A boundary impedance model is implemented to specifically handle cases investigating the acoustical properties of a porous layer of a given thickness d backed by a sound-hard wall. This can be applied instead of explicitly modeling the porous layer, as long as the incident acoustic field is normal to the boundary; as for all the other boundary impedance models, the tangential components of the acoustic field are ignored by this model.

For a porous layer with a user-specified thickness d, the impedance from the porous layer backed by a sound-hard wall is given by (see Ref. 9)

Here ρc and cc are the equivalent fluid descriptions of the porous model. This impedance model applies to any type of porous model which can be written as an equivalent fluid model. All porous models implemented in COMSOL are available for this impedance boundary condition. See details of the poroacoustics equivalent fluid models in the section Theory for the Equivalent Fluid Models.