[paul.fr]

if two sounds are fired at eachother, could they cancell eachother out producing a state of quietness?

[RD]

Noise cancelling headphones use this principle.
http://reviews.cnet.com/4520-3000_7-1017728-1.html

[that mad man]

If the sounds were of the same frequency but at opposite phases then some sounds can cancel out.

A similar thing happens if you wire the home stereo speakers wrong and get one of the terminals mixed up, one speaker cone will be pushing while the other pulling. If that happens then you will get little or no bass as its the lower frequencies that are affected most.

I did read that they were experimenting with a similar principal on planes to cut down on the internal noise.

Bee

[lyner]

The problem with cancelling  acoustic noise (and, indeed, in trying to cancel any wave) is that you can cancel the sound in one restricted region by 'destructive interference'  but the cancellation drops off in places where the  'cancelling' signal  is not in antiphase with the unwanted sound. It's a bit of a knife edge situation.
It works for headphones because your ear stays in the same position relative to the earpiece. It can also work, particularly for low frequencies, in cars and right next to noisy machinery.
There's no substitute, in a home with noisy neighbours, for banging a broom on the ceiling and shouting "Shut the ***** up!" at the top of your voice.