# General equation for the speed of sound?

 P: 23 I've seen stated in many a physics book that the general case for the speed of sound (for general equations of state p(ρ) ) is given by $$c^2 = \frac{\partial p}{\partial \rho}$$ where p is pressure and ρ is density. but I can't for the life of me figure out how on earth to derive that. I've seen tons of derivations for specific cases--gasses, solids, but not for the general case. According to wikipedia, it can be derived using classical mechanics. Can someone point me in the right direction?
 P: 8 c = $\sqrt{P/\rho}$ Where P = coefficient of "stiffness" and $\rho$ = density
 P: 215 General equation for the speed of sound? the equation of state is p=p(ρ,s) thus dp = ($\frac{∂p}{∂\rho}$$)_{s}$d$\rho$ + ($\frac{∂p}{∂s}$)$_{\rho}$ ds I am guessing that because ($\frac{∂p}{∂\rho}$$)_{s}$ has units of "velocity squared", it is looked upon as such; But why this velocity is the sonic one - beats me... Anyone?