School Launch

Sound and Waves Explorer

Sound is a wave: air pushed and pulled back and forth. Make a metal plate ring and watch sand draw its pattern, then race a sound source past a listener, all the way through the sound barrier.

Explanations for:

Make the plate ring

Slide slowly: only some frequencies make a clear pattern.

Notes:
What is happening?

When the plate rings, most of it shakes up and down very fast, too fast to see. The shaking parts bounce the sand away. But along some lines the plate stays still. The sand keeps bouncing until it lands on one of those still lines, and there it stays. The lines it draws are called nodal lines.

Each pitch makes its own pattern. A higher note makes more lines, closer together. Between the notes that fit the plate, the plate barely moves and the sand just sits.

Nobody draws these patterns. They come from the plate's size, shape and stiffness and the pitch it is played at. It is a wonderful thing that so much order is hidden in a simple ringing plate.

A little history

The German musician and scientist Ernst Chladni scattered sand on metal plates and stroked their edges with a violin bow. In 1787 he published the patterns in a book on the theory of sound, and they became famous as “Chladni figures.”

Chladni later toured Europe with his plates. After he showed them in Paris, Napoleon was so impressed that he paid for a prize of 3,000 francs, which the scientific academy in Paris (then the First Class of the Institut de France) announced in 1809 for a mathematical theory of vibrating plates. The problem was hard. Only one person sent in an answer: Sophie Germain, who had taught herself mathematics. Her first try was turned down and her second earned an honorable mention, but she kept working, and in January 1816 her third paper won the prize. She was the first woman to win a prize from the Paris academy, and her work helped begin the science of elasticity used today for bridges and buildings.