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Re: Dispersion of sound waves??



You have discovered "whistlers". Sound is dispersive in a medium in which
the boundary is not infinitely far away. In a long, narrow pipe low
frequency sound travels faster than higher frequency sound. For a pipe of
the proper dimensions one can clap one's hands at one end and listening to
an echo which has a rapidly rising pitch return. The greater the ratio of
the pipe's length to diameter the more pronounced the effect. Some highway
culverts work but the best effect I have seen was in the science museum in
St. Louis where a pipe had been hung for just that purpose.


For the past few days there has been a stack of big steel pipes in the road
outside where they are installing a new gas main. These are steel pipes (I
guess) about 25cm in diameter and 12 metres long as measured by pacing them.

Great, I thought, we can observe reflection from the open end. This is done
by going "boh" into the pipe and listening to the sound bouncing back and
forth. Works well. Today I chanced to click my tongue instead of going "boh"
and observed an interesting effect. What I heard back was a sort of high
frequency "pew" sound, just as if the click had been dispersed, with the
higher frequencies arriving first. Snapping my fingers produced the same
effect.

Now I thought that sound in air is non-dispersive. Any ideas?

Mark.

Mark Sylvester
UWCAd, Duino, Trieste, Italy.


Jim Riley, Department of Physics
Drury College
Springfield, MO 65802
e-mail: jriley@lib.drury.edu
Phone: (407) 873 7233