You take a hollow disk with an ultrasonic transducer and microphone mounted
90 degrees apart on the rim. Hit the resonant frequency of the cavity with
the transducer, such that there is no response from the microphone. Now,
if you start to spin the cavity along the axis of symmetry, you will get a
reponse at the microphone that is proportional to the angular frequency of
rotation.
It's a neat little magic demo.
-G-
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Gordon Smith
National Center for Physical Acoustics
Coliseum Drive
University, MS 38655
slipstk@olemiss.edu