The Sound Hard Boundary (Wall) adds a boundary condition for a sound hard boundary or wall, which is a boundary at which the normal component of the velocity is zero (a slip condition)

Note that this condition is not identical to the Symmetry condition. The condition is also available from the Pairs submenu, as an option at interfaces between parts in an assembly. This effectively introduces a condition similar to the Interior Sound Hard Boundary (Wall) but used on an assembly.