[HN Gopher] How far behind a plane is its noise? ___________________________________________________________________ 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: ___________________________________________________________________ (page generated 2022-06-12 23:00 UTC)