At 07:32 PM 6/21/01 -0700, Fred wrote:
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.
Yes, this is inhererent in every 1-dimensional, single driver approach to
sound cancellation. In the end, if you want to null out a source
for everyone, you need to surround it with drivers. Not practical.
But, with two drivers strategically placed apart, you can cancel
the source along one direction. Oh, and you need more than one
mike along every direction for the same reasons.
carlos.
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Carlos E. Murillo-Sanchez carlos_murillo(a)nospammers.ieee.org