Why do helicopters make a chopping noise instead of a smooth spinning noise?

Question

Why do helicopters make a chopping noise instead of a smooth spinning noise?

In: Engineering

Answer 1

Helicopter rotors operate a low RPM, usually around 600. That’s much much slower than things like fans, and not fast enough to produce a continuous noise:

Each blade create a pressure wave as it passes by, and the time between one wave and the next is long enough that they stay distinct.