K s = ρ ρ ( ∂ ∂ P ∂ ∂ ρ ρ ) s , where P is the pressure and the derivative is taken isentropically, that is, at constant entropy s. This is because a sound wave travels so fast that its propagation can be approximated as an adiabatic process, meaning that there isn't enough time, during a pressure cycle of the sound, for significant heat conduction and radiation to occur.