From: The World In Your Head by Steven Lehar

Fig. 8.1 Chladni figures for a square steel plate (adapted from Waller, 1961) demonstrate the fantastic variety of standing-wave patterns that can arise from a simple resonating system. A square steel plate is clamped at its midpoint and sprinkled with sand. It is then set into vibration either by bowing with a violin bow, or by pressing dry ice against it. The resultant standing-wave patterns are revealed by the sand, which collects at the nodes of the oscillation where the vibration is minimal.