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This may be serving only to expose my own ignorance, but ...
At a retreat over the weekend, I came across a device (which may have
once served as the cabin's dinner bell) that really has me puzzled. It
consists of what looks to be a solid aluminum rod, about 20 cm in length
and 1 cm diameter, suspended by a pair of fine strings (looped around the
rod) at about the 5 cm and 15 cm points from one end. When struck with the
supplied rubber-headed mallet, the rod emitted a nice high-pitched tone -
and beats! The beat frequency was in the 1-2 Hz range and was both clear
and reproduceable. In fact, the beats were considerably more noticeable
than the ones I *try* to produce in class with a pair of few-Hz-apart
tuning forks.
I can't for the life of me figure out why a single rod would produce
two strong tones a Hertz or two apart in frequency when the dominant tone
was in the 800-900 Hz range. Do the strings introduce some weird boundary
conditions? Is there a plausible explanation if the rod were, in fact,
non-uniform? Could temperature differentials across the rod (in a
non-uniformly-heated cabin - but it's a short rod compared to the length
scale of the temperature differences) have anything to do with it? I
didn't have the opportunity to do extensive tests on the device, but this
really has me bothered.
Any insights? Or am I missing something completely obvious?
Nick
Nick Guilbert
The Peddie School
Hightstown, NJ
nguilber@peddie.k12.nj.us