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From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: sci.physics,sci.math,sci.electronics
Subject: Re: noise cancellation
Message-ID: <18203@glacier.STANFORD.EDU>
Date: 22 Mar 89 06:04:37 GMT
References: <723@wucs1.wustl.edu> <7260@fluke.COM> <453@corpane.UUCP> <7996@ihlpf.ATT.COM> <805@neptune.AMD.COM> <1491@petsd.UUCP>
Sender: John B. Nagle <jbn@glacier.stanford.edu>
Reply-To: jbn@glacier.UUCP (John B. Nagle)
Organization: Stanford University
Lines: 16


    1.  It's "Tales from the White Hart", by Clarke.

    2.  Noise-cancelling headphones were used on the Voyager round-the-world
	flight, but the units quit working partway through the flight.  The
	book on the Voyager project gives the maker, but I do not have the
	reference handy.  

    3.  Noise cancelling only works for small volumes of space, smaller than
	a wavelength.  Attempts to produce cancellation over a large space
	with a cancellation source located at a different point than the
	noise source will result in nulls at some points in space and
	twice the sound level at other points.  So headphones work, but
	large-area cancellation doesn't.

					John Nagle