Why do helicopter go "chop chop"?

Factual Questions
21

I’ve noticed that a helecopter hovering (not moving) above me doesn’t make a chop chop sound, just growling and whirring and jet noises, and asked a pilot why.

He said the chop chop chop sound came because of two effects: first, the blade speed through air is different by twice the flight speed for the blades on either side of the craft, and second, you hear the blades coming at you or going away from you.

If it’s all the stuff about the tail rotor, why don’t choppers hovering overhead chop?

22

I’ve heard that the “chop chop” sound that helicopters make is so distinctive that there are audio helicopter detectors that can hear them coming from very far away, long before humans can hear them. I think they use some sorta DSP to analyze the audio spectrum…