for the acoustic pressure, p = p(x, t). Here c is the speed of sound and ρ denotes the equilibrium density, while qd and Qm are dipole and monopole sources, respectively. The density and speed of sound can both be non-constant in space. In contrast, they are assumed to vary with time  on scales much larger than the period for the acoustic waves and are therefore considered time independent in the previous equation. This physics interface is available for 3D, 2D, and 1D Cartesian geometries as well as for 2D and 1D axisymmetric geometries.