barry@ames-lm.UUCP (05/06/84)
[]
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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Electric Avenue: {dual,hao,menlo70,hplabs}!ames-lm!barrypalmer@uw-june (David Palmer) (05/10/84)
fnord From Kenn Barry: > 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 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! At 15 MHz, the wavelength of sound is about 20 microns, so that for this to work, the size of the alpha particle path must be about this size. The length of the path can be longer, but if it gets too long, the microphone becomes ludicrously directional (about 1 degree if the path is 1mm long.) lets call 1 mm the path length. A few weeks ago I did a physics lab where we determined the decrease in energy of alpha particles when air of various pressures was placed between the source and the detector. Since the changes in pressure due to sound are of the order of 1 micron of mercury, we can use my data to estimate the change in alpha energy as a sound wave passes. The change I measured was 0.42 Mev/51 mm Hg/3.3cm path length. Converting that to our microphone, we get 2.5 eV change for a change of 1 micron of pressure over a distance of 1mm. Is this detectable? this requires further analysis. In the same lab, we measured the spreading of the energy (some alpha particles lose more energy than others.) This goes (approximately) as the square root of the distance. We found a spreading of ~250 keV over 33 cm at atmospheric pressure. This would give a spread of about 40keV at atmospheric pressure over a distance of a millimeter. In order to detect a difference, deltaX, in the means of two samples with Gaussian distribution and standard deviation sigma, you need to take approximately (sigma/deltaX)^2 samples. In our case, that's about 2.5e8 alpha particles, twice each cycle (to respond to a sine wave). This works out to 8e15 alpha particles/second. The typical solid contains about 1e22 atoms/cc (give or take an order of magnitude,) so an alpha source 1/2 wavelength on a side (at 15 MHz) would contain (1e-3)^3 cc's, or about 1e13 atoms. Assuming that each atom can emit only one alpha ray, the microphone would be able to work for about a millisecond before its alpha source is totally decayed away. The military may find some practical(?) use for this, but most recording studios will stay with capacative mikes. > 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? There should be no lower limit to frequency response. The longer the cycle time, the more alpha rays can be detected per cycle, and the larger the components (alpha source, path length, etc.) can be. David Palmer P.S. I really have no interest in such a microphone, I can only hear up to 5 MHz :-)