The Acoustic Impedance node adds a boundary condition defining a relation between the local acoustic pressure p and the normal acoustic velocity n·u, that is, the acoustic impedance Z. The condition specifies the inward normal velocity

This condition can be used to model the properties of artificial boundaries. When the impedance is set equal to the characteristic specific impedance of a propagating plane wave ρ0c0, the condition represent the simplest nonreflecting boundary condition. This is also the default value of the impedance when the impedance condition is added.

Note that this condition is not equivalent to the general impedance condition, as given by Myers (see Ref. 4), when a flow is present. This means that, for example, it cannot be used at a reacting surface which has a tangential flow. For this kind of detailed conditions a frequency domain representation is needed.

The acoustic impedance condition (with the default value ρ0c0) should be used at the outer boundary when Absorbing Layers are used in a model.

Enter a value or expression for the Acoustic Impedance Z (SI unit: Pa·s/m). The default expression is cwe.c0*cwe.rho0 which is the characteristic specific impedance ρ0c0 of a propagating plane wave.