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 Zn. The condition specifies the normal velocity

This condition can be used to model the properties of artificial boundaries by adding constant resistive losses (no reactive components can be added). When the impedance is set equal to the characteristic specific impedance of a propagating plane wave ρc, the condition represents the simplest nonreflecting boundary condition. This is also the default value of the impedance when the condition is added.

The acoustic impedance condition (with the default value ρc) should be used at the outer boundary when Absorbing Layers are used in a model.

Enter a value or expression for the Specific impedance Zn (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.