Today is the birthday of Ernst Chladni, a German physicist born in 1756 who helped found the science of acoustics. Chladni, who was a lawyer and music lover as well as a scientist, began studying sound waves in 1786 and worked out mathematical formulas to describe how they travel. His most famous experiment demonstrated that when a thin plate of metal covered with fine sand vibrates, the jiggling sand grains collect along so-called nodal lines--regions where the waves do not bend the plate. These patterns are now called Chladni's figures. He also measured the changes in pitch that occur when an organ pipe is filled with gases other than air, thus showing that the frequency of a sound wave varies according to the molecular composition of the gas through which it passes.
[Source: Roy Porter, Ed., The Biographical Dictionary of Scientists (Oxford University Press, ed. 2, 1994).]