- Get a ceramic cup, fill it with coffee, add coffee creamer (the disgusting powdered, non-lactose kind).
- Start tapping your spoon at the bottom of the cup.
- Observe the pitch of the sound rising by more than an octave as you continue tapping.
The search results showed my theory was clearly way out in left-field - it actually has to do with the molecular-structure of the coffee, not the cup. By tapping, you cause the bubbles in the coffee to rise to the surface, altering the stiffness of the fluid. The best explanation of all of this was provided by Shawn Carlson in response to the same question by a curious Bob Hall in the SAS.org's E-Bulletin from 02 April, 2004: E-Bulletin Backscatter (scroll halfway down)
Another version of the explanation was given in a paper by Andrew Morrison and Thomas D. Rossing, both at the Physics Department, Northern Illinois University: Sound of a Cup With and Without Instant Coffee: A Foam-Filled Acoustics Demonstration
Apparently the same effect can be demonstrated in beer. I should go do that.
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