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Last week I heard a helicopter approaching from a distance.

The sound it made was a very distinct and deep wop wop wop at a remarkably slow two beats per second, more or less.

As it came closer, the sound suddenly changed, to something more like six beats/second, with a quite different and more familiar chattering.

What were we hearing from the ground? What do the two different sounds represent?

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  • $\begingroup$ Maybe doppler effect? $\endgroup$
    – user14897
    Commented Aug 30, 2016 at 22:36
  • $\begingroup$ It's super super super dependent on the specific design of the helicopter - a 50 year old Huey cobra sounds SOOooo much different than a modern chopper with a shrouded tail rotor. TL;DR is below. ;-) $\endgroup$
    – ljwobker
    Commented Aug 30, 2016 at 23:04
  • $\begingroup$ What type? A lot of noise comes from the blade tip vortexes interacting with each other, particularly when turning with a low rate of descent. The tip of each blade can strike the vortex left by the previous blade. Chinooks are famous for the "wop wop" as the vortex from the rear blade moving down strikes the forward blade going in the other direction. Overall, as others have said, helicopter noise in general is a very complex area. $\endgroup$
    – Simon
    Commented Aug 31, 2016 at 12:46

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Rotor noise is relatively complicated, and is very highly dependent on the design of that particular aircraft. Here are at least a few components to consider:

Ultimately what you're "hearing" is of course pressure waves in the air, which in a helicopter are almost entirely caused by the rotors passing through said air[1]. If you're head-on and a fair distance away, you're primarily hearing the pressure wave that's generated as the rotor passes the point where it directly points at you, because that's where you get the strongest component of pressure "pointed" in your direction.

As the aircraft moves, your orientation to the sound profile changes: you get both the doppler effect which changes the pitch, but you also get a very different sound profile because the pressure waves from the main rotor are interacting with both the body of the aircraft and more importantly the pressure waves from the tail rotor in different ways. I don't remember the exact percentage, but a very large part of the sound generated by helicopters is the interaction of the main and tail rotors - this is a main reason that you see a lot of modern helicopters with a "shrouded" tail rotor. If you're truly head-on, it's reasonable to assume that you're getting very little of the interference with the tail rotor, and as you get more side-on with the aircraft you get far more interference noise.

[1] You also, of course, get noise from the engine-- but that's usually drowned out unless you're quite close, OR if you happen to be sitting near a chopper with the rotor at zero-lift... in this case the noise from the rotor is actually pretty low and you can hear the engines and the tail rotor a lot more clearly...

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