Why does the pitch of the noise made when you stir a cup of coffee with a spoon, drop as you stir faster?

Question

When you mix up a cup of cappuchino (say), and begin stirring it round and round, the spoon makes noises as it scrapes around the bottom of the cup. Or a milky drink, ...-whatever.

But as the rotational velocity of the liquid increases, the pitch of the stirring-noises decreases as if a Doppler effect were taking place. Why? The ears hear both receding and approaching wave-fronts simultaneously.....

And as you cease stirring, or stir more slowly, the pitch rises again, and falls again as you stir faster again. You can repeat it over and over... it seems to have something to do with the density and viscosity of the liquid, and its rotational energy, but I can't understand the phenomenon.....
Can you?

Monkey Kick. ... To a Neanderthal, no, of course it dosn't really matter, -but then, he's going nowhere, is he?

With respect, it crosses my mind whether you should really be answering Physics or Science questions? Perhaps Media Studies? or TV Soaps? Or "DUH?" perhaps?

Answer 1

As you stir something, the amount of the side of the cup that's actually in contact with the liquid increases; liquid improves the conduction of sound as compared to air, since it's denser, so a larger distance (bottom to top) of the cup is incontact with the liquid. This acts to drop the pitch, just as a longer or thicker string on an instrument has a lower pitch than a short or thin one.

Answer 2

If you stir a cup of plain tap water, the pitch does not change. It s only when one is mixing more than one ingredient that the pitch changes, due to a change in density as the ingredients mix. Once the ingredients are thoroughly mixed, the pitch stops dropping.

Answer 3

Possibly due to air getting intrained into the coffee. This would change the density.

Good question.

Answer 4

I'm not trying to be mean or anything, but why does it even matter?