The force in Equation 6 acts as a source function upon the domain . Any force acting within the domain causes pressure and density changes, and the fluid nature of the medium will create an equilibrium restoring force. These changes in density and pressure are small.

We consider small perturbations
in the density , the particle velocity
, and the pressure **P** from the initial rest conditions
which are labeled with
subscript 0.
The perturbation Equations 7, 8,
and 9 are for
, , and , the particle velocity, density,
and pressure respectively.

The initial particle velocity is because the domain is at rest.