Chladni FiguresSprinkle sand on a metal plate and play a loud tone through it. The plate flexes, and the sand skips away from the shaking parts and settles along the still lines — drawing the shape of that particular note. Change the pitch and you get a whole new figure.Robert Hooke rubbed a glass plate with a bow in 1680; Ernst Chladni turned it into a touring act in 1787 and mapped hundreds of patterns. Sophie Germain won a Paris Academy prize in 1816 for the equation behind them — a fourth-order elastic wave — and the still lines are simply where two standing waves, cos(nπx)cos(mπy) and its mirror, cancel.