Why do helicopters make more noise coming than going

[roadman65]

Ever since I was kid I always remembered hearing the chopping of air sound of a helicopter about to flyover head at least 45 or so seconds to its arrival above. However once the chopper passes, the chopping sound disappears for the sound of the motor. Then after passing the motor sound fades rather quickly.

Being a rotor on a helicopter revolves 360 degrees, you would figure the sound it makes would also.  Yet physics, only allows it to sound out forward only.

What is the nature or physics behind the sound being loudest ahead of the aircraft and not anywhere else?

[hotdogPi]

Doppler effect. The waves are effectively closer together when they're coming toward you, which means that you perceive more of them and it's therefore louder. (It also affects pitch, too.)

[kalvado]

Helicopters have pretty complex noise footprint.
While  it is flying forward, blades on one side - where blades move forward - move faster with respec to to air than on the opposite side, where blades move backward. That means lower angle of attack on one side than on the other (achieved with swash plate)and some effect of blade turning into the air - they compared it to hand clapping. Result  is a funny noise pattern with more noise forward and towards one side of the helicopter
Example:

[SectorZ]

Doppler effect. The waves are effectively closer together when they're coming toward you, which means that you perceive more of them and it's therefore louder. (It also affects pitch, too.)

Seconding this.