Robbie remarked:

>[ Noise-canceling techniques like you describe were demonstrated
>[ in small aircraft cabins but thus far haven't become commercially
>[ feasible, probably because of the expense.

Bose sells commercial noise canceling earphones.  It really does work.

The principle of having a microphone pickup sound and regenerate that
sound becomes complex in practice, because of the audio feedback, so
familiar as howling.  It works at locations only where the frequency
can be limited to a less than a quarter wavelength of the tone.

The earphones work because the output sound does not get fed around the
cushions and back into the mic on the outside of the earphone.  And the
mike can get most all of the sound because they are so close together.

The principle also works at low frequencies, like pile drivers, and
some heavy equipment, but practically takes a live-in "sound-engineer"
to keep it useful.

Will Herzog