Speed of sound

Summary

The speed of sound is the speed at which small pressure disturbances travel through a fluid at rest. It is fixed by how much the pressure rises when the fluid is compressed without exchange of heat, through $c^2 = (\partial p / \partial \rho)_s$. It sets the scale for all effects of compressibility, and the ratio of the flow speed to it is the Mach number. For an ideal gas it depends only on the temperature, $c = \sqrt{\gamma R T}$.