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Why is it that when you hear an airplane it sounds like it is coming from somewhere behind where the airplane really is? Is it because light travels faster than sound? - S, TX

Yes. The sound seems to come from behind the plane because you're hearing sound that the plane emitted many seconds earlier. The plane is flying ahead of its sound, so what you see and what you hear get out of sync.

This problem becomes more severe as the plane's speed increases and, at a certain point, something totally new happens. When the plane outruns its own sound waves, thus traveling faster than sound itself, its sound begins to pile up in a cone that radiates outward and backward from the supersonic plane. Outside the cone, you can't hear the plane at all. Inside the cone, you hear the plane as it was sometime in the past. But right at the cone, you hear a "sonic boom," a sudden surge in pressure as the piled-up sounds of the plane finally reach your ears. The cone moves along with the plane and as it sweeps over you, you hear the sonic boom. The angle of the cone depends on how fast the supersonic plane is going but the fact that it sweeps along the ground means that not everyone hears it at once. Furthermore, the sonic boom has little to do with actually passing through the speed of sound. Sonic booms can be heard throughout the plane's supersonic flight and they're so disturbing to people and animals that supersonic fight is limited to unpopulated areas or oceans.


Answered by Louis A. Bloomfield of the University of Virginia