I'm just armchair musing here, and I'm definitely no expert on sound waves, but I wonder if they considered the fact that most airliners have more than one engine. Could the effect also be the superposition of multiple engine sounds?
Those have a fixed spatial distance, too, and the effect would (I suppose) change with the lateral angle to the listener during the fly-by. This theory should be pretty easy to falsify, because then the effect would not occur if the plane's path went exactly overhead.
For that, the pressure waves (sound) coming from the engines would have to be somewhat coherent, or correlated in phase. Since what we're hearing is essentially turbulence, that's not going to be the case.
The majority of the engine noise is caused by the turbulent mixing of the exhaust with the surrounding air. Turbulence is chaotic, so even if the engines are phase locked the sound rapidly becomes incoherent.
Do you have a source for that? I’ve heard about something that some twin-props have, but definitely not jets. The engines don’t even run at the same RPM.
I once picked up my memory foam mattress and stood it up against one of the walls ... for cleaning the bed or whatever.
As I walked past the mattress I instantly noticed that the mattress is such a good absorber of audio waves that I could immediately notice a dip in ambient sound in the ear facing the mattress.
The room was already "silent" and this newly discovered lower limit of silence was pretty surprising to me physiologically.
If you are handy to an R&D lab that has a combo Faraday Cage/anechoic chamber you can have a nice experience free of RF and audio noise and stimulus. Even better if it is dimly lit in near-infrared. Even better-better if it has a tank of warm water with lots of epsom salts, although I've never been in a lab that had such a thing as a requirement.
RF anechoic chambers are, as a side effect of their construction, pretty low echo also when it comes to audible frequencies.
I have spent a bit of time in one (EMC testing a product), and it was the quietest room I have been in by far.
The goal is not to prove or disprove any affects on one's physiology, but simply to have the experience of being free of RF and audio for the sake of it.
I think the point is that even saying the "experience" of being free of RF implies a perception which does not exist.
Plus it's well-known that you don't really get the full experience of this unless you manage to shield yourself from neutrinos by surrounding yourself with sufficiently-dense proto-neutron stars.
Those requirements were not specified, so were not designed or built. If you can increase the budget, we can write-up a proposal to wrangle some sufficiently-dense proto-neutron stars.
Don‘t know if this is the same but I went to Death Valley on the Devil‘s Golf Course during summer. There was no wind no nothing. It was so damn silent. Wonder how that compares to an anechoic chamber now.
Edit to add: I've been in an anechoic chamber and also the black rock desert, which is dead flat and thus has very little surface area oriented to reflect sound back at the listener, which makes it similar in that you don't experience environmental reflections.
Devil's Golf Course has more "texture" to it but if you were quiet on a windless day I think the effect would be similar.
Probably similar to whatever its normal frequencies are for you, but perceptually louder. That seems to be my experience when I'm in a location with minimal background noise...
Ground feature echo (mentioned in the article)... possible. Not mentioned here or in the article: thermocline in the atmosphere. Thermocline in the water is traditionally how submarines "hide" from surface ships.
I hear flanging from the planes incoming from quite a distance, and they're pretty low when the fly over where I live. More telling: I can hear the freeway and the busy arterial, "depending on how the wind blows". Sometimes it flanges, too.
So: ground reflection along with thermocline refraction seems a perfectly plausible explanation for one source of the phenomenon; could be several, probably all involving ground and atmospheric factors.
The same effect is responsible for an unavoidable flaw with stereo loudspeakers, where you have differing path lengths between your ears and each speaker. Try playing some mono pink noise on stereo speakers and moving your head, then compare with the same sound hard-panned to a single speaker. It's most obvious when you're close to the speakers and in an acoustically dry environment. If you add lots of additional reflections you'll generate many overlapping interference patterns that will average out to a smoother frequency response. This is one reason why adding a real physical center channel can improve clarity of dialogue in movies.
Real hi-fi enthusiasts sit at the reference listening point of their system (the third corner of an equilateral triangle placed on the speakers). Everyone else won't notice the difference; they listen to 256kbps Spotify anyways.
