[HN Gopher] How far behind a plane is its noise?
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How far behind a plane is its noise?
Author : otras
Score : 71 points
Date : 2022-06-12 16:24 UTC (6 hours ago)
(HTM) web link (alexanderell.is)
(TXT) w3m dump (alexanderell.is)
| DavidPeiffer wrote:
| Neat writeup! From the title, I had my fingers crossed that they
| integrated ADS-B flight tracking data to show a map of where the
| sound of airplanes in the air is currently observable.
|
| If anyone wants to go down that rabbit hole really far, I'm
| imagining general profiles of the sound each airplane makes,
| considering altitude, different sound propagation by frequency
| depending on distance the sound travels, and geography. Might as
| well throw air properties in too, to minimize the overall error.
| The user could provide their location and see the estimated
| arrival time and frequency of the sound from the airplanes in the
| sky.
| kqr wrote:
| Oh it could get messy quickly. One of the playgrounds I like to
| hang out with my son at is right under the most common approach
| to a small local airport. It's also walled off by seven-story
| apartment buildings on two sides.
|
| The noise from approaching planes is dampened by the building
| between me and them, but bounce off the opposite building. So
| it always sounds like they're coming from the opposite
| direction, until they clear that angle, when they sound correct
| again, until obscured by the other building...
| bernulli wrote:
| Air properties are actually super relevant, the speed of sound
| distribution (slower with increasing altitude) can have lensing
| effects for sonic booms.
| bernulli wrote:
| Nice, but very wrong. This describes the case of a plane suddenly
| appearing in mid-air and starting to make noise, something,
| planes rarely do (maybe in the Bermuda triangle). It's like
| thunder after lightning, or seeing a ball fly before hearing it
| being kicked when you're far away.
|
| The aircraft, however, is flying _for a long time_ , certainly it
| was flying and making noise much earlier than when it is passing
| the observer. As long as it flies subsonically, i.e. sound
| outpaces the aircraft - which is the case for every single
| commercial plane - the sound may be able to reach you much much
| earlier than the plane: As an example, take an aircraft flying
| with 100 m/s directly towards you. With every second flying, the
| sound will gain another 200 m distance relative to the aircraft
| (speed of sound ~300 m/s).
|
| If you're 100km away, the aircraft will reach you after 1000s,
| the sound has reached you after 333s, i.e. _ahead_ of the
| aircraft. If you 're 200km away, the aircraft will reach you
| after 2000s and the sound has reached you after 667s.
|
| So, how come it sounds like the sound of the plane is _behind_
| the plane? It 's got to do with sound attenuation in the
| atmosphere and your hearing threshold.
|
| So, it's not at all like in the article.
|
| Somewhat minor nitpicks:
|
| - The aircraft is drawn to essentially fly with Mach 1, i.e. at
| the speed of sound, as the position of the plane relative to the
| wave does not linearly increase with time. Essentially all
| airplanes you see are flying subsonically (unless you're in the
| military).
|
| - "If the plane was moving very slowly, it wouldn't outpace its
| sound by much." That's completely wrong. "very slow" aircraft are
| much slower than their sound, and all commercial aircraft still
| are slower than their sound, all of them are outpaced by their
| sound rather than the other way around.
|
| [Edit: typos & math]
| kqr wrote:
| > So, how come it sounds like the sound of the plane is behind
| the plane? It's got to do with sound attenuation in the
| atmosphere and your hearing threshold.
|
| Wait, does it have to be that complicated?
|
| A plane flying X feet above you ought to make the same noise as
| a plane flying X/2 feet above you, except at 1/4 of the volume,
| and lagging by something like twice as much (meh trigonometry
| was never my forte). What am I missing?
| bernulli wrote:
| So why don't you hear the sound when it all begins, right at
| take-off? That should be the first sound to reach you, no?
| kqr wrote:
| I would assume you do, only it's so far away and your ears
| aren't powerful enough on their own to make it out. (If it
| can even be picked up over the other background noise.)
| bernulli wrote:
| For another thought experiment: if you cannot hear that
| original first sound on take-off, which one _can_ you
| hear? 10 miles from you? 1 mile from you? _That_ will be
| the virtual first sound to you, determined by how much
| weaker the sound has become on its trip through the
| atmosphere, and how that relates to your hearing
| threshold. But it will not _always and exactly_ be at the
| point where the plane has reached its closest point to
| you (as in the article).
| kqr wrote:
| I think I get what you're saying now. Thanks for taking
| the time!
