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HUMMING
PLATES consists of two metal plates, round and square, each supported
in the middle, with fine sand on their surfaces. A taut bow is rubbed
on the edge of the plate, sending out a wave, a vibration that travels
along the surface of the plate. The waves spread out in all direction
and reflect from the edge of the plate. The reflected waves meet
new waves produced by the bow. Where these waves meet, they can
add together and make the plate vibrate a lot, or cancel out so
that the plate is hardly vibrating at all. The sand bounces away
from places where the plate vibrates a lot, and collects in relatively
still areas.
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