The Impedance condition adds a boundary condition defining the relation between the local acoustic pressure p and the normal acoustic velocity n·ut, that is, the specific acoustic impedance Zi. The model includes the option to define the nonlinear or second order pressure-velocity relation to extend the validity of the impedance condition to higher pressure amplitudes.

This condition can be used to model the properties of artificial boundaries by adding 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 represent 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.

Select the Pressure-particle velocity relation used for the impedance formulation First order (default) or Second order. Enter a value or expression for the Impedance Zi (SI unit: Pa·s/m). The default expression is nate.c*nate.rho which is the characteristic specific impedance ρc of a propagating plane wave (linear theory).