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From: inc@tc.fluke.COM (Gary Benson)
Newsgroups: sci.physics,sci.math,sci.electronics
Subject: Re: noise cancellation
Keywords: anti-noise
Message-ID: <7260@fluke.COM>
Date: 10 Mar 89 16:45:47 GMT
References: <723@wucs1.wustl.edu>
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Organization: John Fluke Mfg. Co., Inc., Everett, WA
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The theory says that the *best* you can hope for is total cancellation of
the offending noise.

The idea behind "anti-noise" as you call it is that you generate an exact
replica of the offending noise, except opposite in polarity. It must be the
same frequency and the same amplitude, just reversed. As the tops of the
wave fronts collide, they dissipate each other's energy, and the same thing
happens all down the trough and up the next wave. The dynamic case would
monitor the signal and generate it's equal-but-opposite counterpart in
real-time. It sounds like you are looking for a "static" anti-noise
generator, so you'll monitor the signal first, and replicate it via some
kind of noise generator-amplifier arrangement.

I have never heard either of these in action, so I have no idea what the
result might sound like, or how successful you are likely to be in blocking
it out. However, I am really interested in that whole topic, and would
appreciate it if you post a summary of your responses, and also a
description of your final setup and the results.

Off the topic, but while you are researching and building the anti-noise
gizmo, you might consider adding more absorptive material in the room.
Curtains, drapes, and rugs will help absorb some of that sound energy.
Also, in the past I have successfully blocked noise (traffic noise in my
case) with a little "surf synthesizer". It was just a little 15 transistor
circuit -- a white noise generator modulated by three asynchronous
multivibrators. You might consider something like that, too.

Gary Benson
John Fluke Mfg. Co. Inc.