From: Paul Doherty (pauld@exploratorium.edu)
Date: Thu Sep 23 2004 - 16:58:40 PDT
Message-Id: <l03110777bd791297a53b@[192.168.112.30]> Date: Thu, 23 Sep 2004 16:58:40 -0700 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Hot chocolate phenomenon
The "Hot chocolate effect is a well known phenomenon.
You can often do it simply by filling a tall glass with hot water from the tap.
Lift the glass and tap the bottom with the handle of a spoon liusten for
the frequency of the "thonk". The pitch will initially decrease then
increase again. The pitch decrease is accompanied by visible bubble
formation in the glass.
The speed of sound in a liquid is proportional to the square root of the
bulk modulus of elesticity divided by the density. The bulk modulus tells
you the fractional change in the volume when you squeeze the liquid. Pure
water is pretty "stiff' squeeze it and it doesn't change volume much. Air
is very compressible. Add bubbles of air to liquid water and you get a
material with the compressibility of air and the density near that of
water. This worst case scenario greatly decreases the speed of sound.
The resonant frequency of the glass is the (inverse of) time it takes sound
to travel the length of the glass twice. So if the speed of sound decreases
the frequency of the resonance decreases.
Paul D
>Hey, a friend asked me this question and I have NO
>idea.
>
>Stir a cup of hot chocolate (swiss miss
>instant in hot water) with a metal spoon.Then, with
>the spoon in the mug, tap the spoon with an even
>rhythm against the bottom of the mug. You will hear
>the sounds of the spoon clanking against the mug
>increase in pitch.
> What causes this increase in pitch?
>
>We tried this with tea and there was no pitch increase.
>
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