The Impedance condition adds a boundary condition defining the relation between the local total acoustic pressure
pt and the normal acoustic velocity
n·ut, that is, the specific acoustic impedance
Zi. The condition specifies the normal velocity
Enter a value or expression for the Impedance Zi (SI unit: Pa·s/m). The default expression is
pate.c*pate.rho which is the characteristic specific impedance
ρc of a propagating plane wave.