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Re: speed of sound experiment



One of you (thanks, Kyle) suggested trying the experiment with another
gas. Well, homebrewer that I am, I went down to the kitchen and took the
CO2 cylinder, and "poured" CO2 into a vertical tube with the bottom
sealed. I got a 10% change in speed, and I estimate the measurement
uncertainty to be only a few percent. Now, the speed of sound in CO2 is
actually about 30 or 40% faster than in air, but I'm sure I didn't have
the tube full of just CO2. All I did was pour.

The speed of sound in CO2 is less than the speed of sound in air at the
same temperature. The best demonstration of this I have see is the use
of a CO2-filled balloon as an acoustic lens, entirely anaogous to the
focussing of light by a glass sphere in which the speed of light is
slower than it is in air. The two demonstrations can be set up at the
same time. The demonstration I saw, incidentally, was of a CO2-filled
weather balloon about ten feet in diameter. A more modest size should
work, however, if only high frequency sound (like the ticking of an
alarm clock) is used as an object. The lens, of course, must be several
wavelengths in diameter. By measuring image and object positions and
applying the lensmaker's formula one can find the index of refraction
for acoustic waves this way.

Leigh