Thanks, now I'll be hearing and thinking about the effect for the rest of my life.
Call me a dummy, but this was exactly how I thought Doppler works.
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.
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 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.
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.
People explaining doppler don't want to have to explain this to a bunch of nit pickers, so they use an ambulance.
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.
It is what the "Sync" switch on the panel does.
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.
https://aviation.stackexchange.com/questions/71738/is-engine...
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.
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.
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.
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!”