need additional specification if the implication
| is violated. That's how language works - shortcuts
| everywhere, even with really important things, because
| people figure it out. There are also lots of examples of
| this biting people in the ass - it doesn't always work,
| even if most of the time, it does.
| jchw wrote:
| You don't drop the unit for months, so it's not ambiguous
| in context.
| sanderjd wrote:
| If you were writing a research paper or engineering
| artifact rather than having a casual conversation, you
| should specify the units ("years old") for age as well.
| jayd16 wrote:
| When you say 35 it would be strange for it to be anything
| but years.
|
| 3 days, weeks, months or years are ironically all common
| units when someone is "3".
| seanhunter wrote:
| No one actually says "my child is 3" meaning anything
| other than 3 years. They would say "3 days", "3 weeks" or
| "3 months" meaning the other lengths of time.
| MyOutfitIsVague wrote:
| Not where I live. You'd never specify how old somebody is
| with a bare number and have it mean anything other than
| years in the US. "My kid is 3" is always 3 years. So is
| "How old is your Tammy?" "Three". That only ever means
| years. Every other unit is always explicit. In my decades
| as a parent and being around other parents of kids and
| newborns, I've never experienced an exception to this.
| sanderjd wrote:
| That's the point. If you are talking colloquially to
| someone where you live, sure, whatever, this doesn't
| matter. But if you are writing something down in some
| publication for a wide and unpredictable audience, you
| should use units.
| bigstrat2003 wrote:
| Most of the time if someone says "he's 3", it is a good
| bet that they mean 3 years. People _usually_ specify if
| they mean days /weeks/months with respect to someone's
| age. Not always, of course, but it's definitely uncommon
| to drop the unit when it's anything except years.
| timeinput wrote:
| But for insertion loss and amplifier gain it is "just" dB,
| it's the ratio of the input to the output. The amplifier
| has a gain of 35 dB means its output is 35 dB higher than
| the input. If the input is -30 dBm the output is +5 dBm,
| etc. The reference for an amplifier, or insertion loss is
| clear in context since you're talking about the gain / loss
| of a device, and isn't referenced to any fixed scale like
| db relative to 1 mW, SPL (A-weighted), or 1 volt.
|
| On detailed spec sheets they list the gain of amplifiers as
| xxx dB.
| drob518 wrote:
| Just because the author is ignorant of the context doesn't
| mean that the engineers working in those fields are. They use
| it because it all makes sense in context.
| sanderjd wrote:
| This is literally the same as saying that you don't need to
| explicitly specify units in any field because "it all makes
| sense in context" to "engineers working in those fields".
|
| No. We've painstakingly figured out the right answer to
| this through the generations of doing science and
| engineering: You always specify units.
| drob518 wrote:
| But obviously we don't. So, there's your counter proof.
| strbean wrote:
| Except the various disasters caused by assuming the wrong
| units (Mars Climate Orbiter, for example).
| monster_truck wrote:
| The team that wrote their code in English units instead
| of Metric defied specifications, that has nothing to do
| with this.
|
| > The Software Interface Specification (SIS), used to
| define the format of the AMD file, specifies the units
| associated with the impulse bit to be Newton-seconds
| (N-s). Newton seconds are the proper units for impulse
| (Force x Time) for metric units. The AMD software
| installed on the spacecraft used metric units for the
| computation and was correct. In the case of the ground
| software, the impulse bit reported to the AMD file was in
| English units of pounds (force)-seconds (lbf-s) rather
| than the metric units specified.
|
| From https://llis.nasa.gov/llis_lib/pdf/1009464main1_0641
| -mr.pdf
| seanhunter wrote:
| Hey don't blame the English for that. I would be prepared
| to wager you couldn't find a single English engineer who
| uses lbfs or anything similar. Everyone in physics or
| engineering uses metric for everything to do with forces
| even those who might use mph for a speed informally.
| drob518 wrote:
| Which proves what? That misunderstandings happen? Yes
| they do. Get over it. But most amplifiers and recordings
| don't crash and burn, so there's your counter proof. Use
| units when they might be ambiguous. But in many fields
| they aren't.
| ashoeafoot wrote:
| No, this are the sounds of underspecification.
| Nevermark wrote:
| > But in many fields they aren't.
|
| I am lost. What fields you are talking about?
|
| 1. I am unaware of any field operating within its own
| echo/context chamber using unit-less numeric notation for
| anything but actual unit-less quantities. Except for
| informal slap-dash arithmetic, on trivial calculations.
|
| 2. Units indicate the dimension being measured, not just
| the relative magnitude within that dimension. Nobody is
| going to know from any shared context, except in person,
| what a bare number measures.
|
| 3. Virtually every measurable quantity has multiple
| possible units of different relative magnitude, depending
| on micro context, so even people within a field, who
| agree on the dimension measured, still need units.
| Meters, light years, AU, angstroms?
|
| 4. You cannot apply standard formulas of physics, or
| anything else, without specific units. Formulas operate
| on dimensions, but to interpret and calculate any
| numbers, you need to know the specific unit being used
| for each dimension.
|
| (In any context, but a late night napkin argument between
| two well acquainted colleagues in a bar, units are
| universally used. And in that case, the opportunity for
| serious misunderstandings is more likely to be from
| missing units, than the quantity of scotch each has
| imbibed, or how much they have spilled on the napkin.)
| drob518 wrote:
| You're arguing both sides. If nobody does it then it's
| not a problem.
| sanderjd wrote:
| The meaning of my last sentence is "You (should) always
| use units (or else there will inevitably be some mistake
| eventually due to confusion about the units)".
| LastTrain wrote:
| Yeah hence his comment "If you know you know." The author
| is far from ignorant on the subject, he's pointing out the
| unit is often used without context.
| hulitu wrote:
| > The author is far from ignorant on the subject
|
| But he surely forgot to search what a decibel is and in
| which context is used.
| marcosdumay wrote:
| You may have missed the short article explaining how it
| could mean 3 different things in an audio context, or 3
| other different things in a radio context. Or how it
| doesn't actually mean anything by itself and yet people
| insist on using it that way.
| Nevermark wrote:
| The point of units is to indicate both what dimension, and
| relative magnitude in that dimension, is being talked about
| clearly.
|
| In virtually any other situation, leaving off units and
| counting on context to fill them in would be considered to
| be at the extreme end of unacceptable.
|
| The unit problems in question, are only accepted because
| they are an historically created anomaly. Not because they
| are a good idea, or anyone wanted that outcome.