| bernulli wrote:
| Sorry, I thought the first answer was so short it seemed
| rude, which is not what I intended.
| bernulli wrote:
| Precisely, "sound attenuation in the atmosphere and your
| hearing threshold".
| dredmorbius wrote:
| > This describes the case of a plane suddenly appearing in mid-
| air
|
| False.
|
| > and starting to make noise
|
| False
|
| > As long as it flies subsonically, i.e. sound outpaces the
| aircraft...
|
| > If you're 100km away, the aircraft will reach you after
| 1000s...
|
| These statements exhibit a fundamental misunderstanding of the
| phenomenon. It's not that the sound outpaces the _aircraft_. It
| 's that _light_ from the aircraft (reflected or transmitted,
| e.g., by landing / navigational lights) _travels faster than
| sound_.
|
| When _light_ from the aircraft reaches you, the sound is
| lagging behind _the light_.
|
| At the height of a jet airliner (~FL30, 30,000 feet), light
| reaches you in 30 microseconds. At the height of a small plane,
| about 3,000 feet, say, it's 3 microseconds.
|
| Sound takes 27 seconds to reach you from the jetliner, and 2.7
| seconds to reach you from the small plane.
|
| If the jetliner is flying at 600 mph (~mach 0.8, ~515 knot)
| _the aircraft has travelled 4.6 miles (7.4 km) from the
| position from which its sound was emitted before that sound
| reaches you._ The _apparent_ position indiciated by _vision_
| and _sound_ don 't match.
|
| If the small aircraft is travelling at 122 knots (140 mph)
| (cruise speed for a Cessna 172), it has travelled about 1/10 mi
| (0.16 km) before the sound reaches you. That's about 550 feet.
|
| Both cases are for _when the aircraft its directly overhead_.
| The apparent difference will _increase_ as the aircraft is
| closer to the horizon (arriving or departing).
|
| Again, the _visual position_ and _apparent aural position_ of
| the aircraft are not the same.
|
| You can determine this yourself, if you're outside and _hear_ a
| jet aircraft flying at altitude. If you _look to where the
| sound appears to be coming from_ you _will not see the
| aircraft_. It is going to be nearly 5 miles further along its
| path of travel. It can be surprisingly difficult to _visually_
| find the aircraft if you 've only first _heard_ it. If instead
| you 're watching the sky and first _see_ the aircraft, it will
| be quite some time, about 30 seconds, before the sound reaches
| you, and that sound will seem to be considerably far back along
| the aircraft 's path of travel.
| bernulli wrote:
| Ok, you can adjust my illustrative example of O(100s-1000s)
| by the 30ms to account for a finite speed of light if you
| think that makes any difference to the argument. Let me know.
| dredmorbius wrote:
| https://jkorpela.fi/wiio.html
| bernulli wrote:
| > " These statements exhibit a fundamental
| misunderstanding of the phenomenon. It's not that the
| sound outpaces the aircraft. It's that light from the
| aircraft (reflected or transmitted, e.g., by landing /
| navigational lights) travels faster than sound."
|
| Explain to me again how any of that explains why the
| airplane passes me (and I see it 1e-6s later which seems
| to be somehow super important to you) _before I hear it_
| , even though its _sound is traveling towards me much
| faster than the plane_.
| dredmorbius wrote:
| You're demonstrating Wiio's Law.
| FPGAhacker wrote:
| This has got to be a troll. I have trouble believing you
| are being serious saying things like this.
| [deleted]
| Dave_Rosenthal wrote:
| Slight tweak: it's 30 microseconds, not 30 milliseconds. But
| I wholeheartedly agree with your points!
| bernulli wrote:
| For all practical purposes it's really completely unrelated
| to the speed of light. Nothing would change in the argument
| if light travelled instantaneously. Sure, the numbers would
| change by O(1e-6s), but I'll admit that I wouldn't be able
| to tell the difference when watching an aircraft.
| dredmorbius wrote:
| Precisely my point.
|
| I'd meant to edit my post to note that for the purposes
| of this phenomenon, light speed is instantaneous.
|
| Though in the more general case, the phenomenon would
| apply to any case in which two signals or channels travel
| at different rates or speeds. Light and sound are the
| examples most familiar to us, though other alternatives
| exist.
|
| Neutrinos can tell us what is occurring at the core of
| the Sun with an ~8 minute delay whilst the propagation of
| EMR effects _from the Sun 's core_ is thought to take
| 10,000 to 170,000 years, as these travel through repeated
| collisions, absorption, and re-emission.
