Re: pinhole Re: Donald Duck & He

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Fri Dec 01 2000 - 10:31:19 PST 

Message-Id: <l0311070bb64d9d80eb99@[192.174.2.173]>
Date: Fri, 1 Dec 2000 10:31:19 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole Re: Donald Duck & He

Prof. Brian Holmes at San Jose State is the expert here, but the frequency
of the vocal cord vibration changes little in the presence of helium.
(Think about the aquanauts breathing helium under 10 atmospheres of
pressure their voices sound high pitched even with that highly compressed
gas.)

The speed of sound in helium is higher than the speed of sound in air by a
factor of 2.7. (The square root of the ratio of the molecular masses)

This changes the resonant frequency of the mouth and vocal cavity, which
remains the same size but now contains a medium with a higher speed of
sound.

The vocal cords vibrate with a range of frequencies (see the Exploratorium
exhibit "Frequency Analyzer.") The frequencies that match the resonant
frequencies of the vocal tract are amplified. With helium higher
frequencies are amplified than with air. And so the voice sounds higher
pitched.

Paul D

Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld

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