Re: things to do with dry ice?

[William Beaty <billb@ESKIMO.COM>]

On Thu, 16 Sep 1999, Stefan Jeglinski wrote:

> > Fill balloons with CO2 and use them as sound lenses (sound telescopes)
>
>
> do you have a reference for this? I've never heard of it.

It's a classic museum exhibit:  fill a small weather balloon with CO2,
then have two people sit on either side of it at the 2F distance, and they
can whisper to each other, and the "lens" concentrates the sound waves in
both directions like a "cone of silence" effect.  (The 2F distance is
easily determined by experiment.)

The same thing works with any small rubber or mylar balloon or
pillow-shaped plastic bag. Hold a CO2 balloon up to a microphone and aim
it at distant sound sources to listen in.  It's the "lens version" of the
Big Ear parabolic sound mirror.  You don't need a microphone, just hold it
a distance from your ear (move it around to find the 1F distance, whatever
it might be), then aim yourself sideways to focus upon a distant sound
source.  (If you see somebody with a balloon on the side of their head,
they might be attempting to overhear a distant conversation!)


I've always wanted to build an "acoustic camera" based upon this effect.

Cover a spinning drum with microphones, sample the microphone amplitudes
continuously with an A/D card (and display the results as a raster on the
computer screen), and you've formed a sort of "acoustic phosphor" which
reveals the intensity of sound on the surface of the entire drum.  Now
hold a CO2 balloon between the drum and the rest of the world, and you
will "see" the world in glowing sound waves (maybe we'd want to
"illuminate" the world with a broadband high-frequency "white light" sound
source, if the ambient sound wasn't "bright"  enough.)  If the microphones
are filtered to pick up 10KHz and above, then the resolution on the
surface of the drum will be a couple of cm or so.  Blurry, but no doubt
interesting.   Add some bandpass filters and more A/D channels, and build
yourself an acoustic "color camera."

This is based on the Winston Kock book "Seeing Sound"  ASIN: 047149710X,
where he scanned a single microphone back and forth, piped the amplified
signal to an NE-2 bulb mounted next to the moving microphone, and recorded
the raster on film via a long camera exposure, thus revealing the
intensity profile of the sound in space.  (By generating "coherent"
illumination with an audio generator, and by adding a "reference" wave
from the generator to the microphone, he was able to produce pictures of
"frozen sound waves" and not just pictures of the distribution of audio
amplitude.)

Kock is a personal hero of mine just because he refuses to say that laser
light is "in phase light" or "coherent light".  Instead he says that laser
light is "sharper" than the light from other sources (which is exactly
correct.  Point-source light can be focused nearly to a geometrical point,
while the light from non-laser sources cannot.)


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William J. Beaty                                  SCIENCE HOBBYIST website
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