Re: An Acoustics Question (fwd)

[William Beaty <billb@eskimo.com>]

On Fri, 14 Mar 1997, Pete Lohstreter wrote:

> I received this message from a teacher at a nearby school...

> coffee, once she has dumped the powder into the hot liquid and
> stirred, she can tap on the top of the cup with the spoon repeatedly, and
> with each successive tap, the pitch made by tapping the cup with the spoon
> rises.

This was in the SAS newsletter (Soc. for Amateur Sci.) a few issues ago in
an article by Shawn Carlson. It's called the "Hot Chochlate Effect" and is
a consequence of the reduced speed of sound in foams.  If you fill water
with gazillion tiny bubbles (for example by pouring in powder which
dissolves and leaves behind the trapped air), the compressibility of the
medium increases drastically, even though the density only decreases a
little.  You end up with a material which has a sound propagation velocity
much lower than that of either water or air.  The cup acts like it's
acoustically enormous inside. If you clank a spoon around inside, you will
hear deep thunking noises as if the cup was many times larger.  As the
cloud of bubbles rises, the volume of the foam-filled portion of water
shrinks, and the resonant frequency of the foam-glob goes up.  If you stir
the liquid up again before the bubbles all rise and pop, the frequency
decends again.

Now that Dr. Carlson has sensitized me to the phenomena, I keep noticing
it in different places.  If you make merangue (whipped egg whites) or
homemade whipped cream, you'll notice that the beaters make a low pitched
sound as they knock against the sides of the bowl.  Same thing  happens
with a spoon in milkshake if the mixture is in a metal can rather than a
paper cup.

While experimenting recently at a public pool, I found that if I knocked
my knuckles together in the bubble-jet underwater in the Jacuzzi, I could
hear an unexpectedly loud clunking noise.  The underwater cloud of bubbles
forms a sort of resonant chamber.  Also, if I formed a cloud of bubbles in
the pool by splashing with clawed fingers, I could duck my head under and
shout into the bubble cloud at low pitch, and the noise coupled right into
the water and made people look at me strangely.  The cloud of bubbles
forms an acoustic matching transformer, and lets sounds in the air travel
into the water without simply bouncing off the surface.

If you're swimming in a lake where small rocks are available, try
splashing up a bubble cloud, then knock rocks together repeatedly in the
cloud.  You'll hear a very loud version of the rising-pitch spoon clinks
in hot chochlate.  Don't pinch your fingers when trying to increase the
volume by using larger and larger rocks!   ;)


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