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From: gmark@ihlpf.ATT.COM (Stewart)
Newsgroups: sci.physics,sci.math,sci.electronics
Subject: Re: noise cancellation
Keywords: anti-noise
Message-ID: <7996@ihlpf.ATT.COM>
Date: 17 Mar 89 15:47:19 GMT
References: <723@wucs1.wustl.edu> <7260@fluke.COM> <453@corpane.UUCP>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 33

In article <453@corpane.UUCP>, sparks@corpane.UUCP (John Sparks) writes:
> In article <7260@fluke.COM>, inc@tc.fluke.COM (Gary Benson) writes:
> ] The theory says that the *best* you can hope for is total cancellation of
I'll settle for that.

> 180 degrees out of phase with one another. Make sure you feed the same sound
> into both speakers.
> 
> But in reality this won't work. Since each sound source is the center of
> a 3 dimensional spheroid of sound waves, They would have to both be located
> in the exact same space in order to cancel out each other. 

Which leads me to believe that one might design headphones (as is
currently used by the British system, and, I guess, Bose), but
with the amp. inside (or in a pocket/walkman-type unit), with the
microphone(s) on the set (ideally, in locations near and approximating
the quality of the human ears.  Coupling to the output of the earcup
speakers would be a problem.  But, it should minimize noise quite
a bit.  Now, the processing comes in when you decide what you do
want to hear.  You might have several switches on the control to
allow you to select music (using a circuit similar to the ones
that remove vocals from music), voice, or to remove these (for who
knows what reason).  This would be nice to wear in a noisy bus, for
instance, and could be sold with a walkman attached to allow
music in a noisy environment to sound like sitting in a quiet
room.  'Course, safety for joggers would be quite a spectre of
doom.

				- Mark

				G. Mark Stewart
				ATT_BTL, Naperville, Ill. ix1g266
				ihlpq!gms (312)979-0914