Waves and Resonance

Visible Vibrations

Visitors select a shape from an assortment of thin plates (square, circular, triangular), mounting it on the post of a vibrating mechanism. A pinch of sand is sprinkled on the plate, and the vibration is started. Users can change the vibration with a frequency control. Depending on the shape of the plate and the frequency of vibration, the sand will form into many surprisingly different patterns. When a plate vibrates at a given frequency, parts of it move a lot while other parts are almost still. Sand tends to jump away from vibrating places and settle at the still spots, revealing different resonant patterns. Patterns occur because vibrations moving outward from the mounting post bounce off the edges of the plate and interfere with each other. Lower frequency waves make simpler patterns, because the waves are longer and fewer of them "fit" on the plate.