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NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Pitch of Cocoa when Stirred
Name:  Justin
Status:  other
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001

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 "clunk."

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.
Assistant Director
PG Research Foundation, Darien, Illinois

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