Tuesday, 12 May 2015

Sound Propagation


In a compressible fluid medium, usually air, the sound propagates as a pressure change created by the sound source. A speaker, for example, uses this mechanism. Compression spreads, but the air particles oscillate only a few micrometers around a stable position, in the same way that when a stone is thrown into water, the waves travel away from the point fall, but the water remains in the same place, it only move vertically and not follow the waves (a cap placed on the water stays in the same position without moving). In fluids, the sound wave is longitudinal, that is to say that the particles vibrate parallel to the wave traveling direction.

Solid, vibrating, can transmit a sound. The vibration propagates as in fluids, with a low oscillation of the atoms around their equilibrium position, resulting in a stress of the material, equivalent to the pressure in a fluid, but more difficult to measure. The rigidity of the material enables the wave transmission of transverse stresses.

Similarly, although to a lesser extent, the viscosity of a fluid may vary, especially in extreme conditions, the propagation equations derived for an ideal gas.

Propagation or sound célérité1 speed depends on the nature, temperature and pressure of the medium. In a perfect gas the sound propagation velocity is given by the equation:

c = \ frac {1} {\ sqrt {\ rho \ chi_S}} where \ \ rho is the density of gas and \, \ chi_S its isentropic compressibility.

It is seen that the sound propagation speed decreases

when the density of the gas increases (inertia effect)
when its compressibility (the ability to change the volume under the effect of pressure) increases.

In water, the speed of sound is 1482 m / s. In other settings, the vibrations can spread even faster. Thus in the steel, the vibrations they propagate 5,600 m / s to 5900 m / s. The sound does not travel in a vacuum, for lack of material whose vibration could spread into sound waves (sound insulation).