| BobaFloutist wrote:
| I mean it's always a little bizarre when the default unit
| used to measure something is dimensionless. Why not set a
| default (like STP) that's assumed to be the baseline unless
| otherwise specified? It would be like if Celsius had no
| reference point and every time you said water boils at 100
| degrees Celsius people came out of the word work to smugly
| correct you "Only if you first say 0 degrees C is when water
| freezes, which you can't assume. What if 0C is actually
| absolute 0?"
| beloch wrote:
| Many scientific measurements are entirely contextual. e.g.
| What is 0 V? Ground? Is ground across the globe always at the
| same potential? Nope. What's ground on a space station?
| Ground potential is whatever we want it to be. You don't need
| to define what ground potential is in, say, a computer,
| relative to some global standard for things to work.
|
| How about velocity? What's 0 m/s? What does it mean to be
| absolutely still? All motion is relative, and being still is
| entirely a matter of perspective. You might be sitting still
| on a train, but traveling very quickly relative to a cow
| standing still while you blow by.
|
| Bels are a relative measure that confuse some because they
| pop up in different contexts that seem unrelated. However,
| they are useful when dealing with quantities for which most
| pertinent relationships are exponential. e.g. They work for
| sound because humans perceive exponential increases in volume
| in a linear fashion. Something that is 3dB louder is _twice_
| as loud in terms of pressure levels, but we only perceive it
| as a little bit louder. Sound pressure levels are both
| relative measures and an attempt to reflect human perception
| . That makes them, necessarily, a bit odd.
| sanderjd wrote:
| None of these other examples are contextual in the same
| way.
| xeonmc wrote:
| Interviewer: "So what's the fastest you've gone on your bike?"
|
| Kid: "Thirty."
|
| Interviewer" "Thirty what?"
|
| Kid: "Umm..."
|
| Kid: "Speed."
|
| [0] https://www.youtube.com/watch?v=OYt1kqDNlMY
| dotancohen wrote:
| KSP measures funds in some unitless value, it drives me nuts.
| Everything else is almost (but not quite) perfect.
| viraptor wrote:
| Just pretend it's a Jane Austen novel and you're earning
| 500. Pounds? Pieces of gold? Sheep? Who knows really ;)
| BlueTemplar wrote:
| https://en.wikipedia.org/wiki/Currency_sign_(generic)
| UncleMeat wrote:
| "I was doing 70 down the highway" is absolutely the sort of
| thing people say. From context (this conversation takes place
| in the US, most highways have a speed limit somewhere around
| 60-70mph) people understand that you are talking about miles
| per hour.
| viraptor wrote:
| And yet, on the internet without the extra context, I can't
| tell if you're going slowly in km/h or the usual in mph.
| With dB that depends on whether you're taking to a person
| familiar with the context-specific conventions or not.
| JoeDaDude wrote:
| Maybe you can say "6 dbm/s"
| skrebbel wrote:
| To be frank, your comment just reads to me as a "stockholm
| syndrome" type reaction to a needlessly complex unit that
| you're intimately familiar with.
|
| You see the same in HN threads where people complain that eg
| Git or Rust are needlessly complex, there's a swath of people
| who are so emotionally invested in how well they understand the
| ins and outs of Git resp Rust that any suggestion that maybe
| things could be better makes them angry.
|
| It's possible for decibels to be usable and generally fine _and
| also_ for them to needlessly complex, ie for there to exist
| better alternatives in each place they 're used.
|
| As an example, it makes no sense to me that eg in audio
| software, volume sliders start at 0 dB and then go _down_ to
| negative $MUCHO, until complete silence at -Infinity. And then
| this same unit is also used to measure how loud my coffee
| machine is, somehow, but then it 's suddenly _positive_ and not
| a relative number at all? That 's just weird shit, it's like
| expressing the luminosity of a pixel (in HSL terms) in lumen
| instead of a unitless percentage.
|
| In the audio software context, it would be much more intuitive
| for "no sound" to be 0, and "full volume" to be 100, a bit like
| percentages. The "but volume needs to be logarithmic because
| that's how we hear it!" argument doesn't disallow that at all.
| Just because a slider goes from 0 to 100 doesn't mean that a 10
| must mean 10% of the power output. Decibels are ridiculous.
| amluto wrote:
| > As an example, it makes no sense to me that eg in audio
| software, volume sliders start at 0 dB and then go down to
| negative $MUCHO, until complete silence at -Infinity.
|
| This one doesn't bother me. Those sliders, and especially the
| real analog sliders they're modeled after, don't have an
| absolute scale -- they are attenuators that reduce voltage.
| So 0dB is the same as no slider at all, -20dB reduces voltage
| by a factor of 10, etc.
| CamperBob2 wrote:
| dB is always relative, even when it appears to be an absolute
| unit. The 0 dB marking at the top of the volume slider on pro
| audio gear (more likely +3 or +6 or something similar to
| leave some headroom) means "0 dB relative to the maximum
| rated power level." In pro gear this will be an absolute
| industry standard of some sort, likely one where the load
| impedance is also defined. 1 milliwatt into 600 ohms or
| something like that. The distinction between voltage and
| power is always going to confuse people, but that's not the
| dB's fault.
|
| A major reason decibels are used is to make it easy to assess
| the overall gain or loss of an entire chain of processing
| stages: you simply add the numbers. The equipment's output
| can only go down from 0 dB, so the rest of the scale is
| negative.
|
| As for sound pressure levels in dB, those are given relative
| to a 0-dB point that corresponded originally to the faintest
| sound people were generally considered capable of perceiving.
| These days "0 dB" refers to a specific amount of acoustic
| power, which I don't know off the top of my head, and that
| might or might not be near the threshold of perception for a
| given listener. But the reasoning still applies:
| amplification or attenuation of power levels is a simple
| matter of addition when expressed in dB. Arbitrarily defining
| a system's reference level to be 100 dB instead of 0 dB would
| be of no use to anyone.
| drob518 wrote:
| Exactly. dB is just a way to apply a logarithmic scale on a
| quantity that would have orders of magnitude range if
| working with a linear scale. It allows an engineer to
| quickly add up all the amplification and attenuation
| through a series of amps and filters without having to do a
| lot of uglier math. It's not really a physical unit. It's a
| marker that says that we're working in the domain of
| logarithmic quantities of some other units.
| nyeah wrote:
| It makes perfect sense that the sliders start at 0dB and go
| down to -inf. Maybe you don't understand it, but it
| definitely makes sense. Everyone who uses dB has also tried a
| % scale with 100% as 0dB, and then later made a conscious
| choice to figure out how dB work.
|
| Maybe they're all in a conspiracy to make things needlessly
| complex. But that's not the only possibility.
| viraptor wrote:
| > and then later made a conscious choice to figure out how
| dB work.
