Watch as Chladni patterns form on a metal plate, a teaching demonstration “spiced up” with colored sand by associate professor of physics at UNC Charlotte skullsinthestars. The video is accompanied by Prelude from Suite No. 3 by Johann Sebastian Bach. (If you want to hear the tones that accompany the demo, watch this video from St. Mary’s University in Halifax.)
The technique was first discovered by Robert Hooke, who ran a violin-style bow along the edge of a plate with flour on it. Ernst Chladni took it one further by experimenting with and recording the nodal patterns, now called Chladni figures. From Skulls in the Stars in 2013:
A metal plate, supported by a post in its center, is vibrated at a single frequency by use of a mechanical driver. For most frequencies, nothing at all happens; when certain special frequencies are hit, however, standing waves appear on the plate, driving the sand away from the points of large vibration to the points of no vibration. By varying the frequency of oscillation, we can find a large number of the so-called resonance frequencies and their accompanying patterns, which become increasingly complex and beautiful as we up the rate of oscillation.
Next, watch Resonance, forced vibration, and a tuning forks demo, Mercury Hz: Sound waves passing around & through mercury, and Making sounds visible: Sound vibrations transform colorful sand patterns.
Four centrally mounted brass plates are driven into various resonant states with a violin bow. Sand sprinkled on the plates helps to visualize the nodes and antinodes for each state. Pressing a fingernail against the edge of the plate imposes a boundary condition (as does the location of the bow, for certain plates).
h/t Boing Boing.