Movie File Size - 2.1 MB Movie Duration - 39.29 Seconds File - MOVIES/Chladni/ChladniSquSweep.mov (Click here for larger version of movie)

Discussion

When waves with the same frequency, but traveling in opposite directions, pass each other they can interfere to create standing waves, so-called because the resulting wave does not appear to move. Ideally, a standing wave oscillates in a stationary envelope, with nodes, points at which wave displacement is zero, and antinodes, points where wave displacement is maximal.

A Chladni plate (named after Ernst Chladni, 1756-1827, a German physicist known as the father of acoustics) is simply a thin sheet or plate of wood, metal, or other stiff material that can be made to vibrate in one way or another. In these movies, square and round plates of metal are mounted on a wave driver. To begin an experiment, sand is sprinkled on the plate. When the frequency of the wave driver matches a resonant frequency of the metal plate, three things happen: the sound level rises dramatically from that at off-resonant frequencies (energy transfer to the plate is much greater at resonant frequencies), two dimensional standing waves are established, and sand particles dance away from regions where wave displacements are large to collect in nodal regions, where there is little or no movement of the plate. Sand-covered nodal curves enclose bare antinodal regions of the plate in patterns that become more complex and intricate as the driving frequency increases. Because a sheet of metal lacks the uniformity of, for example, a string or pool of water of constant depth, there is no simple relationship between driving frequency and the number of nodal regions, such as those we found for waves on strings or in ripple tanks, other than the increasing complexity observed with increasing frequency.

In this movie, the driving frequency is increased from 932 Hz to 1459 Hz, and passes through three resonances (955 Hz, 1055 Hz and 1377 Hz) of the square plate. As a resonant frequency is reached, the sound level increases and a new nodal pattern forms.