|
| You're just projecting your ideas here. I've not made that
| choice, it's just the only option in a lot of software -
| I'd like my % slider back.
| nyeah wrote:
| Right. Decibels are my idea.
| viraptor wrote:
| Projecting in that context means you claimed what other
| people think/do, because it's what you think/do. It's
| about describing the conscious choices of others (where
| my experience disagrees for example) not about decibels
| specifically.
| nyeah wrote:
| Agreed, you are using the word "projecting" fine. But
| you're ignoring the actual behavior of people using dB in
| the real world.
| formerly_proven wrote:
| Windows has 0-100 volume sliders if you like that better.
|
| They are still some kind of faux-logarithmic*
|
| *behavior depends on drivers/hardware.**
|
| **for some hardware 50 in Windows will be neutral and 100
| will be something like a +30 dB digital gain, that's
| probably in part because Windows is mapping the 0-100
| range in some way to the USB audio control range, which
| is at most +-127 dB or something like that.***
|
| ***with some audio interfaces (the non-USB-Audio-class
| kind) the 0-100 actually becomes a linear factor of 0-1,
| making the windows controls very useless indeed, as 70%
| of the slider range does approximately nothing.
| seba_dos1 wrote:
| % slider sounds like a good idea until you actually have
| to use it.
| regularjack wrote:
| There is no such thing as full volume, and there is not such
| thing as zero volume.
| nyeah wrote:
| To be fair, since we can go to +3dB out of 0dB, we could
| also go to 200% on a 100% scale. In the era of software it
| would not be completely insane to allow users to choose how
| their faders are marked. (It _would_ be insane to use 3 /4
| of a slider's travel for the range 100% to 400%. So the
| _locations_ of the markings on usable sliders would remain
| mysterious to %-only folks.)
|
| Ultimately people who use this a lot would choose to become
| familiar with dB, as they always have. But there's no rush.
| DonHopkins wrote:
| There such a thing as going to 11.
|
| https://www.youtube.com/watch?v=uMSV4OteqBE
| jameshart wrote:
| 'There is no such thing as zero volume' - can you expand on
| that?
|
| Surely constant unchanging air pressure has zero volume?
|
| Volume is the range of variation in air pressure, right?
| That can surely go down to zero.
|
| It can also only meaningfully go up to about double the
| absolute mean air pressure, before what you are talking
| about becomes shockwaves of overpressure.
| tadfisher wrote:
| Air pressure is a statistical measure; given a room with
| zero net variation in pressure, I can always find a
| volume in that room with positive pressure, down to
| measuring Brownian motion of atoms. Now try to design a
| measurement apparatus that can only sample a small volume
| to measure variations in pressure, and you can understand
| why (sound) volume can never go to zero.
| jameshart wrote:
| Over meaningful statistical sampling areas, like an
| eardrum, say, pressure can be effectively constant,
| surely?
|
| Volume can be a statistical property, like temperature.
| crazygringo wrote:
| I don't know what you're talking about. Both of those
| exist.
|
| Zero volume is when a speaker's diaphragm is still. On an
| 8-bit PCM audio file, it corresponds to a value of zero.
|
| In the context of a signal, full volume corresponds to the
| 127 value in an 8-but PCM audio file (or arguably -128). In
| the context of a speaker, those values should push the
| diaphragm no further than how far it can travel linearly
| without distortion. Obviously the user may want to turn
| down the volume from this full volume.
|
| I hope you understand that even when using an audio editor
| that displays values in dB, the underlying values are
| integers (or floats) that _absolutely_ have zero and "full
| volume" meanings, and conventionally map respectively to
| -[?] dB and 0 dB.
| klodolph wrote:
| If you want 0 = no sound, then you can't use a logarithmic
| scale. You end up with sound being measured as Pascals in the
| micro to unit range.
|
| "0 dB SPL" is 20 micro pascals which is roughly the threshold
| of hearing. A loud rock concert at 120 dB SPL is 20 pascals
| (no micro). The dB figures are a lot more convenient to work
| with.
|
| It's intuitive for 0 to be silent and 100 to be full, but if
| you work with audio you learn that dB are more convenient.
| Long-term convenience for experts tends to win out over
| short-term intuition for non-experts. This is why musicians
| continue to use sheet music and all of its seemingly
| ridiculous conventions--and likewise, decibels only seem
| ridiculous to people who don't work in audio.
|
| I don't know what more usable alternative there would be, to
| decibels.
| _kb wrote:
| > volume sliders start at 0 dB and then go down to negative
| $MUCHO, until complete silence at -Infinity. And then this
| same unit is also used to measure how loud my coffee machine
| is.
|
| They're not the same unit, at all.
|
| The audio software is a skeuomorphism from an analogue mixing
| console that is applying a change to a signal. 0 is unity
| gain and deviation from this describes an amplitude
| variation. This is important, as it means you are either
| discarding information by lowering the level and reducing
| dynamic range, or interpolating new information (/ decreasing
| SNR) by applying gain. This is less important today with
| floating point, but has strong historical reasons for
| existence across both analogue and digital domains.
|
| If you look at an audio power amp, you will likely have some
| form of positive number as this is applying gain. Depending
| on the context this may have some specific meaning or it may
| be a screen print of a Spinal Tap logo and the numbers 1..11.
| These are all just UI decisions and part of doing that well
| is presenting coherent information for the target user group.
|
| When you're talking about an acoustic noise source this is dB
| SPL which is a quantifier against a physical reference. That
| reference level quantifier is omitted a lot, which leads us
| to a lot of the angst in this post and the comments here.
| These are precise measurements, with very specific meaning.
| Their expression is often sloppy, but the units aren't to
| blame.
|
| (excuse me while I got "full HN" here - I appreciate the
| irony in this response noting your first few sentences)
|
| The reason people respond strongly to comments like this (or
| those about Git, or Rust) is because details matter. When you
| immerse in a domain, you learn the reason for those details.
| That does not mean things can't be improved, but this also
| does not imply those details can be removed or are wrong. A
| lot of the world, particularly when working outside of the
| bounds of a computer, depends on necessary complexity.
| lxgr wrote:
| > They're not the same unit, at all.
|
| Exactly, so why label two different things using the exact
| same letters in a potentially ambiguous context.
| _kb wrote:
| Because it's not ambiguous.
|
| If I pay for something in Australia and the bill comes to
| $50 this has meaning within that context.
|
| I receive a bill in Zimbabwe for $50 this also has
| meaning within that context.
|
| These values are not equivalent.
|
| Ditto if I were to say it's 30 degrees out. You may
| interpret that as either a good day for the beach, nice
| weather for ice skating, or we need to bear north-
| northeast depending on what context we share.
