ABSTRACT
The physical phenomenon known as 'sound' in a fluid (i.e. a gas or a liquid) essentially involves time-varying disturbances of the density of the medium from its equilibrium value: these changes ofdensity are in most cases extremely small compared with the equilibrium density (typically of the order of 10-7_10 - 5). They may be attributed to changes in the volume of space occupied by a given mass of fluid, changes of shape not being of consequence in the case of sound waves: it is the volumetric strain, or dilatation, undergone by an elemental mass of fluid which matters. The static elastic nature of air in its response to volumetric strain is easily demonstrated by closing the outlet hole in a bicycle pump with a finger and depressing, and then releasing, the plunger; the plunger returns almost to its original position on release. It is not so easy to find an everyday phenomenon to demonstrate that liquids, such as water, are also elastic; this is because, in response to a given volumetric strain, they generate much larger internal stresses, and hence, reaction forces. Surprisingly, it took little longer to establish the 'compressibility', and hence sound speed, of water than that of air. The Swiss physicist Daniel Colladon measured its value in 1826 in response to the offer of a prize by the Paris Academy of Science, and the result was validated by a measurement of the speed of sound in Lake Geneva by Colladon and his colleague, the mathematician Charles Sturm, in the same year. The 'correct' speed of sound in air was evaluated theoretically by Pierre Simon Laplace in 1816, and the ratio of specific heats was accurately determined in 1819, a century or more after Isaac Newton and Leonhard Euler, among others, had presented incorrect theoretical predictions of the sound speed by assuming an isothermal process.
