The Sound of a DrumA drumhead can only ring in certain shapes. Each has still lines — rings and diameters that never move (the “nodes”) — and its own pitch. Sprinkle sand on a real drum and it collects along exactly these lines. Drag to orbit the surface; pick a mode with the sliders.The wave equation on a disc separates into an angle part (m nodal diameters) and a radius part, a Bessel function Jₘ. The membrane can vibrate only where Jₘ hits zero at the rim, so the frequencies are fixed by Bessel’s zeros — not evenly spaced, which is why a drum sounds un-pitched.