|
| Language is messy.
| skrebbel wrote:
| This is unnecessary complexity. My rant is against that
| exactly. You're defending confusing shit that doesn't get
| any better from being confusing. If all countries had a
| different currency, things would be clearer too. Ask any
| Australian shopping for digital products on international
| sites. Half of the sites write $ but forget to specify
| whether it's "we geolocated you and guessed AUD" or "haha
| it's USD but we just wrote $ because we forgot that
| there's a world outside the US". If Switzerland would
| rename the CHF to Euro but not change its value to match
| the existing Euro, everybody would agree that that's a
| terrible idea. There wouldn't be edgy HN commenters
| explaining that well, actually, there's precedent so it's
| fine! No, it'd just be bad. The dB ambiguity is a mess
| for the same reason. The situation has no benefit and in
| the world of units, where most other things (eg the most
| of the SI) are actually relatively usable and non-
| ambiguous, it's a fuckup. And the power vs voltage aspect
| of it makes it even worse than the $ situation.
|
| Your argument that it isn't so bad in practice doesn't
| change the fact that it has no benefits whatsoever.
|
| It's _just_ ambiguity for the sake of it, because way
| back when people started measuring sound stuff, nobody
| bothered to go "but wait is this actually handy?" and
| then we got stuck with whatever the first guy came up
| with. It's just like the whole kilobyte/kibibyte crap and
| the whole Wh vs mAh vs kilojoule soup. It's all downside.
| jwagenet wrote:
| How about using kg for both mass and weight? At least as
| an American we learn lb is actually lb_f and lb_m or slug
| is used for mass. The weirdness is consistent with the
| rest of the system. In metric Newton exists as a separate
| and sane measure of force...
| davrosthedalek wrote:
| Ok, I bite. Please come up with a mapping of 0 to 100 to -inf
| dB to 0 dB attenuation.
|
| And then put two in series. Is there a simple formula to
| calculate the total attenuation?
|
| This works flawlessly with dB. Just add. And it doesn't
| matter how you break it: -20dB and -20 dB in series is the
| same as -40dB and 0dB.
| baxtr wrote:
| _> People who assume that everyone is an idiot but themselves
| are rarely correct._
|
| Print this, frame this, put this up on walls in schools,
| offices, heck even outdoor on large billboards!
| anthomtb wrote:
| And before doing any of that, get it tattooed onto your own
| forearm.
| DonHopkins wrote:
| Some people just hate certain numbers.
|
| Many buildings don't have a 13th floor because superstitious
| people think it's unlucky.
|
| Some people lose their shit at 8647 because paranoid delusional
| people think it's out to kill them.
| lxgr wrote:
| > It's a bit like hating numbers, and saying "What do you mean
| by 'three'; what is it - three volts, three amps, three metres?
|
| No, it's a bit like saying "for chocolate M&Ms, 3 obviously
| means 9, to compensate for the fact that they're much smaller
| than the peanut ones".
| timerol wrote:
| Can I get 3 M&Ms? Obviously I mean 3 serving sizes, how else
| would you measure M&Ms?
| DonHopkins wrote:
| At least the Mel-frequency cepstrum is honest about being a
| perceptual scale anchored to human hearing, rather than posing
| as a universally-applicable physical unit.
|
| https://en.wikipedia.org/wiki/Mel-frequency_cepstrum
|
| >Mel-frequency cepstral coefficients (MFCCs) are coefficients
| that collectively make up an MFC. They are derived from a type
| of cepstral representation of the audio clip (a nonlinear
| "spectrum-of-a-spectrum"). The difference between the cepstrum
| and the mel-frequency cepstrum is that in the MFC, the
| frequency bands are equally spaced on the mel scale, which
| approximates the human auditory system's response more closely
| than the linearly-spaced frequency bands used in the normal
| spectrum. This frequency warping can allow for better
| representation of sound, for example, in audio compression that
| might potentially reduce the transmission bandwidth and the
| storage requirements of audio signals.
|
| https://en.wikipedia.org/wiki/Psychoacoustics
|
| >Psychoacoustics is the branch of psychophysics involving the
| scientific study of the perception of sound by the human
| auditory system. It is the branch of science studying the
| psychological responses associated with sound including noise,
| speech, and music. Psychoacoustics is an interdisciplinary
| field including psychology, acoustics, electronic engineering,
| physics, biology, physiology, and computer science.
| jampekka wrote:
| > For instance, in the case of loudness of noise to human ears
| in air, the unit can be taken to be dBA (in all but rare cases
| which will be specified) measured with an appropriate
| A-weighted sensor, relative to the standard reference power
| level.
|
| This was covered in the article. But also it was discussed why
| things aren't this simple.
|
| > People who assume that everyone is an idiot but themselves
| are rarely correct.
|
| Indeed.
| timerol wrote:
| > multiplier for some known value of some known quantity
|
| Of course, unless it's a multiplier for an unknown value of a
| known quantity, like every amplifier, filter, and sensor
| specified in dB.
|
| > For instance, in the case of loudness of noise to human ears
| in air, the unit can be taken to be dBA
|
| Unless you're using dBB or dBC, but of course we all know
| exactly why you'd use dBC (more suitable for figuring out
| safety of high impulse, short events) or dBB (it's somewhere in
| between - you'll know it when you see it).
|
| > The apparent excessive complexity is necessary, not invented
| to create confusion.
|
| Wait, so writing dB instead of dBA is now necessary, not just
| convenient?
| WhitneyLand wrote:
| People who assume things are not complex because they
| understand them are rarely correct.
|
| The complexity absolutely is not necessary. Maybe you mean to
| say it's understandable or it's coherent if you know the rules.
|
| Physics doesn't require us to create ambiguity by assigning DB
| to mean multiple possible ratios. If it needs to be
| disambiguated, then they both didn't need to be DB in the first
| place.
| mystified5016 wrote:
| Exactly, the log(x) function is stupid and useless! It's the
| log of _what_? 6? Pi? Little-g? Same for logn(x). Who the hell
| is n?!
| IvyMike wrote:
| > People who assume that everyone is an idiot but themselves
| are rarely correct.
|
| This is an off-by-one error.
| ok_computer wrote:
| I think college undergrad physics and engineering is an
| important time to gain exposure to a variety of concepts and
| measures. Then you learn the people who discovered or invented
| these concepts were writing by candlelight and not washing
| their hands and didn't have modern mathematics or computers and
| didn't want to piss off the church. Oftentimes working at the
| pleasure of a rich sponsor family or king.
|
| Like you have to describe relative intensity of waves in some
| way and these were experimental scientists, not commerce
| merchants looking for absolute interchangeability like weights
| and measures.
