Chladni and his plates

In the 1780s a young German scientist stood in front of audiences with a violin bow and a sheet of metal covered in sand. He drew the bow along the edge, a clear tone rang out, and the sand jumped into sharp geometric figures. People had never seen sound before. His name was Ernst Chladni.

From law to sound

Ernst Florens Friedrich Chladni was born in 1756 in Wittenberg, Germany. His father wanted him to become a lawyer, and he dutifully earned a law degree. After his father died, Chladni turned to what he really loved: music and physics. He played the piano and was curious about why instruments sound the way they do.

He had read about Robert Hooke's experiment with flour on glass. Chladni repeated it carefully and systematically, with plates of different shapes and sizes, and recorded every pattern he found.

The experiment

The setup was simple. A thin plate of brass or glass is clamped at its center. Fine sand is sprinkled over it. A violin bow is drawn along the edge, and the plate starts to sing. Where the plate vibrates, the sand is thrown away; where it stays still, the sand gathers. Touching the edge with a finger at different points, or bowing at different places, makes the plate ring at different notes, and each note draws its own figure.

In 1787 he published his results in Discoveries in the Theory of Sound, with page after page of drawn patterns. These figures are still called Chladni figures today.

Chladni did not just see the patterns. He catalogued them, so others could predict them.

A show for Napoleon

Chladni toured Europe giving demonstrations, and his fame grew. In 1809 he showed his plates to Napoleon in Paris. Napoleon was so impressed that he funded a prize from the French Academy of Sciences for anyone who could explain the patterns with mathematics.

The problem was hard. The prize was finally won in 1816 by the mathematician Sophie Germain, one of the very few women working in mathematics at the time. Her work became an early step toward the theory of how plates and surfaces bend, which engineers still rely on.

More than plates

Chladni wrote Die Akustik (1802), one of the first complete books on acoustics, and is often called the father of acoustics. He invented musical instruments, including the euphon, played by rubbing glass rods with wet fingers. He also argued, years before most scientists agreed, that meteorites fall from space and are not formed in the atmosphere. He was right.

His plates, today

More than two hundred years later, his idea is still in use. Instrument makers test the wooden plates of violins and guitars with Chladni patterns, and every physics museum seems to have a plate and a bow. The SeeSound Cymatics generator is a digital version of his experiment: the same sand, the same plate, driven by any sound you choose.