The three spatial coordinates are for the domain .
Particle velocities are for each direction .
The body forces are negligible, . Notice that the repeated index used in the equations indicate a tensor summation. 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. We consider small perturbations in the density , the particle velocity , and the pressure from the initial rest conditions which are labeled with subscript 0.