|
| Computer blog people like standards and languageisms but
| science isn't determined by big tech sponsored committee. It's
| the best tool put forward so far and db is a physics concept
| and with a reference denominator you can calculate the absolute
| value. It's fine. If you dabble with physics and expect the
| universe to make intuitive sense then you need an education.
|
| Read books not blogs.
| ttoinou wrote:
| Love this kind of articles but I wished the author suggested
| solutions.
|
| I go even further than this author : sometimes decibels are
| computed using logarithms and what we put inside the logarithm
| has a physical unit. But I can prove mathematically that this is
| wrong and that whats given to a log function has to be
| dimensionless. Hence a lot of dB calculus is mathematically wrong
| and physically meaningless
| adrian_b wrote:
| Decibels are not ridiculous, but very frequently the notations
| for quantities expressed in decibels are ridiculous.
|
| The decibel is an arbitrary unit for the quantity named
| "logarithmic ratio".
|
| Logarithmic ratio, plane angle and solid angle are 3 quantities
| for which arbitrary units must be chosen by a mathematical
| convention and these 3 units are base units, i.e. units that
| cannot be derived from other units. For a complete system of base
| units for the physical quantities, there are other 3 base units
| for dynamic quantities that must be chosen arbitrarily by
| choosing some physical object characterized by those quantities,
| i.e. a physical standard (originally the 3 dynamic quantities
| were length, time and mass, but in the present SI the reality is
| that mass has been replaced by electric voltage, despite the fact
| that the text of the SI specification hides this fact, for the
| purpose of backward compatibility), and there also are other 2
| base units for discrete quantities (amount of substance and
| electric charge) which must be established by convention.
|
| Like for the plane angle one may choose various arbitrary units,
| e.g. right angles, cycles, degrees, centesimal degrees, radians,
| or any other plane angles, for the logarithmic ratio one may
| choose various arbitrary units, e.g. octave, neper, bel, decibel.
|
| So if we choose decibel all is OK. Decibels have the advantage
| that for those used to them it is very easy to convert in mind
| between a logarithmic ratio expressed in decibels and the
| corresponding linear ratio, so it is very easy to make very
| approximate computations in mind, but good enough for many
| engineering debugging tasks, in order to replace multiplications,
| divisions and exponentiations with additions, subtractions and
| rare simple multiplications, for a quick estimate of what should
| be seen in a measurement in a lab or in the field.
|
| The problem is that whenever a logarithmic ratio is specified in
| decibels, it must be accompanied by 2 quantities, what kind of
| physical quantities have been divided and which is the reference
| value. Humans are lazy, so they usually do not bother to write
| these things, assuming that the reader will guess them from the
| context, but frequently the context is lost and guessing becomes
| difficult or impossible.
|
| An additional complication is that one never uses logarithmic
| ratios for electric voltages or currents, but only for powers.
| When it is said that a logarithmic ratio refers to a voltage or a
| current, what is meant is that the logarithmic ratio refers to
| the power that would be generated by that voltage or current into
| an 1 ohm resistor. A similar problem exists for sound pressures,
| because logarithmic ratios are used only for sound intensities,
| so where sound pressure is mentioned, actually the corresponding
| sound intensity is meant.
|
| This complication has appeared because voltages, currents and
| sound pressures are what are actually measured, but powers and
| sound intensities are frequently needed and using logarithmic
| ratios with different values for related quantities, while
| omitting frequently to mention the reference value, would have
| caused even more confusion than the current practice.
| FRidh wrote:
| The problem with using decibels is that since it is a relative
| unit you need to know what it is relative to, and that is often
| not just the unit but also the quantity. Unfortunately, this part
| is (as expressed) at times omitted or put in the wrong place. And
| its often also the engineers in the respective fields that keep
| using this incorrect notation spreading the confusion for those
| outside the domain.
|
| E.g., acoustic engineers often write db(A) for A-weighted sound
| pressure levels. Yes, it is often noted this way, but it is
| incorrect. The correct way is to specify the quantity and that
| the quantity is A-weighted, `L_{p, A} = 80 dB` for example to
| express an A-weighted sound pressure level of 80 dB.
|
| Regarding sound pressure and sound power. Sound power is not
| expressed using A-weighting because it does not make sense. Sound
| power is a property of the source. A-weighting is a property of
| the receiver, that is, the human listener.
| rightbyte wrote:
| > The problem with using decibels is that since it is a
| relative unit you need to know what it is relative to
|
| Ye but that is also the point. The author seem to prefer to
| memorize dozens of absolute values.
| thinkingtoilet wrote:
| Not surprisingly, Tom Scott has an interesting video on this:
|
| https://www.youtube.com/watch?v=Is_wu0VRIqQ
| lamename wrote:
| I appreciate the sentiment, but I've found this resource [0] much
| more direct and comprehensive. It explains all of the nuance
| regarding dB and related terminology from audio engineering to
| perception (voltage, power, intensity, volume, loudness, etc.)
|
| The format is a bit circular; just enjoy getting lost in it for
| half an hour.
|
| https://sengpielaudio.com/TableOfSoundPressureLevels.htm
| HPsquared wrote:
| It's a bit like "percent". A shorthand for dealing with ratios,
| often with hidden assumptions.
| colejohnson66 wrote:
| Sure, but you never say "the production is at 50%". 50% of
| _what_? Peak possible output? Typical output? That 's what the
| author's complaint is: dB, while arguably useful, is annoying
| because people leave out the part that it's relative _to_. And
| then people, when talking about dB, assume the other party has
| knowledge of what their ratio is relative to. dBm, dbSPL, etc.
| davrosthedalek wrote:
| In every communication, the sender assumes that the other
| party has some knowledge. For example, I assume that you can
| read my badly worded English text. There is rarely confusion
| with the people in the fields that use dB a lot. That other
| people often get confused by dB is not a failure of dB, not
| even of its common use, the same way I can't blame English
| for not being Chinese, or for Americans use F instead of C,
| or K, for temperatures.
| BlueTemplar wrote:
| Insert mandatory complaint about percentages being misused :
| their point is the approximation : 1.02x1.03~1.05 <=>
| +2%+3%~+5%, which only holds up to low tens +/-%.
| kragen wrote:
| This is an incredibly valuable article for anyone who's trying to
| make sense of decibels in some context. Michal clearly explains
| almost all of the gotchas you have to understand.
|
| I realized recently, after years of doing it for signal powers,
| that dB are a pretty convenient way to do mental logarithmic
| estimates for things that have nothing to do with power or
| signals, with only a small amount of memorization. Logarithms are
| great because they allow you to do multiplication with just
| addition, and mental addition isn't that hard. For example, if
| you want to know how many pixels are in a 3840x2160 4K display,
| well, log10(3840) [?] 3.58 (35.8dB-pixel) and log10(2160) [?]
