[Phys-L] Re: sound frequency in helium

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 04:21 PM 4/18/2005, Moses,  you wrote:
(1)
>   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.
(2)
> 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.
(3)
>   If this is true, the same effect should be observed in air under higher
> pressure.
(4)
> 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


Moses's para (1) seems reasonable - still, there is the distinction between
the generating fundamental and the intensity of the envelope of harmonics,
  which he does not explicitly mention, however.

A sonogram is often featured on websites that I have examined - in connection
  with voiced word recognition. This kind of diagram would be helpful.

I do not take it (2),  as evident that the generating fundamental, and the
formants would shift 'ensemble', however.

I do not think Moses intended to assert (3), that air at the same temperature
and composition, but different pressure promotes a change in speed of sound.
Indeed, it does not.

It is often observed (4) that deep divers sound high-pitched.
Their diving gas is nitrogen-reduced - for the usual reasons.


Brian Whatcott    Altus OK    Eureka!
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