The Interior Sound Hard Boundary (Wall) node adds a boundary condition for a sound hard boundary or wall on interior boundaries. Add this node from the Interior Conditions submenu. A sound-hard boundary is a boundary at which the normal component of the acceleration is zero:

This is applied to the two sides of the boundary (the up and down sides). For zero dipole source qd and constant fluid density ρc, this means that the normal derivative of the pressure is zero at the boundary. On an interior sound hard boundary, the pressure is not continuous but is treated as a so-called slit feature.