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I am completely blind and have heard this several times when I was at air shows. Then my friend sent me this recording of a crop duster spraying the fields, and I heard the same type of sound.

It starts out by slowly ascending, and then the plane's engine rapidly decreases its speed.

I know that because of the Doppler effect, sounds can sometimes be deceiving, so I wondered what I would hear if I were inside the plane.

Here's the recording. It's audio, unfortunately, but hopefully someone with experience will be able to provide clarification.

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  • $\begingroup$ There is no link for the audio recording I'm afraid. $\endgroup$
    – John K
    Commented Jan 31, 2021 at 1:08
  • $\begingroup$ I'm not sure why, but when I post an Iframe, it shows up in the edit body, but it doesn't post as an actual frame where the embedded player should be. clyp.it/a3mczu2m $\endgroup$
    – The Harmonic Rainbow
    Commented Jan 31, 2021 at 1:23
  • $\begingroup$ I fixed it for you by editing your post to put in the link. $\endgroup$
    – John K
    Commented Jan 31, 2021 at 2:23

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Ok I listened and what you are hearing is a turbine powered ag plane. Most of the rising and decreasing pitch you are hearing that sounds so dramatic is doppler effect from the propeller. The propeller speed will be constant all this time as the pilot will set the RPM for his activity and leave it, just changing power with throttle.

So in the cockpit, the noise from the prop will more constant without the rising and falling sound, just a steady OOOMMMM of somewhere around 100 hz (three blades at 2000 rpm say), not as intense as the sound heard from the ground, with a roar/hiss from the air passing that rises and falls somewhat with speed as he dives and zooms.

The intensity of the propeller sound will also increase or decrease a lot with the power, but not the overall pitch; because the propeller is constant speed, that remains constant unless the speed is changed, and that won't happen during this maneuvering.

Then the very high pitch zeeeeeeeeee whining sound under the big booming sound is the turbine engine itself. It's sound will change a little bit from within the cockpit as power is added and removed, with some change in the pitch of the whining sound and its intensity.

This video is from the inside of a plane like the one you heard, but the inside sound is being recorded through an active noise cancelling headset, so 80% of the cockpit sound is being suppressed. But you can hear faintly the airstream, and a bit of the prop's sound and can tell that it's pretty constant to the pilot the entire time. Way less dramatic for sure.

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  • $\begingroup$ So is it a turboprop? I've been in one (a Cessna-208) for my sky-dive, and its propeller RPM sounded like 105-110 RPM. $\endgroup$
    – The Harmonic Rainbow
    Commented Jan 31, 2021 at 9:41
  • $\begingroup$ @HeavenlyHarmony on Cessna 208 Caravan, the propeller spins 1600 to 1900 RPM in cruise, a bit more on take-off and landing. Generally the faster the aircraft, the slower the propeller spins, independent of the engine type. So in small airplanes up to about 2700 RPM while regional turboprops like ATR-72 go down to somewhere around 1000 RPM. But nothing goes much lower than that in flight, and the slower propellers tend to have more blades, which raises the sound pitch again. $\endgroup$
    – Jan Hudec
    Commented Jan 31, 2021 at 12:56
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    $\begingroup$ @HeavenlyHarmony Yes in this case it's a turboprop. The whine of the turbine is quite distinctive. The dominant sound you are hearing in the cabin is propeller wave pulses from the blades passing impacting the windshield, so the frequency is the number of blades passing per second. If it sounds like 110 Hz sound, and the prop has three blades, it's spinning at 2200 RPM, which is probably about right for a Pratt & Whitney PT-6 turboprop not at takeoff power. $\endgroup$
    – John K
    Commented Jan 31, 2021 at 16:13
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What you are hearing is indeed the doppler effect. As the plane approaches you, the pitch rises, and as it flies away from you, the pitch falls. Inside the cockpit the pitch would stay constant.

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  • $\begingroup$ So why, then, does it sound more dramatic than if it went normally? Usually when you hear a plane approaching, it's already at the peak of its pitch, so all it does is decrease the pitch much more slowly as it receeds into the distance. So why does it sound more dramatic in this case? Is it because the plane is going by much faster, or because it is doing some other manoeuvre? $\endgroup$
    – The Harmonic Rainbow
    Commented Jan 31, 2021 at 9:27
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    $\begingroup$ @Heavenly The plane approaches much closer than usual. When it comes this close, it goes from coming straight at you (high pitch) to going away (low pitch) almost instantly. Normally planes are much further away and the transition from "incoming" to "outgoing" is much slower. $\endgroup$
    – Sanchises
    Commented Jan 31, 2021 at 11:57

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