Noise cancellation (was: Boom cars (ot for sure...)

For years I have used noise-cancellation headphones at COMDEX. The slowness of the speed of sound limits the practicality of the technology to situations where you can have the added signal provided to the receiver (your ear) at the same place as the original sound (in the headphones). If you tried to produce a unit to silence the neighborhood, alas, the delay of the sound reaching the mike, added to the delay of the "anti-sound" getting back to the original source, would render it impossible to synch up adequately. Even if the mike were at the source, eliminating THAT delay, people at different locations would experience different phase combinations of the two signals. In fact, at 500 feet away there would be almost a half a second of delay. Since you want either no time delta between the signals (if inverted), enough distance to give half the frequency of the sound would completely bollix it. In other words, you could diminish the sound for yourself, but not cancel out the noise level even a few feet away.