The finite-difference model is decomposed into a set of discrete cells, indicated by the solid lines, each with uniform static sound speed c and ambient density ρ_o. The acoustic pressure and density perturbations are defined at the nodes at the centre of each cell, indicated by the filled circles. The horizontal particle velocity v_x (triangles pointing right) and vertical particle velocity variables v_z (triangles pointing up) nodes straddle the cell boundaries of the spatially staggered grid, shown in two dimensions.