It doesn't take much head movement to cause audible flanging. The real hi-fi enthusiasts will have to use head clamps like in A Clockwork Orange (or just use headphones).
Always a huge pleasure when Oona posts something. Her posts are the sort of magic you get when a genuinely curious person has the competence to satisfy and explore those idle curiosities. Glad she's still going strong after all these years.
We have planes pass overhead at about 6000ft. When the conditions are right they'll make a completely different sound, I've always assumed it's the Doppler effect mixed with the valley we live in but I'm always very curious when it does happen.
They make their usual sound but then there's a second sound that arrives, a lot higher pitched. Sounds like they've struck it in reverse or something (they haven't they're just doing a normal decent).
I think it’s some engine type that makes the sound at some specific speed / throttle setting. But I can’t remember the specifics. Some planes passing us make the sound, most don’t.
Very interesting question. I thought that was when they extend their flaps so the wings become a bit wider for the slower flight and that creates some resonance.
I strongly suspect that the unexpected doppler shift is from jetwash.
That is, the principle source of noise from a jet aircraft isn't the engines directly (turbine spool), or the fuselage's passage through the air (turbulent white noise), but the stream of hugely-accelerated air which has exited the turbine(s) and is now shredding itself against the stationary surrounding air. The noise source therefor isn't a point (engine) but a linear source (the turbulent shred-wall interface between the jetwash and surrounding air), and it is moving rapidly backwards from the aircraft.
Which means that as the aircraft approaches you, the jetwash / shred turbulence is moving away from you, and is doppler-shifted toward lower frequencies, and once the aircraft passes minimum distance, the jetwash is streaming toward you, at a high fraction of the speed of sound, and should therefor be doppler-shifted upwards.
The insight that it was jetwash and not engines themselves making noise became clear to me when I lived near an airport with a road passing immediately behind the runway. I happened to be cycling past one day as a jet lined up for take-off, heading away from me. I was positioned directly behind it (and out of immediate reach of the jetwash). My first thought as the engines spooled up was "this is going to be loud" ... but it wasn't. Rather than the roar you'd hear when you were alongside the plane, all I heard was a loud spooling turbine whine ... until the jetwash roar itself returned to me echoed off mountains a few kilometers distant.
TL;DR: Jet engines don't make (much) noise, their exhaust does, and it has a markedly different velocity vector than the plane itself, or its engines, accounting for a different doppler signature.
TL;DW: The apparent tone shift is a result of interference from a bounce-path echo (from the ground) to the listener of the white noise emission of the aircraft, rather than expected doppler effect. This can be experimentally verified several ways, e.g., changing the bounce length (by moving closer or further from the ground) or by monitoring similar noises of known origin near an acoustically-reflective surface.
As a sound comes towards you (say an ambulance) the sound waves arrive squashed (higher pitch) and as it goes away the sound waves are stretched (lower pitch).
Let's say the mistake is understandable, because it happens to coincide with the observation of a passing jet. ;)
I guess that's why Doppler explanations nearly always use an ambulance as their example.
The jet example will needlessly confuse people and create fodder for low effort "akshually" type commentary because the air getting sucked into the front and crammed out the back of the engine sound very different so you can get like most the same effect without the aircraft actually moving relative to the observer.
People explaining doppler don't want to have to explain this to a bunch of nit pickers, so they use an ambulance.
Pretty sure this is also why, when you stand at the right spot in a techno concert, the music starts to sound like a jet engine.
We also have this in game development, where if two sound effect emitters play the same effect at the same time with just a bit of offset, phase, whatever, they sound like that.
If the offset is fixed, the effect is called a comb filter. If the offset is changing, the effect is called flanging. The name stems from recording engineers rubbing their fingers against the flange of a reel-to-reel recorder's tape reel, to brake it slightly, which adds increasing delay to the sound.
