Use the Impedance node to specify a (normal) specific impedance Zn on a boundary. Add this node from the Mechanical submenu. This feature is useful outside the viscous boundary layer, as this condition mimics the behavior of a corresponding Pressure Acoustics Model with an impedance condition. The boundary condition reads:

Enter a value or expression for the Specific impedance Zn (SI unit: Pa⋅s/m).

Select a condition for the Tangential velocity — Slip (the default) or No slip. The No Slip option will apply an additional constraint for the velocity in the tangential direction on the boundary.