Re: pinhole helium

Date view Thread view Subject view Author view Attachment view

From: Paul Doherty (pauld@exploratorium.edu)
Date: Thu Jan 15 2004 - 15:52:44 PST 

Message-Id: <l03110727bc2cd52cc003@[192.168.111.197]>
Date: Thu, 15 Jan 2004 15:52:44 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole helium

Hi Irene

Helium molecules are less massive than nitrogen and so the speed of sound
in helium is faster than the speed of sound in air. (Helium is 4 amu while
nitrogen is 28 and the average of air is 29 amu) The speed of sound is
proportional to the square root of the molecular mass so the speed of sound
in helium is (29/4)^0.5 times greater than that in air = 2.7 times greater.

The resonance frequency of your mouth depends on the time it takes sound to
cross your mouth cavity. When you fill your mouth with helium the time it
takes sound to cross your mouth (at the high sound speed in helium) is less
than the time it takes when your mouth is full of air. By 1/2.7.

The resonant frequency of your mouth goes up like the inverse of the
crossing time, it goes up by a factor of 2.7.

So your voice goes up by more than an octave.

The increase in frequency has nothing to do with changing the vibration
frequency of your vocal chords. This is a standard science urban legend.

Paul d

>I was wondering if anyone can give me any information on what causes your
>voice to sound squeeky when you "talk" using the helium gas. Irene
>Hirota
>

Date view Thread view Subject view Author view Attachment view

This archive was generated by hypermail 2.1.3 : Mon Aug 02 2004 - 12:05:34 PDT