What is Sound?

Vibration Creates Form

Ernst Chladni (1756–1827) was a German physicist and musician.

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". Chladni's technique, first published in 1787 his book, Entdeckungen über die Theorie des Klanges ("Discoveries in the Theory of Sound"), consists of drawing a bow over a piece of metal whose surface is lightly covered with sand. The plate is bowed until it reaches resonance and the sand forms a pattern showing the nodal regions.

The following is a drawing from Chladni's original publication.

He illustrated it with diagrams of the vibrations of thin metal plates.














Chladni Figures and Vibrating Plates
An applet that demonstrates the mode patterns of vibrating circular and rectangular plates :
http://www.phy.davidson.edu/StuHome/jimn/Java/modes.html

Sound = Electricity

A Sound Way To Turn Heat Into Electricity

ScienceDaily (Jun. 4, 2007) — University of Utah physicists developed small devices that turn heat into sound and then into electricity. The technology holds promise for changing waste heat into electricity, harnessing solar energy and cooling computers and radars. Five of Symko's doctoral students recently devised methods to improve the efficiency of acoustic heat-engine devices to turn heat into electricity.

"You have heat, which is so disorderly and chaotic, and all of a sudden you have sound coming out at one frequency," Symko says.

University of Utah physicist Orest Symko demonstrates how heat can be converted into sound by using a blowtorch to heat a metallic screen inside a plastic tube, which then produces a loud tone, similar to when air is blown into a flute.

Source: http://www.sciencedaily.com/releases/2007/06/070603225026.htm