Chris J Dixon wrote:

Any other explanations you can think of?


The stirring causes the liquid level to rise at the edges and
thus increase the damping of the cup. Let it settle and it is
back as it was. Repeat at will.

That is one suggestion I have seen, along with a suggestion that the
swirling causes the spinning liquid to reduce the amount of exposed cup
height thus altering the resonant frequency of the tuned pipe air column
above the liquid.

I am not convinced by these, because with powders that disolve
completely into the liquid the change in pitch is permenent - letting
things settle does not reverse the pitch change. Also the air column
height is so short that it would represent a quarter wave resonator for
a much higher pitch that you hear in real life.


--
Cheers,

John.

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