| 3.33 (33.3dB-pixel), and 3.58 + 3.33 = 6.91 (69.1 dB-square-
| pixel), and 106*91 [?] 8.13 million. The correct number is 8.29
| million, so the result is off by about 2%, which is precise
| enough for many purposes. (To be fair, though, 4000 x 2000 =
| 8000, which is only off by 3.5%.)
|
| The great difficulty with logarithms is that you need a table of
| logarithms to use them, and a _mental_ table of logarithms is a
| lot of rote memorization. You can get pretty decent results
| linearly interpolating between entries in a table of logarithms,
| so you can _use_ a lot more logarithms than you _know_ , but you
| have to know some.
|
| It's pretty commonplace in EE work to make casual use of the fact
| that a factor of 2x [in power] is about 3 dB, which is a
| surprisingly good approximation (3.0103dB is a more precise
| number). This is related to the hacker commonplace that 210 =
| 1024 [?] 1000 = 103; 1024x is 30.103dB, while 1000x is precisely
| 30dB.
|
| To the extent that you're willing to accept this approximation,
| it allows you to easily derive several other numbers. 4x is 6dB,
| 8x is 9dB, 16x is 12dB, and therefore 1.6x is 2dB. 1/2x is -3dB,
| so 5x is 7dB (10-3). So with just 2x = 3.01dB we already know the
| base-10 logarithms of 1, 2, 4, 5, and 8, to fairly good
| precision. That's half of the most basic logarithm table. (The
| most imprecise of these is 8: 100*9 is about 7.94, which is an
| error of about -0.7% when the right answer was 8.)
|
| If we're willing to add a second magic number to our
| memorization, 3x [?] 4.77dB. This allows us to derive 6x [?]
| 7.78dB and 9x [?] 9.54dB. So, with two magic numbers, we have
| fairly precise logarithms for 1, 2, 3, 4, 5, 6, 8, and 9.
|
| The only multiplier digit we're missing is 7. (Shades of the
| Pentium's x3 circuit: http://www.righto.com/2025/03/pentium-
| multiplier-adder-rever....) So a third magic number to memorize
| is that 7x [?] 8.45dB. And now we can mentally approximate
| products and quotients with mentally interpolated logarithms.
|
| You can do my example above of 3840x2160 as follows. 3.8 is 80%
| of the way from 3 (4.8dB) to 4 (6.0dB), so it's about 5.8dB. 2.2
| is 20% of the way from 2 (3.0dB) to 3 (4.8dB), so about 3.4dB.
| 35.8dB + 33.4dB = 69.2dB, which is between 8 million (69.0dB) and
| 9 million (69.5dB), about 40% of the way, so our linear
| interpolation gives us 8.4 million. This result is high by 1.2%,
| which is much better than you'd expect from the crudity of the
| estimation process.
|
| For a more difficult problem, what's the diameter of a round
| cable with 1.5 square centimeters of cross-sectional area? That's
| 150mm2, half of 300mm2, so 24.77 dB-square-millimeters minus
| 3.01, 21.76dB. _A_ = _pr_ 2. Divide by _p_ by subtracting 5dB
| (okay, I guess that 's a fourth magic number: log10( _p_ ) [?]
| 4.97dB) and you're at 16.76dB. Take the square root to get the
| radius by dividing that by 2: 8.38dB-millimeters. That's less
| than 7x [?] 8.45dB by only 0.07dB, so 7-millimeter radius is a
| pretty decent approximation, 14mm diameter. The precise answer is
| closer to 13.82mm.
|
| For approximating small corrections like that, it can be useful
| to keep in mind that ln(10) [?] 2.303 (a fifth magic number to
| memorize), so every 1% of a dB (101*001) is a change of about
| 0.23%. So that leftover 0.07dB meant that 7mm was high by a
| couple percent.
|
| More crudely: 150mm2 is 22dB, /p is 17dB, [?] is 81/2 ( _pace_
| Fellini), 7x.
|
| It's pretty common in engineering and scientific calculations
| like this to have a lot of factors to multiply and divide,
| increasing the number of additions and subtractions relative to
| the number of logarithmic conversions; this is why slide rules
| were so popular. Maybe you derived the 1.5cm2 number from
| copper's conductivity and a resistance bound, or from the yield
| strength of a steel and a load, say. 3840x2160 pixels x 4
| bytes/pixel / (10.8 gigabytes/second), as I was calculating last
| night in https://news.ycombinator.com/item?id=44056923? That's
| just 35.8 dB + 33.3dB + 6dB - 100.3dB = -25.2dB-seconds, which is
| 3.0 milliseconds to memcpy that 4K framebuffer. (I didn't do that
| mentally, though.) Even 36 + 33 + 6 - 100 = -25, so p ms, is a
| fine approximation if what you want to know is mostly whether
| it's more or less than 16.7 ms.
|
| So here's a full list of the seven magic numbers to memorize for
| these purposes: 2x [?] 3.01dB ([?] 4x, 8x, 5x)
| 3x [?] 4.77dB ([?] 6x, 9x, 1.5x) 7x [?] 8.45dB px [?]
| 4.97dB ln(10) [?] 2.303 ([?] 0.01dB [?] 0.23%, etc.)
| 1.259x [?] 1dB (+1dB [?] +25.9%) (1 - .206)x [?] -1dB (-1dB
| [?] -20.6%)
|
| I haven't been applying this approach long; I'll try to report on
| results later.
| elfrinjo wrote:
| I'll summarize: It's annoying that dB is often used as a physical
| unit without the necessary suffix. It's annoying that the suffix
| seems quite random for most units It's annoying that the factor
| for power differs from the factor for voltage, etc. It's slightly
| annoying that it's usually decibel instead of Bel.
|
| However, we all agree that dBs are really useful.
| Aachen wrote:
| > However, we all agree that dBs are really useful.
|
| Which part of the article was that again? I was surprised to
| see that conclusion as summary because it's not what the
| takeaway seemed like to me, so went back to the article and I
| don't see this mentioned, perhaps I'm overlooking it because I
| didn't do a careful read a second time
| elfrinjo wrote:
| oh sorry; thats the comments
| DonHopkins wrote:
| Almost as annoying as Euler angles, which force you to choose an
| arbitrary ordering (XYZ, ZYX, ZXZ, ...), while each choice is
| equally valid yet mutually incompatible, so there is no canonical
| "true" yaw-pitch-roll.