|
| Diffusion processes such as smell or other chemical
| materials both travel more slowly than either light or
| sound, _and_ at different rates for different compounds
| --- heavier compounds diffuse more rapidly than lighter
| ones. This is incorporated into the chemical signalling
| processes evolved by insects such as ants, in which some
| compounds are heavy and complex (usually for food or
| other valuable resources), others are light and fast
| (danger or alert signals). Again, for a moving or
| propagating phenomenon, these will move at different
| rates.
|
| For more complex phenomena, you might note that there are
| early / rapidly-moving indicia and those which move more
| slowly. Again, understanding the difference between
| these, the rates at which they travel, and their
| association and interactions with the processes
| originating and surrounding them will assist in drawing
| an accurate inference of the root phenomenon.
|
| All sensation is mediated, not direct, and that mediation
| has a direct effect upon sensation.
| dredmorbius wrote:
| Gah! Thanks, corrected in original.
| Deritio wrote:
| Isn't that only correct if the plain is heading towards you?
|
| The article states that the sound of the plane is not were you
| hear it. At least this is true
| bernulli wrote:
| Use the construction the author is using, i.e. the emanating
| sound waves, but you'll have to start them where it all
| starts, i.e. at take-off, and not simply appearing right next
| to you. Then, correct the drawing by having the plane move
| slower than the sound waves. The first sound you should be
| hearing as observer will be the take-off (if you could hear
| it) at the airport, and the aircraft will be wherever it is
| afterwards.
| korantu wrote:
| Suppose observer just woke up / heard plane that is passing
| overhead.
|
| Very reasonable assumption, as the plane might have taken
| off thousands of miles away, and will land thousands of
| miles away in the other direction.
|
| In this case, no point talking about takeoff sound, as it
| is not detectable already at these distances.
|
| Humans can detect reasonably well which _direction_ the
| sound comes from. This direction _will not match_ the
| direction they observe the airplane at.
|
| The article is describing the mismatch between plane real
| position and the plane position we would detect if we were
| just listening to it.
| smoyer wrote:
| Ignore the idea that the plane and it's sound are in different
| locations ... The key to understanding this phenomena is that
| there seems to be a greater discrepancy the further YOU are
| from the plane. Ignore the planes sound and consider the case
| where someone on the plane set off a firecracker. When you hear
| the sound from the firecracker,the plane will have moved away
| from that point!
| [deleted]
| dignick wrote:
| Unfortunately, this is very wrong! Why does it have to be a
| sudden sound? The effect the article describes is the same as
| eg thunder, except an aircraft is continuously moving and
| emitting sound. The aircraft in the article is not heading
| directly towards the observer. It simply takes time for the
| sound produced at a given moment to reach the observer, but the
| light from the aircraft travels much faster, which is why the
| lag is observed. It is not 'sound attenuation' or 'hearing
| threshold'.
| bernulli wrote:
| > "Why does it have to be a sudden sound? The effect the
| article describes is the same as eg thunder"
|
| Well yeah, that's a _sudden sound_. My point precisely.
|
| So why don't you hear from your observation point the
| airplane (or _all airplanes_ for that matter) as it takes
| off, which is when it makes its first noise? And by all
| means, account for a few ms of light movement if that makes
| you happy.
| dignick wrote:
| > Well yeah, that's a sudden sound. My point precisely.
|
| But you are saying that isn't like an aircraft - why?
|
| > So why don't you hear from your observation point the
| airplane (or all airplanes for that matter) as it takes
| off, which is when it makes its first noise? And by all
| means, account for a few ms of light movement if that makes
| you happy.
|
| That is attenuation! The aircraft is far enough away that
| all the energy from the sound is absorbed by the air and
| objects between observer and aircraft. Attenuation does not
| affect the speed the sound travels. But when the aircraft
| is closer to you, the attenuation is lower so you can hear
| the sound.
| bernulli wrote:
| Because an aircraft does not make a sudden noise? At
| least where I'm from aircraft don't sound like discrete
| booms. I'm not sure I understand your question.
|
| > But when the aircraft is closer to you, the attenuation
| is lower so you can hear the sound.
|
| So we agree after all.
| danachow wrote:
| > But you are saying that isn't like an aircraft - why?
|
| Because last I checked airplanes in cruise flight have a
| pretty constant engine noise, that's why.
| otras wrote:
| Like any general explanation, I think some simplification is
| helpful :)
|
| I may have been assuming that it was clear in the article, but
| to help show the effect, the diagrams show only the sound
| emanating from the plane at _one instant in time_. In reality,
| the aircraft is continuously moving, and there 's a
| continuously changing "where is the sound coming from" vector.
