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Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!mit-eddie!uw-beaver!tektronix!reed!craig
From: craig@reed.UUCP (Craig Deforest)
Newsgroups: sci.physics,sci.math,sci.electronics
Subject: Sound holography (was Re: noise cancellation)
Keywords: anti-noise audiophile sound acoustic holography
Message-ID: <12109@reed.UUCP>
Date: 20 Mar 89 05:24:37 GMT
References: <723@wucs1.wustl.edu> <7260@fluke.COM> <453@corpane.UUCP>
Reply-To: craig@reed.UUCP (Craig Deforest)
Organization: Reed College, Portland OR
Lines: 36

In article <453@corpane.UUCP> sparks@corpane.UUCP (John Sparks) writes:
>Hmm, In theory you can try taking two speakers from a stereo system and
>wire one of them opposite polarity from the other. [and get nothing out]
>...
>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. 

Hmm.  This brings up the interesting topic of sound holography, which was 
(I seem to remember) going on at U.C. San Diego some years ago...  Sound
holography works just like optical holography, except that the wavefront can
be constructed 'by hand' with a wall full of speakers, since speakers are 
about (!) the same size as sound waves.  Of course, there's the small ;-) matter
of feeding the right sounds, with the right delays, to all of those speakers, 
even after you've figured out what you need to do to produce the 
desired image...  

The possibilities are obvious.

I even recall a simple (trivial) case of sound holography for the home audio-
phile:  in 1985, some high-fidelity *electrostatic* speakers were put on the
British market:  a pair of plastic membranes had several metal foil capacitor
plates which charged and discharged, producing sound waves.  The neat part was
that there were several such capacitors on the membranes, which were fed 
through linear phase (read: 'constant time') delay amplifiers, to reproduce
the wavefront that *would* happen if the sound were coming from a point source
15cm (or so) *behind* the membrane.  They sounded great! :-)

Does anyone know anything about other applications or experiments with
acoustic holography?  Please followup or send email; I'd like to know more...

-- 
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"Well, the universe _IS_ linear, |  craig@reed.BITNET    -or- 
 at least to first order..."     |  ....tektronix!reed!craig 
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