|
| Not to mention gimbal-lock singularities, wrapping
| discontinuously at +/-PI, mapping one orientation to many
| triples, and forcing you to juggle trig identities just to
| compose two spins.
|
| To pure mathematicians obsessed with elegant unambiguous
| coordinate-free clarity, Euler's pick-any-order gimmick is like
| hammering tacky street signs onto the cosmos: a slapdash, brittle
| hack that smothers the true geometry while quaternions and
| rotation matrices sail by in elegant, unambiguous splendor.
| 49pctber wrote:
| The idea that helped make decibels click for me is that they're a
| way to quickly do both dimensional analysis and gain/attenuation
| calculations at the same time.
|
| "Plain" decibels are simply (power) ratios. These can describe
| multiplicative changes in power. These are positive for gains
| (like in a power amplifier) or negative for attenuations (like
| path loss). They are unitless quantities.
|
| Decibels add. A ten 10 dB gain (x10) followed by a 20 dB loss
| (x0.01) is -10 dB (x0.1).
|
| "Flavored" decibels are in reference to some power quantity. For
| example, dBm uses one milliwatt as its reference. So 2 mW / 1 mW
| = 2 = 10^(3/10) = 3 dBm. These quantities have associated units,
| but they're still technically dimensionless.
|
| Here's the key insight. You can only have one "flavored" decibel
| value per computation. Say you have some 3 dBm signal (2 mW). You
| can add as many regular decibel values as you want, but the unit
| is still dBm. 3dBm + 4 dB - 7 dB = 0 dBm. In linear units, 2 mW *
| 2.5 * 0.2 = 1 mW
|
| If you were to do something like 3 dBm + 0 dBm, the linear units
| would be 2 mW * 1 mW = 2 mW^2, which is probably not what you
| want.
|
| dBs _are_ confusing. Different fields have slightly different
| conventions. People talk about any factor of 2 as a 3 dB change,
| when technically it should only be relative to power-like
| quantities. It 's weird that some of these "units" can be added
| together, while others can't. The factors of 10 and 20 can be
| confusing.
|
| But if you consider the units from a dimensional analysis
| standpoint, decibels are much more sane and intuitive than they
| appear.
| timerol wrote:
| > 2 mW / 1 mW = 2 = 10^(3/10) = 3 dBm
|
| It's worth noting that this is wrong, in exactly the way that
| makes decibels confusing. 3 dBm is an absolute power figure
| (about 2 mW). 2 mW / 1 mW is a ratio of 2 (about 3 dB).
|
| 2 mW / 1 mW = 2 = 10^(3/10) = 3 dB.
|
| 2 mW = 2 * 1 mW = 10^(3/10) * 1 mW = 3 dB (1 mW) = 3 dBm.
| 49pctber wrote:
| Yeah, that's a better way of thinking about it. That way you
| don't have any phantom bookkeeping like the way I was taught.
| The units are still right there.
| tbihl wrote:
| Similarly, but in a very different context:
|
| I have to teach non-engineers C programming for an undergrad
| course, which is basically trying to teach very explicit
| attention to punctuation (among many other things). "Watch
| those double quotes!", "single quotes, not double!", "where's
| your semi-colon??", and so on.
|
| Then, three weeks into the course, we're passing values by
| reference with &, and I get the question, "isn't that _scanf_
| missing the and-sign in front of the string name? ", and I'm
| forced to answer, "that punctuation doesn't matter, this
| time," because the C standard makes & _do nothing_ in front
| of a string specifically because so many people were confused
| about that fact that a string 's variable name is already
| passing by reference.
| etskinner wrote:
| Here's another related one that always bothers me: When you say
| something's loudness in decibels, you also need to specify a
| measurement distance.
|
| The author of this article even accidentally makes this omission:
|
| > It's 94 dB, roughly the loudness of a gas-powered lawnmower
|
| And that distance is very important; the actual sound pressure
| measured is proportional to distance^2. So for a lawnmower
| measuring 94dB, let's say we assume that we're measuring at 1m.
| At 2m away, the sound is actually 91dB.
|
| And don't get me started about the fact that a halving in power
| is 3dB, that's just wacky. I wish we used base 2.
| duped wrote:
| > And that distance is very important; the actual sound
| pressure measured is proportional to distance^2.
|
| While we're sniping nerds, the inverse square law only applies
| in the far field (which is tautologically "far enough away for
| the source to behave as a point source and follow the inverse
| square law"). That's probably a good bit further than 1m for a
| lawnmower in the physical world. For loudpseakers you have to
| be about 2m away before the inverse square law kicks in, unless
| they've been designed to operate as line sources which decay
| linearly for a very long distance. For loud sound sources near
| barriers like the ground they behave like half point sources,
| which will eventually act like point sources but there's a good
| bit of distance before it is really measurable.
| RicoElectrico wrote:
| This difference in mindset makes me comfortable as an EE that all
| the laid-off SWEs aren't going to take my job. Granted, demand
| side of the job market isn't rosy due to macro conditions, but at
| least supply isn't.
| stdbrouw wrote:
| One way to illustrate some of the weirdness of dB is to look at
| the documentation of unit libraries, e.g.:
|
| https://pint.readthedocs.io/en/stable/user/log_units.html
|
| https://painterqubits.github.io/Unitful.jl/v0.7/logarithm/
| karmakaze wrote:
| The writing style is antagonistic like a clickbait title. I find
| it bothersome. I would have much preferred an organized breakdown
| of all the different ways dB are used and what they mean instead
| of a random rant. It complains about how unscientific 'the' unit
| is, knowing that it's not a single unit then adds little to sort
| it out.
| timerol wrote:
| The one thing that really resonates with me from this article is
| also the silliest thing: Why decibels instead of bels? Everyone
| has multiplied in scientific notation before, and making a power
| ratio of 10^4 = 4 B would be so much more intuitive for
| conversion to absolute figures. It's a completely silly
| complaint, given that it's literally just a decimal point instead
| of a d, but it would make mental math of absolute conversions
| that much faster, especially for people who don't live and
| breathe amplifier chains all day.
|
| I want to live in a world where a radio can be specified as a
| -10.2 Bm sensitivity. -9 is three SI prefixes down from 1 mW, so
| less than 0.1 pW.
| aimor wrote:
| Yes, dB is a mess and if we could do it again we ought to use
| something more explicit like just annotating the units with log10
| such as log10(W). Then it's easy to use other convenient units
| like log2(W) or if you want to reproduce dBV: 20log10(V). dB is
| fine shorthand, but it gets used all the time in places where
| things should be explicit.
| stevage wrote:
| Gosh, I didn't know it was that bad - I didn't know it was used
| for so many different things.
