[Phys-L] Re: sound frequency in helium

[Julie Quah <juliequah@GMAIL.COM>]

hi thanks for the enlightening explanation. Does it follow that if a =
pure=20
sound wave (i.e. one produced by tuning fork) is used in such activit=
y, the=20
pitch will not appear to change, since there is only one frequency?

On 4/18/05, Folkerts, Timothy J <FolkertsT@bartonccc.edu> wrote:=20
> =20
> Julie,
> =20
> It is only sort of a change in pitch.
> =20
> Suppose you sing a particular note - i.e. produce a particular fund=
am=3D
> ental frequency for the vibrations of the vocal cords. Of course, t=
h=3D
> ere are also harmonics of this frequency that are produced. The par=
t=3D
> icular sound that comes out depends on the shape of the mouth/tongu=
e/=3D
> throat. The size and shape of the vocal tract creates different res=
=3D
> onances which enhance or repress different harmonics. The relative =
a=3D
> mount of different harmonics determines determines the quality (or =
ti=3D
> mbre) of the sound.
> =20
> When you have a mouth full of helium, the speed of sound is much hi=
gh=3D
> er. Even if the vocal tract is the same shape, the resonace frequen=
c=3D
> es are much higher. The lowest frequencies are no longer in resonan=
c=3D
> e, so they are attenuated. New, higher pitched harmonics are in res=
o=3D
> nance, so they are enhanced.
> =20
> In a nutshell - the original vibrations are the same, but the reson=
an=3D
> t frequencies are much higher.
> =20
> Tim F
> =20
> =20
> -----Original Message-----
> =3D46rom: Forum for Physics Educators on behalf of Julie Quah
> Sent: Sun 4/17/2005 8:56 PM
> To: PHYS-L@LISTS.NAU.EDU
> Subject: sound frequency in helium
> =3D20
> Dear experts, pleasse help me out with this issue of sound pitch in=
 h=3D
> =3D3D
> elium=3D3D20
> gas.
> When one breath helium gas, and speak, it is found that the pitch i=
nc=3D
> =3D3D
> reases.=3D3D20
> How do we explain this? But if we compare the water wave travelling=
 t=3D
> =3D3D
> hrough=3D3D20
> deep to shallow water, its speed changes without frequency chage, a=
s =3D
> =3D3D
> the=3D3D20
> source (motor frequncy) is the same.
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