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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. The frequency of the produced sound, however, should depend on its speed. Since the helium atoms are lighter than the air molecules, the speed of sound in helium, at the same pressure and temperature, is greater than in air. Hence the higher pitch. So I would not expect the change in spectral composition of the produced sound, rather the whole hierarchy of the fundamental frequency and its harmonics shift up in the same proportion.
If this is true, the same effect should be observed in air under higher pressure. I do remember vaguely, as a schoolgrader, reading stories to that effect about deep underwater experiments, but what is stuck in my memory, is only mentioning that people working deep underwater (under accordingly higher air pressure in the chamber) hear their own and others' voices sounding weird. Maybe, someone in contact with those working underwater have more detailed information about this effect.
Moses Fayngold,
Physics Department, NJIT
-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu]On
Behalf Of Chuck Britton