Guy King wrote:

What I can't find is the explanation for why the sound of tapping a
spoon on the bottom of a fresh poured drink rises in pitch as you
continue to tap. I saw one a while ago, but I can't remember the
details. Something about microbubbles changing the speed of sound in the
liquid but that doesn't always seem to fit what's happening.

I have seen a number of debates on this subject none that claim to be
conclusive.

The one that interested me was why the pitch rose as you stirred a
powder like drinking chocolate into hot milk or water. The most
noticeable one I found was stirring honey into hot milk - the pitch
rises as you stir, but the falls again if you pause and restart -
probably as the honey does not emulsify in the liquid and some starts to
settle out again.

My best guess for the cause is that as you hit the cup you cause it to
ring. It will ring at its "natural" resonant frequency. Control theory
tells us that as you add damping to a system you also cause the
frequency it will resonate at to move away from the natural frequency. I
guess that is what is happening here. As the powder dissolves into the
liquid it alters its viscosity and hence the damping effect it has on
the ring and the more pitch shift.

Any other explanations you can think of?


--
Cheers,

John.

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