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[Phys-L] Re: sound frequency in helium



I do not know much about the anatomy of the vocal chords, but if we
can assume that their sizes are fixed and in first approximation do
not depend on muscular effort, then each cord (more accurately,
corresponding resonating cavity?) must produce a system of standing
waves of fixed lengths regardless of surroundings.

That doesn't follow from the assumption that muscular effort is
irrelevant. Moreover muscular effort IS relevant. As John Denker
and others have pointed out, in vocal sound the frequency is primary.
The vocal cords vibrate at a specific frequency that can be altered
via muscular effort and produce a source waveform that is rich in
harmonics. The wavelength of each harmonic is a secondary quantity
determined by the frequency and the speed of sound for the gas in the
vocal tract. Harmonics with wavelengths that resonate well within
the vocal tract given its shape and size are reinforced and those
that do not are suppressed. Usually there are two or three bands of
harmonics that are emphasized via resonance. These are called
"formants." A little googling on "formants" may be useful.

BTW, pipes are different. In the case of pipes it is the wavelength
that is the primary quantity. It is determined by the construction
of the pipe. The frequency is a secondary quantity determined by the
wavelength and the speed of sound for the gas inside the pipe.

John Mallinckrodt
Cal Poly Pomona
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