The vibrations of an idealized circular drum head—essentially an elastic membrane of uniform thickness attached to a rigid circular frame—are solutions of the wave equation with zero boundary conditions.
There exist infinitely many ways in which a drum head can vibrate, depending on the shape of the drum head at some initial time and the rate of change of the shape of the drum head at the initial time. Using separation of variables, it is possible to find a collection of "simple" vibration modes, and it can be proved that any arbitrarily complex vibration of a drum head can be decomposed as a series of the simpler vibrations (analogous to the Fourier series).