| The main idea is that this "where is the sound coming from
| vector", if you will, may be behind the plane's light vector
| (which we can just say is the plane's position) if you're some
| distance away, leading to this oddity.
|
| > the sound may be able to reach you much much earlier than the
| plane
|
| I completely agree! I didn't mean to say that a plane's sound
| is always behind it -- it very much depends on the position of
| the observer. If a plane is flying in any other direction than
| perfectly perpendicular, the math and the effect will be
| different.
|
| For "If the plane was moving very slowly, it wouldn't outpace
| its sound by much.", I meant in the sense that we, as
| observers, are perceiving the sound. More that the plane's
| position vector wouldn't outpace the "where is the sound coming
| from vector" by as much (a smaller X in the diagram, if you
| will), leading to "where is the plane" being closer to "where
| is the plane's noise". Going faster than the speed of sound
| leads to all sorts of very interesting questions, but I don't
| believe it would affect this in the simple case we're looking
| at.
| bernulli wrote:
| Thanks for chiming in!
|
| My point is that your _one instant in time_ is completely
| arbitrary. You _do not know_ where the position is, and you
| _do not know when the sound was emanated_. I.e., you cannot
| calculate anything.
|
| Your math would work iff you observe a discrete event where
| you can tie sound and light - engine blow-up, for example. In
| all other cases, it means nothing.
|
| Again - why don't you hear the sound of the aircraft taking
| off if speed of sound is the only effect?
| roelschroeven wrote:
| I feel you're overcomplicating things and/or are describing
| some different phenomenon. The point isn't the question whether
| it's the sound, the light or the airplane itself that reaches
| you first. The question is which direction is the sound coming
| from.
|
| Imagine a plane flying 3 km high, circling around your location
| in a circle with radius 4 km. In other words, the plane is
| consistently sqrt(32 + 42) = 5 km away from you. Let's assume
| the speed of sound is at a constant 343 m/s in this scenario,
| so sound takes 5e3 m / (343 m/s) = 14.58 seconds to travel from
| the plane to you. The direction of the incoming sound that you
| detect will change all the time, at the same speed as the
| direction of the plane itself, but it will lag behind. The
| sound that you hear at each moment is the sound that the plane
| generated 14.58 seconds before, and you detect it as coming
| from the location the plane was in at that moment, and not the
| current time. Your eyes (or a camera, or a radar) detect the
| plane from one position, your ears (or a directional
| microphone) detect it from another, older, position.
|
| All that is true independently from the speed of the plane, be
| it subsonic or supersonic (except when the plane is flying very
| slowly, or if it's a hovering helicopter: in those cases the
| sound is still delayed, but the location it's from is hardly
| changed or not at all).
|
| Strictly speaking the same happens with the visual image, but
| since light is so much faster we can neglect the delay it
| causes in every-day situations like this.
| bernulli wrote:
| Haha, yes, you found the one case (which is unrelated to the
| article) in which sound weakening is not a function of time,
| as the distance to you stays constant.
| skogsbonde wrote:
| The article is right
|
| > So, how come it sounds like the sound of the plane is behind
| the plane? It's got to do with sound attenuation in the
| atmosphere and your hearing threshold.
|
| > So, it's not at all like in the article.
|
| On the contrary it is indeed because of what the article is
| getting at. It's because the sound emitted by the airplane at
| one position reaches you significantly later than the light the
| plane reflects from that position reaches you. Maybe what
| you're describing is that the sound emitted when the airplane
| took off reaches you faster than the airplane reaches you which
| sure, it's correct - but the light still reaches you way way
| faster.
|
| > - "If the plane was moving very slowly, it wouldn't outpace
| its sound by much." That's completely wrong. "very slow"
| aircraft are much slower than their sound, and all commercial
| aircraft still are slower than their sound, all of them are
| outpaced by their sound rather than the other way around.
|
| Even if the sound plane (edit: meant plane) travelled faster
| than sound, you would still see the airplane passing over you
| before the sound emitted from the airplane when it passed over
| you reaches you.
|
| Minor nitpick: - As an example, take an aircraft flying with
| 100 m/s
|
| 200 m/s would be a better example as the Boeing 737 (the most
| common commercial passenger jet) cruises at around 230 m/s
| bernulli wrote:
| > "Even if the sound travelled faster than sound [sic] you
| would still see the airplane passing over you before the
| sound emitted from the airplane when it passed over you
| reaches you."
