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From: barry@ames-lm.UUCP
Newsgroups: net.audio,net.physics
Subject: Want information on a "super-microphone"
Message-ID: <235@ames-lm.UUCP>
Date: Sun, 6-May-84 16:36:33 EDT
Article-I.D.: ames-lm.235
Posted: Sun May  6 16:36:33 1984
Date-Received: Wed, 9-May-84 01:59:54 EDT
Organization: NASA-Ames Research Center, Mtn. View, CA
Lines: 47

[]
     I've got a question that's been bugging me for years -
anybody have more information on this one?

     In his editorial in the June, 1969 issue of ANALOG ,John
Campbell describes a microphone that has a flat frequency
response up to at least 15 MHz (that was the limit of
accuracy of the test equipment). It would be cheap to make,
and ultra-reliable, since the design was very simple, and
involved no expensive materials or moving parts.
     It worked like this: take a hollow cylinder, like a
tin can, and coat the inner surface with a semiconducting
material; then put a rod of some material that emits (low-
level) beta radiation at the axis of the cylinder. Hook
wires to the semiconductor, which will act as a beta-
particle detector. The device will then respond to changes
in the air pressure of the air between the semiconductor
on the inner surface of the cylinder, and the rod in the
center. Voila, a microphone!
     Campbell indicates that his "informant" (unidentified)
actually saw a prototype being tested. He also stated that
the response of the device was phase-flat as well as
frequency-flat.
     So, my question (obviously) is this: what ever happened
to this super-microphone? I am not well-informed on matters
audial, but I have looked for further references to this
device, and have never found another mention of it. I
assume there must have been some unobvious flaw which made
the design impractical, but in 15 years I've not been able
to discover or deduce what it is. My only guess is based
on the fact that no lower limit of flat frequency response
was mentioned in the editorial; perhaps adequate low-end
response would require too large a cylinder?
     Well, as I said, I'm ignorant of audio. Can someone
better-informed venture a guess on what the fatal flaw
must be? Or has anyone seen any other mention of this
microphone? Any information or educated guesses would be
much appreciated.
	P.S. I know Campbell rode some peculiar hobby-horses
in his time (Dianetics, dowsing, Dean Drive), but his one
unimpassioned reference in a single editorial to this
microphone, just doesn't sound like one of his weird trips.
                                                Kenn Barry
                                                NASA-Ames Research Center
                                                Moffett Field, CA
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