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

From: Fred Cisin <>
Date: Thu Jun 21 21:32:42 2001

> > It might be difficult but it's not impossible. They're using systems
> > like that on US Army tanks right now.
On Thu, 21 Jun 2001, Chris Kennedy wrote:
> Not to mention the active nose cancellation headsets that I and most
> of my friends were while flying. They're designed only to deal with
> low frequency noise; when you turn them on it's like someone stuffed
> high-pass cotton into your ears. Voices sound perfectly normal but
(well, a little "hollow")
> the drone of the engine sounds like it's in the next county.
> The big problem is power density -- these things only have to provide
> cancellation in the confines of a headset ear cup. Cancellation in the
> large would obviously require much higher power levels.
(and relocation of the mike and speaker)

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.

IF you could get the originator of the sound to put a mike next to his
speakers and add your speaker there, then you could cancel most of his
noise. But only one person here seems to live in an area where you could
get that level of cooperation.

In theory, it might be possible to put a microphone in your exhaust header
pipe and a speaker in the muffler (giving your circuit a headstart), and
significantly reduce outgoing vehicle exhaust noise!
What percentage of the public in your neighborhood appreciate good

Grumpy Ol' Fred
Received on Thu Jun 21 2001 - 21:32:42 BST

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