|
| Absolutely not. It _depends_ on the Mach number, distance,
| sound weakening, and your hearing threshold.
|
| You cannot hear some crazyman running at you, screaming,
| until he has passed you? You cannot hear the stereo in some
| guy's car until after he passed you? You cannot hear a siren
| of police until the car has passed you? Or are what you
| describe special magical airplane-only physics?
| skogsbonde wrote:
| Oops, I mean to write even if the _plane_ travelled faster
| than sound, not sound travelled faster than sound.
| bernulli wrote:
| Well yeah, if the plane is faster than its sound (and
| flying towards you), the plane will reach you earlier
| than the sound does. The plane does not get attenuated by
| flying farther, and your seeing threshold is helped by
| the sun or the lights the aircraft turns on at night.
| FPGAhacker wrote:
| Of course you hear the siren or crazy man or anything,
| before it passes you if the component of the velocity
| vector pointing to you is slower than the speed of sound.
|
| But it still takes time for the sound to reach you. And in
| that time the source has continued to move. So it will be
| as if you are watching a video but hearing with a tape
| delay.
|
| If some one was standing 1000 meters away from you, and had
| a sign that flashed a sequence of numbers, 1,2,3,4,... once
| per second, and at the same time as the number flashed,
| they shouted the number loud enough that you could hear it,
| do you think what you heard and what you saw would be in
| sync?
| danachow wrote:
| Did you read the entire article? I think where you're
| getting mixed up is that the article is using some poor
| assumptions and a broken thought experiment to derive a
| scheme for calculating or estimating the distance based
| on the sound/light mismatch. I don't think anyone is
| claiming sound and light don't travel at different speeds
| but the explanation in the article is pretty misguided.
| bernulli wrote:
| Thank you.
| bernulli wrote:
| > "Of course you hear the siren or crazy man or anything,
| before it passes you if the component of the velocity
| vector pointing to you is slower than the speed of
| sound."
|
| So, only in aircraft it is different? Magical aircraft
| physics after all?
|
| > " If some one was standing 1000 meters away from you,
| and had a sign that flashed a sequence of numbers,
| 1,2,3,4,... once per second, and at the same time as the
| number flashed, they shouted the number loud enough that
| you could hear it, do you think what you heard and what
| you saw would be in sync?
|
| Of course not.
|
| But to humor you: which is the distinct event in a
| normally flying aircraft in which you can tie the exact
| point at which the light and sound signal leave the
| aircraft towards you so you can use that to calculate the
| distance? Spoiler: there isn't _, you cannot, and that is
| precisely the point.
|
| _ There are some examples currently in Ukraine, in which
| you could use your argument.
| Retric wrote:
| The article is correct, their explanation is poor. When you
| hear a sound with your eyes closed you can normally locate
| where it's coming from. As in close your eyes and snap your
| fingers. Now suppose someone sets off a bomb some distance from
| you. You see the explosion or lightning flash etc but it takes
| a while for sound to show up. For stationary objects it doesn't
| really matter you can still locate direction just fine.
|
| Aircraft in level flight are also loud enough to be heard at
| distance sufficient to notice a delay. If the aircraft is
| flying by you hear sound from exactly one instant, but it like
| the explosion it was produced in the past. So if you close your
| eyes and try to locate the aircraft by sound you will point to
| wherever it was when it produced that sound not where it is
| right now.
|
| The same is true of every sound you hear, but normally
| distances are short enough and speed are low enough it just
| doesn't matter.
| bernulli wrote:
| It's just completely unrelated to why the aircraft passes you
| before you hear it.
| Retric wrote:
| They never suggested the first time you would hear it was
| the aircraft was already past you.
|
| However, the maximum difference in angle between it's
| current location and the location the sound comes from is
| just after it flew past you.
| bernulli wrote:
| The title is "How far behind a plane is its noise?" It's
| not. It's ahead of the aircraft.
| Retric wrote:
| The noise is always _pointing_ behind the aircraft or any
| moving object due to lag.
|
| Light also encodes the direction to an objects past
| location, even though light is always moving faster than
| the object.
| mav88 wrote:
| I was in the boondocks last week and had a striking example of a
| jet climbing up to its ceiling around 2 or 3 miles away. I tried
| to figure out the answer to this in my head and didn't get
| anywhere - was hoping a kind geek would come up with something.
| temptemptemp111 wrote:
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(page generated 2022-06-12 23:00 UTC)