Sprinkling sand on a vibrating plate drew a picture of sound itself

In 1787, Ernst Chladni bowed metal plates sprinkled with sand and watched the grains bounce off the shaking regions to settle along still 'nodal' lines, revealing the invisible standing-wave pattern of the vibration. Different pitches and bowing points produced completely different geometric patterns on the same plate. The technique still visualizes vibration modes today, from violin bodies to spacecraft panels.

— Ernst Chladni, Discoveries in the Theory of Sound (Entdeckungen über die Theorie des Klanges) — 1787