Pitch of Cocoa when Stirred
Over the years, friends of mine and I have tried to
figure out why when you have a mug of hot chocolate and tap the bottom of
the mug with a spoon repeatdly, while holding the handle, does the pitch
increase (the sound of the tapping seems to rise and rise). This does not
happen with coffee or tea. Any idea as to why?
I have seen this effect attributed to small air bubbles in the liquid. It
DOES happen with coffee, for instance, after the addition of powdered
creamer or sugar. The tone of the spoon against the cup is initially dull
and low, and brightens in tone and rises in pitch until all of the bubbles
have escaped. After this point, the sound is a simple "clink" rather than a
Exactly WHY air bubbles have this effect I don't know. The bubbles are much
smaller than the wavelength of the sound in the liquid. It may be an effect
of the different speed of sound in the liquid and in the air bubbles. Not
having any equations to support my speculation, however, I acnnot offer any
further explanation or justification.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012