In the 1960s through 1980s, producers of audio recordings were striving to achieve something which they called “Wall of Sound”, which I suppose was influenced by producers like Phil Spector’s releases with the Beatles. It is a dense, all-encompassing sound with the drums, bass, guitars, keyboards and sometimes vocals too having equity in the mix, and not too much separation from left to right but a spacious “soundstage” effect, and just a solid onslaught of music for the duration of the track.
Well, there was a legend about a certain band fronted by Stan Ridgway. In the late 1970s they were in the studio and the producers were tweaking knobs and sliders to approach the desired sound. And one of them asked whether it was a “Wall of Sound” yet, but another replied, “it sounds more like a Wall of Voodoo!”
I'm just armchair musing here, and I'm definitely no expert on sound waves, but I wonder if they considered the fact that most airliners have more than one engine. Could the effect also be the superposition of multiple engine sounds?
Those have a fixed spatial distance, too, and the effect would (I suppose) change with the lateral angle to the listener during the fly-by. This theory should be pretty easy to falsify, because then the effect would not occur if the plane's path went exactly overhead.
For that, the pressure waves (sound) coming from the engines would have to be somewhat coherent, or correlated in phase. Since what we're hearing is essentially turbulence, that's not going to be the case.
Engines in modern aircraft are phase-locked, though.
The majority of the engine noise is caused by the turbulent mixing of the exhaust with the surrounding air. Turbulence is chaotic, so even if the engines are phase locked the sound rapidly becomes incoherent.
Do you have a source for that? I’ve heard about something that some twin-props have, but definitely not jets. The engines don’t even run at the same RPM.
https://www.osti.gov/biblio/6356085
It is what the "Sync" switch on the panel does.
Yes, but it seems like no turbofan synchrophaser systems have been implemented yet. Only on turboprops.
https://aviation.stackexchange.com/questions/71738/is-engine...
Planes also have flaps, slats, and landing gear, which can have a huge effect on the sound heard on the ground.
Tangentially related
I once picked up my memory foam mattress and stood it up against one of the walls ... for cleaning the bed or whatever.
As I walked past the mattress I instantly noticed that the mattress is such a good absorber of audio waves that I could immediately notice a dip in ambient sound in the ear facing the mattress.
The room was already "silent" and this newly discovered lower limit of silence was pretty surprising to me physiologically.
Everyone should try a real anechoic chamber once. The silence there is deafening.
If you are handy to an R&D lab that has a combo Faraday Cage/anechoic chamber you can have a nice experience free of RF and audio noise and stimulus. Even better if it is dimly lit in near-infrared. Even better-better if it has a tank of warm water with lots of epsom salts, although I've never been in a lab that had such a thing as a requirement.
I'm skeptical of shielding yourself from RF noise having any detectable effect.
Unless you have amalgam tooth fillings, that anecdotally can act as a crude diode, and demodulate strong enough AM signals.
RF anechoic chambers are, as a side effect of their construction, pretty low echo also when it comes to audible frequencies. I have spent a bit of time in one (EMC testing a product), and it was the quietest room I have been in by far.
The goal is not to prove or disprove any affects on one's physiology, but simply to have the experience of being free of RF and audio for the sake of it.
I think the point is that even saying the "experience" of being free of RF implies a perception which does not exist.
Plus it's well-known that you don't really get the full experience of this unless you manage to shield yourself from neutrinos by surrounding yourself with sufficiently-dense proto-neutron stars.
Neutrinos, cosmic rays, and extraterrestrial subatomic particle streams are not considered RF, right?
If we're going out of our way to eliminate things that cause zero perceptual experience I don't see why you would exclude them.
Those requirements were not specified, so were not designed or built. If you can increase the budget, we can write-up a proposal to wrangle some sufficiently-dense proto-neutron stars.
Don‘t know if this is the same but I went to Death Valley on the Devil‘s Golf Course during summer. There was no wind no nothing. It was so damn silent. Wonder how that compares to an anechoic chamber now.
Having experienced both, it's very similar.
