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PICKET
FENCE BEATS has two slightly differently spaced picket fences
attached to each other. Because the picket spacing is slightly
different, the pickets, which start out in step, are soon out
of step. They then return back to being in step, but different
now by one whole picket. From a few steps back, an observer sees
a moire pattern or a visual beat pattern. If a slat of wood is
run across the top of the picket fence the wood clacks where it
falls between the pickets (where they line up) and slides with
little noise where the pickets don't line up, making an audible
beat pattern.
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