When I stir my coffee, why does the sound gradually change pitch?

Dear Cecil:

When I poured myself a cup of coffee this morning I noticed something unusual. As I stirred the hot coffee, the sound of the spoon hitting the sides and bottom of the cup gradually changed pitch. First it was low, then it got higher as, I assume, the coffee cooled minimally. Why?

Cecil replies:

You don’t mention whether the coffee was perked or instant, or whether you put sugar or powdered creamer into it. Assuming you poured something into it, my buddy Jearl Walker at Scientific American says the pitch changed because tiny air bubbles were released from the powder grains. Sound waves move a lot more slowly through air than they do through liquid. Ergo, the frequency of the standing wave set up inside the cup is lowered, and the sound you hear at first is low in pitch. As the bubbles burst, the pitch rises. If you didn’t pour anything into your coffee, I haven’t the foggiest. Ask Jearl.

Send questions to Cecil via cecil@straightdope.com.

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