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Chladni Plates

Historical Context

The German scientist Ernst Chladni was one of the pioneers of experimental acoustics. His research on different kinds of vibrations served as the basis for the scientific understanding of sound that later emerged in the 19th century.

One of Chladni’s inventions was a technique to study the motions of vibrating plates. Starting with a metal plate whose surface had been lightly sprinkled with sand, he found that bowing it produced characteristic patterns that could be related to the physical dimensions of the plate. “Chladni’s Plates”, as they came to be called, provided an early way to visualize the effects of vibrations on mechanical surfaces. Chladni was even able to produce a formula that successfully predicted the patterns found on vibrating circular plates. The success of Chladni’s research, combined with the popularity of his many public demonstrations, did much to improve the standing of acoustics during his lifetime, and inspired many of the acoustic researchers who later extended his work.

Once Chladni’s patterns began to be understood, it was found that they could also be used analytically, to provide information about the conditions that formed them. For example, violin makers have long used Chladni figures to provide feedback as they shape the critical front and back plates of the instrument’s resonance box. Fine metal filings are sprinkled on the wooden plates, which are then vibrated (at as many as seven different frequencies) to produce a series of patterns. Much of the final shaping of the plates is directed towards ensuring that the patterns on both of them match and are symmetrical. This symmetry is what allows the resonator to move as a single mass and to produces the richest, most beautiful tones.

Chladni patterns are still of scientific interest, although their analytical uses have been mostly replaced by other technologies. Today these figures are more likely to be produced by a virtual imaging program than by an actual vibrating plate.

Supporting Artifacts

Chladni Plate

(PH*2010.3095.097)

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Demonstration (41 sec.)

Chladni Plate

This instrument was used in an Ohio high school and probably dates from the late 19th or early 20th centuries. The replication of classic experiments was a common way to teach science at this time, and Chladni’s figures were considered to be both instructive and beautiful.

To “play” this instrument, first sprinkle a thin layer of sand (or salt) on it and then activate the plate by bowing the middle of one edge with a well-rosined bow. A pattern will quickly form because, as the plate vibrates, the sand on the moving areas bounces off and accumulates on the places that aren’t moving (the “nodes”). With a little practice it becomes easy to produce the plate’s “fundamental” tone and this forms the sand into a large “X” that covers the entire plate. Bowing harder and faster on the same place produces a much higher tone and creates a more detailed pattern that has more nodes and smaller open spaces.

“Communication of Vibrations”

These instruments demonstrate the “communication of vibrations” between connected plates. This was a topic first investigated by the French scientist Felix Savart, in the 1820s. Savart experimented with a pair of identical glass disks that were connected by only a single glass rod. When the two disks were sprinkled with sand and the first one vibrated, both disks formed identical patterns.

“Communication of Vibrations”

(PH*314,581.1 & .2)

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“Vibration of Plates”

(PH*314,581.3)

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“Vibration of Plates”

This rack has 3 square and 3 circular brass plates of varying dimensions. It was used to demonstrate the effect of changes in the size and thickness of plates on both their tone and the Chladni figures that they produce. A plate that is the same size as the one next to it, but double the thickness, will produce a note twice as high, while a plate that is half the area of the one next to it, but double the thickness, will sound a note that is four times higher.

“Forked Tube”

This kind of fork-shaped tube was used to study the motions of different parts of a vibrating Chladni plate. If the ends of the tube were held over two zones that were vibrating in the same direction, a sound would be heard, but if the vibrations were in opposite directions they would cancel each other out (interfere) and the sound would be greatly reduced.

“Forked Tube”

(PH*326,944)

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