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.