Cymatics

Several years ago I was using a saber saw to cut a thin piece of plywood. There was sawdust on the wood that started to form geometric patterns due to the vibration. The saw had a variable speed trigger and when the speed increased or decreased the pattern changed. Here is a demonstration of the effect using a metal sheet and table salt.



Salt Sound Waves - video powered by Metacafe
It sounds like a sine wave generator is being used since the tone is pure without overtones. This link takes you to a more complex demonstration of what is called cymatics. Here’s a Wikipedia link.

Now I know what causes the odd patterns in the fish fountain when the goldfish start humming for dinner.