In chapter Wave Equation and Green's function, we mentioned that the motion of small amplitude acoustic waves in fluids is governed by the wave equation. In this chapter we intend to formally prove this statement.
Linear Acoustic Waves in an Ideal Fluid at Rest
Let's start from the governing equations of fluids motion, namely conservation of mass (continuity) and momentum (Navier-Stokes).
The continuity equation for a compressible fluid is
Navier-Stokes equation is also given by
where is the Kronecker delta and is the deviatoric stress tensor. For the time being let's assume that we are dealing with wave propagation of an ideal (inviscid) fluid at rest. Later, we will extend our analysis to consider the moving, viscous fluid. Further assume that the motion of acoustic waves cause small amplitude fluctuations such that the instantaneous density, pressure, and velocity components at any point can be written as
where and are the mean pressure and density, respectively, which are independent of time and position. It should be noted that mean velocity has been set to zero, because we have assumed that the fluid is at rest. Substituting these quantities in the conservation of mass and momentum, and neglecting second and higher order fluctuation terms, we obtain
Now let's take the space derivative of the linearized momentum equation and subtract it from the time derivative of the linearized conitinuity equation to obtain
We can obtain the pressure fluctuation from a Taylor expansion
Since we assumed that the fluid is inviscid and fluctuations are small in amplitude, it is safe to assume that the motion of acoustic waves does not generate entropy and is an isentropic process[]. Thus,
which is our celebrated wave equation. Exactly similar equations can be obtained for pressure and velocity fluctuations as well. It is interesting to note that the acoustic perturbations propagate at speed
which is the famous speed of sound .
Velocity Potential and Wave Equation
In the previous section we obtained a wave equation in terms of denisty fluctuation. An alternate formulation can be obtained in terms velocity potential.
Taking the curl of linearized momentum (Euler) equation, and noting that the curl of a gradient is zero, we obtain
which means vorticity  is constant in time. If we consider the initial vorticity to be zero, the velocity vector can be written as the gradient of a potential function at any moment of time
The linearized continuity and momentum equation can be used then to obatin