Edit to add: I've been in an anechoic chamber and also the black rock desert, which is dead flat and thus has very little surface area oriented to reflect sound back at the listener, which makes it similar in that you don't experience environmental reflections.
Devil's Golf Course has more "texture" to it but if you were quiet on a windless day I think the effect would be similar.
I shudder to think what tinnitus would sound like in an anechoic chamber…
Probably similar to whatever its normal frequencies are for you, but perceptually louder. That seems to be my experience when I'm in a location with minimal background noise...
Ground feature echo (mentioned in the article)... possible. Not mentioned here or in the article: thermocline in the atmosphere. Thermocline in the water is traditionally how submarines "hide" from surface ships.
I hear flanging from the planes incoming from quite a distance, and they're pretty low when the fly over where I live. More telling: I can hear the freeway and the busy arterial, "depending on how the wind blows". Sometimes it flanges, too.
So: ground reflection along with thermocline refraction seems a perfectly plausible explanation for one source of the phenomenon; could be several, probably all involving ground and atmospheric factors.
The same effect is responsible for an unavoidable flaw with stereo loudspeakers, where you have differing path lengths between your ears and each speaker. Try playing some mono pink noise on stereo speakers and moving your head, then compare with the same sound hard-panned to a single speaker. It's most obvious when you're close to the speakers and in an acoustically dry environment. If you add lots of additional reflections you'll generate many overlapping interference patterns that will average out to a smoother frequency response. This is one reason why adding a real physical center channel can improve clarity of dialogue in movies.
Real hi-fi enthusiasts sit at the reference listening point of their system (the third corner of an equilateral triangle placed on the speakers). Everyone else won't notice the difference; they listen to 256kbps Spotify anyways.
It doesn't take much head movement to cause audible flanging. The real hi-fi enthusiasts will have to use head clamps like in A Clockwork Orange (or just use headphones).
Always a huge pleasure when Oona posts something. Her posts are the sort of magic you get when a genuinely curious person has the competence to satisfy and explore those idle curiosities. Glad she's still going strong after all these years.
Next time I see a plane coming, I’m going to lie on the floor to see if the whoosh sound does it fact change.
Check out his video. It's educational! https://www.youtube.com/watch?v=Amj4UevyRfU
She's a woman, btw.
My apologies.
We have planes pass overhead at about 6000ft. When the conditions are right they'll make a completely different sound, I've always assumed it's the Doppler effect mixed with the valley we live in but I'm always very curious when it does happen.
They make their usual sound but then there's a second sound that arrives, a lot higher pitched. Sounds like they've struck it in reverse or something (they haven't they're just doing a normal decent).
I think it’s some engine type that makes the sound at some specific speed / throttle setting. But I can’t remember the specifics. Some planes passing us make the sound, most don’t.
Could also be FOPPs, or both combined! (Fuel Overpressure Protection) https://aviation.stackexchange.com/questions/23701/what-caus...
Some do it at certain airspeeds, others don't. Depends on their shape, and can be mitigated/eliminated, which is sometimes done, other times none.
Very interesting question. I thought that was when they extend their flaps so the wings become a bit wider for the slower flight and that creates some resonance.
Mmm could be. I need more data. Eg is it when landing on one runway or are they going to the other one, cloud layers etc.
Sound is also reflected off a cloud layer if present.
I strongly suspect that the unexpected doppler shift is from jetwash.
That is, the principle source of noise from a jet aircraft isn't the engines directly (turbine spool), or the fuselage's passage through the air (turbulent white noise), but the stream of hugely-accelerated air which has exited the turbine(s) and is now shredding itself against the stationary surrounding air. The noise source therefor isn't a point (engine) but a linear source (the turbulent shred-wall interface between the jetwash and surrounding air), and it is moving rapidly backwards from the aircraft.
Which means that as the aircraft approaches you, the jetwash / shred turbulence is moving away from you, and is doppler-shifted toward lower frequencies, and once the aircraft passes minimum distance, the jetwash is streaming toward you, at a high fraction of the speed of sound, and should therefor be doppler-shifted upwards.
