Date: Tue, 12 Nov 2002 09:07:16 -0500

Author: Tom Ford

Subject: Re: Hot Chocolate Effect (a correction)


I really expected Bill Beaty to wade in on this one. He sent me some
helpful ideas on the subject many years ago. I am also mindful of the
research chemist turned musical instrument maker who use the words
"colloidal suspension" in his explanation back in nineteen seventy something.

Here's what I do know.

I cannot duplicate Wolf's results with only hot tap water, but in the
process did notice there is an optimal position to hit the side of the mug
with the spoon and that it is about a third of the way from the top edge.
At that distance, where around the side one hits the mug makes a tonal
difference. There are places that give the same sound and places where it
is different, but LIKELY the same as some other places. Makes me think of

I am always successful with very hot water added to Swiss Miss. As soon as
the cup is filled, tapping on the top edge will produce tones that ASCEND
in pitch over time, but rapid stirring will allow one to repeat the process
several times --- with what sounds like the very same range of tones. With
Swiss Miss, I HAVE JUST FOUND hitting the bottom or the side of the mug IN
THE RIGHT PLACE, THE SAME bell-like clarity as hitting the top edge. Over
the years, I have challenged my students to try other combinations (sand
and water, tea, coffee...) but they have never presented me with something
comparable. The Swiss Miss trick does not seem to depend on nuceation sites
with super-heated water, or even the temperature of the water in any
absolute way. I wonder if there is a single explanation that would cover
all of the phenomena described in this thread of discussion, or if each is
doing something different. I am grateful for all the contributed ideas.

Since last posting this, I have found some of the things Wolf mentioned by
letting the water run continuously, the part attributable to bubbles, but
cold water does the same thing. IMO, this would not be the demonstration to
start with. I'm sticking with Swiss Miss for that. Can't wait to see all
this in an FFT window and to drive the mug with a signal.

Tom Ford

At 05:18 PM 11/7/02 -0500, you wrote:
>> This demo is 3D30.77. Maybe the references will provide a
>>lead. A web search for "hot chocolate effect" yielded 8 hits,
>>including the one listed below which explains the effect:
>The link above gives the explanation that I have always offered in
>the past to my students until one day I tried something new. Now
>when I hear the explanation I say, "not so fast -- let me show you
>something ..."
>Try this for yourself. You need your coffee cup, a metal spoon, and
>fresh hot water out of the tap. That's all -- no other ingredients.
>Fill the cup with hot water (it comes out of the tap aerated) and tap
>the lip of the cup with your spoon (continuously). You'll hear the
>pitch go DOWN, stay down for a while, and then go UP (a couple of
>octaves or so). The pitch depends on the speed of sound in the fluid
>and that, in turn, is dependent on the overall density of the fluid.
>But if the change in pitch is due only to the bubbles in the fluid
>(which are rising to the surface), why does the pitch go down first
>and then up? BTW, this doesn't happen with hot chocolate stirred in
>-- it just goes up.