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46 chlaDni PaTTerns vibrating surfaces So far we have only considered vibrating strings and other simple systems, but surfaces also can be made to vibrate, and they too can display harmonic or resonant patterns. In 1787 Ernst Chladni found that if he scattered fine sand on to a square plate, and bowed or otherwise vibrated it, then certain notes, generally harmonics of each other, each gave rise to different patterns in the sand on the plate. Like the harmonograph, other disharmonic tones produced a chaotic mess. Sometimes he found that further patterns could be created by touching the side of the plate at harmonic divisions of its length shown below. This created a stationary node like the feather on page 8. Later work revealed that circular plates gave circular patterns, triangular plates triangular patterns and so on. The six pictures opposite are from Hans Jennys book Cymatics, one of the seminal texts on this subject. The vibration picture appears gradually, the sand finding its way to stationary parts of the plate as the volume steadily increases through the sequence. 46 47 47
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