The insight that it was jetwash and not engines themselves making noise became clear to me when I lived near an airport with a road passing immediately behind the runway. I happened to be cycling past one day as a jet lined up for take-off, heading away from me. I was positioned directly behind it (and out of immediate reach of the jetwash). My first thought as the engines spooled up was "this is going to be loud" ... but it wasn't. Rather than the roar you'd hear when you were alongside the plane, all I heard was a loud spooling turbine whine ... until the jetwash roar itself returned to me echoed off mountains a few kilometers distant.
TL;DR: Jet engines don't make (much) noise, their exhaust does, and it has a markedly different velocity vector than the plane itself, or its engines, accounting for a different doppler signature.
There was a pretty good video on this a couple of years ago: https://www.youtube.com/watch?v=QFv3QPNU6hw
TL;DW: The apparent tone shift is a result of interference from a bounce-path echo (from the ground) to the listener of the white noise emission of the aircraft, rather than expected doppler effect. This can be experimentally verified several ways, e.g., changing the bounce length (by moving closer or further from the ground) or by monitoring similar noises of known origin near an acoustically-reflective surface.
> it's like the pitch goes down at first, but when the plane has passed us, the pitch goes up again. That's not how Doppler works!
Call me a dummy, but this was exactly how I thought Doppler works.
As a sound comes towards you (say an ambulance) the sound waves arrive squashed (higher pitch) and as it goes away the sound waves are stretched (lower pitch).
Let's say the mistake is understandable, because it happens to coincide with the observation of a passing jet. ;) I guess that's why Doppler explanations nearly always use an ambulance as their example.
The jet example will needlessly confuse people and create fodder for low effort "akshually" type commentary because the air getting sucked into the front and crammed out the back of the engine sound very different so you can get like most the same effect without the aircraft actually moving relative to the observer.
People explaining doppler don't want to have to explain this to a bunch of nit pickers, so they use an ambulance.
Pretty sure this is also why, when you stand at the right spot in a techno concert, the music starts to sound like a jet engine.
We also have this in game development, where if two sound effect emitters play the same effect at the same time with just a bit of offset, phase, whatever, they sound like that.
If the offset is fixed, the effect is called a comb filter. If the offset is changing, the effect is called flanging. The name stems from recording engineers rubbing their fingers against the flange of a reel-to-reel recorder's tape reel, to brake it slightly, which adds increasing delay to the sound.
Huh so a flange is just a combing effect that shifts around? I'd never considered this. Also neat fact about flanges, didn't know that either. Thanks!
In the 1960s through 1980s, producers of audio recordings were striving to achieve something which they called “Wall of Sound”, which I suppose was influenced by producers like Phil Spector’s releases with the Beatles. It is a dense, all-encompassing sound with the drums, bass, guitars, keyboards and sometimes vocals too having equity in the mix, and not too much separation from left to right but a spacious “soundstage” effect, and just a solid onslaught of music for the duration of the track.
Well, there was a legend about a certain band fronted by Stan Ridgway. In the late 1970s they were in the studio and the producers were tweaking knobs and sliders to approach the desired sound. And one of them asked whether it was a “Wall of Sound” yet, but another replied, “it sounds more like a Wall of Voodoo!”
https://en.wikipedia.org/wiki/Wall_of_Voodoo#Formation
Hm, I suppose that's why a flanger[1] (guitar effect pedal) can sound a little like a jet plane.
[1] https://en.wikipedia.org/wiki/Flanging
it's also the effect that lets you kinda know if you're near a wall (for example when you're fumbling around in the dark)
I've been hearing and thinking (occasionally) about this effect for years, so this explanation is very welcome.
So who is up for turning the last graph of the article into a synthesizer?
very curious whether there are potential (or already existing) military applications based that...
How about the back and the front of the plane?
Thanks, now I'll be hearing and thinking about the effect for the rest of my life.
[dead]