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jolieb@gr*.ci*.up*.ed* (Jolie Bookspan) writes:

>About the Voice Change With Masks And Helium questions:
>With helium, density of the breathing medium determines 
>springiness of the vocal folds (cords) and so their vibrational 
>frequency.

I've seen this stated a number of times and it is quite incorrect.  The 
medium is irrelevant to the vibrational frequency of a vibrating string 
fixed at both ends (similar to vocal cords).  The perceived sound pitch from 
someone talking is primarily a function of the resonant cavities in the 
head, including the sinuses, throat, mouth, and even structures outside the 
mouth (i.e. gas mask, etc.).  The resonant frequency of a cavity is 
definitely a function of the sound media and that is what accounts for the 
voice change in question.  The resonant frequency of a vibrating string is 
strictly a function of the physical characteristics of the string itself, 
such as mass/unit length, and tension.  A hollow cavity has a resonant 
frequency determined by its physical dimensions and the speed of sound of 
medium inside.  Thus when Helium, or other less dense gas is breathed, the 
perceived pitch is higher.

Best regards,
Barry Lanier
lanier@ex*.ds*.ti*.co*

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