Among the most famous experiments which contributes to make the sounds visible, the Chladni’s plate technique is the easiest way to proceed. Ernst Florens Friedrich Chladni (1756–1827) was German physicist and musician. Following Wikipedia, his important works include research on vibrating plates and the calculation of the speed of sound for different gases. For this some call him the “father of acoustics”. He also did pioneering work in the study of meteorites, and therefore is regarded by some as the “father of meteoritics” as well. One of Chladni’s best-known achievements was inventing a technique to show the various modes of vibration of a rigid surface. A plate or membrane vibrating at resonance is divided into regions vibrating in opposite directions, bounded by lines of zero vibration called nodal lines. Chladni repeated the pioneering experiments of Robert Hooke of Oxford University who, on July 8, 1680, had observed the nodal patterns associated with the vibrations of glass plates. Hooke ran a violin bow along the edge of a plate covered with flour, and saw the nodal patterns emerge.
Chladni’s technique, first published in 1787 in his book, Entdeckungen über die Theorie des Klanges (“Discoveries in the Theory of Sound”), consisted of drawing a bow over a piece of metal whose surface was lightly covered with sand. The plate was bowed until it reached resonance, when the vibration causes the sand to move and concentrate along the nodal lines where the surface is still, outlining the nodal lines. These patterns are now called Chladni figures.
Variations of this technique are still commonly used in the design and construction of acoustic instruments such as violins, guitar and cellos.
Since the 20th century it has become more common to place a loudspeaker driven by an electronic signal generator over or under the plate to achieve a more accurate adjustable frequency.
- Resonance Experiment: Fun With Chladni Patterns (sparkonit.com)