| amelius wrote:
| Speaking of which. Does anyone know why, in Physics, we only use
| the operators +, * and exponentiation? And why higher operators
| in the hyperoperation sequence are never used?
|
| https://en.wikipedia.org/wiki/Hyperoperation
| mik1998 wrote:
| You would use it if it was useful or convenient
| amelius wrote:
| It was more of a question about the nature of our universe,
| similar to: why are there only n spatial dimensions?
| gregschlom wrote:
| "The bel is named in the honor of Alexander Bell; this is in the
| same tradition that prompted us to name the "wat" in honor of
| James Watt."
|
| This line killed me. I literally laughed out loud.
| teknopaul wrote:
| "This is nuts: it's akin to saying that the milli- prefix should
| have different meanings depending on whether we're talking about
| meters or liters."
|
| Were were here recently with "mega": Sometimes mega is squared as
| in megapixels. Sometime not as in megabytes.
|
| No biggie.
|
| Db in audio is a relative scale and that makes perfect sense. If
| you mixer goes + or - 6db that makes sense but can't be measured
| as power, your mixer might not be plugged in to any speakers so
| relation to real power is moot in the digital realm.
|
| 3 eq bands with -+6db makes sense too. Doesn't need to be
| precisly specified to be of immediate value, +-12db is clearly
| something else and users know what.
| perching_aix wrote:
| > Sometimes mega is squared as in megapixels.
|
| Is that right? A pixel is a 2D object already. It's not like
| e.g. with centimeters, where it's a 1D unit, so it becomes
| centimeters squared to form a 2D unit.
| davrosthedalek wrote:
| it's (cm)^2, not c(m^2). If there would be the one-
| dimensional equivalent to pixels, say pix, then 1pixel =
| 1pix^2, and then 1 megapixel = (kpix)^2
| perching_aix wrote:
| After putting significantly more effort than perhaps
| socially acceptable into this, I completely agree so far,
| but I'm still horribly confused about GP's point about
| "mega" being a "squared unit in megapixels".
|
| That to me implies that "mega" in "megapixels" is a planar
| ("[pre?-]squared") scaling factor, but it's... not really?
| Are those even a thing?
|
| I think this is what the debate is about at least. Mega
| does line up with kilo squared, but that's not because mega
| becomes a planar scaling factor, but because it just so
| happens that 1000 times 1000 is 1 million. It's kind of a
| coincidence? Like it's literally 1 million pixels, that's
| what's being meant. Just like with cm squared, the
| ohhhhhhhhhhhhhhhhhhhhhhhhh
| davrosthedalek wrote:
| I believe the point is that in "centimeters squared" you
| also square the cm. So it's 1/100 of a meter, then
| squared. While the mega in megapixels applies to the
| area. So presumably there are two length multiplied to be
| a pixel, and then you have a million of those.
|
| I don't think it's a very deep point, and I would say the
| mega is not squared, but the centi is squared.
| perching_aix wrote:
| What finally clicked it into place for me was trying to
| perform a unit conversion. It's kind of annoying cause we
| don't really use mega with anything squared usually (I
| don't recall anything at the moment at least), which
| added to the confusion.
|
| When one converts from square kilometers (km2) to square
| meters (m2), one needs to undo the kilo (x1000) not once,
| but twice, accounting for both dimensions. So as you say,
| it's actually here where the scaling factor is secretly
| squared, it's k2, just not written out. Hence despite
| kilo being a 1000x bump, you need to divide by 1 million,
| because it's actually squared in kilometers squared.
|
| So if mega in megapixels was behaving "normally", that
| would imply similar semantics, so to convert into
| kilopixels, you'd divide by 1000 not just once, but
| twice. But no, 1 megapixels is 1000 kilopixels. I guess
| the idea is that instead of being "secretly" squared it's
| "explicitly" already "square" as a result? So instead of
| resolving to mega2 pixels2, it's just mega pixels, since
| pixels is 2D, and so squaring it is unnecessary.
|
| I think I get it now at least, but yeah, I agree with
| your sentiment. It actually reminds me, when I first
| learned converting between units of area in primary
| school, I had quite the troubles with wrapping my head
| around it exactly because of this.
| StableAlkyne wrote:
| > Sometimes mega is squared as in megapixels. Sometime not as
| in megabytes.
|
| Even worse is Mega in Megabytes could be 1,000,000 or 1,048,576
| and it's more or less up to you to know what's what
|
| (Yeah, there are formally megabytes/mebiibytes/MiB/MB, but I
| honestly cannot recall the last time I heard anyone use
| anything other than just "megabyte" for 2^20 bytes... Or even
| wanted to refer to exactly 1,000,000 bytes. Other than decades
| ago when disk manufacturers wanted to make their hardware seem
| higher capacity than it really was)
| BlueTemplar wrote:
| Wait, when did they stop ?!
| jameshart wrote:
| Megapixels aren't 'squared' though?
|
| Ah, unless you're trying to make 'pixels' the same unit as in
| 'pixels per inch'...
|
| The problem there isn't how 'mega' is applied but how 'pixel'
| means both an area pixel as well as the linear size of a pixel.
| strbean wrote:
| I think the biggest issue by far is that many of the different
| contextual uses for dB are all in the same domain, or very
| close.
|
| When you're talking about the loudness of sound, in the same
| exact context you might care about SPL, perceived loudness, AND
| gain.
|
| If it was just a matter of "in electrical engineering /
| physics, dB implies this unit + baseline, when dealing with
| acoustics, it implies this other unit + baseline", it would be
| less problematic.
| waffletower wrote:
| This is an armchair rant with a very narrow complaint. The
| decibel unit has objective utility in both analog and digital
| signal processing and the field of audio engineering. It is far
| from meaningless in filter design and a useful reference for
| software and hardware interfaces that employ them.
| bsteinbach wrote:
| The power vs voltage thing I think comes from the historical
| connection of dB to sensing waves. Because you can change
| impedance to match any sensor, but power is conserved, power was
| just more often more important to discuss than voltage. I still
| have to look it up every time whether 10x is 10 or 20 dB.
| vt240 wrote:
| I see dB scale units used without contextual issues in near
| uniformity. Unfortunately, I have to agree with the OP, that
| microphone capsule manufactures seem to be an edge case. I'm not
| sure where dBV/Pa became the standard. I can understand why given
| 94dBSPL@1000Hz calibration standards, and the measurement
| equipment of the time, but I've run into my own fair share of
| datasheets with lines such as 'Sensitivity -45dB' with no units
| or other call outs for the standard in use. Thankfully, it seems
| like most modern datasheets use mV/Pa which seems like a much
| better unit in my book.
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