A thin metal plate shaken vertical by a post in the centre will vibrate in a certain mode pattern. The pattern can be made visible by sprinkling white sand or salt on the plate. The grains are shaken off the areas where the plate is moving most violently (the anti-nodes) and collects on the non-moving regions (nodal lines). This technique was originally invented by Robert Hooke, but made famous by Ernst Chladni in the late 18th century.
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