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.

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.

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.

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.

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 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

> 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.

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.

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?

How about the back and the front of the plane?

very curious whether there are potential (or already existing) military applications based that...

Thanks, now I'll be hearing and thinking about the effect for the rest of my life.

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