Interior Impedance/Pair Impedance
The Interior Impedance and Pair Impedance nodes add an impedance boundary condition on interior boundaries or boundaries between the parts of pairs. Add the Interior Impedance node from the Interior Conditions submenu. This condition is a generalization of the sound-hard and sound-soft boundary conditions. The condition corresponds to a transfer impedance condition, relating the pressure drop across the boundary pt,down − pt,up to the velocity at the boundary. In the frequency domain, it imposes the following equations:
Zn is the specific transfer impedance, which from a physical point, is the ratio between pressure drop and local normal particle velocity. On pair (interior) impedance conditions, the up and down, which refers to the up and down sides of an interior boundary, is replaced by a dst and src subscript in the equation display. These refer to the destination and source faces in the pair. In the pair feature, the normal on the left-hand side in the equation refers to the destination normal direction ndst.
Additional information is found in Identity, Contact, and Sector Symmetry Pairs in the COMSOL Multiphysics Reference Manual.
Interior Impedance/Pair Impedance
Select the (interior) Impedance model as User defined (the default) or Perforated plate.
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For the User defined option, enter the value of the Specific transfer impedance Zn (SI unit: Pa·s/m). The default is 0 Pa·s/m.
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For the Perforated plate option, see the options and details for the Interior Perforated Plate/Pair Perforated Plate feature. The two ways of setting up an interior perforated plate are identical.
When coupling a FEM and BEM based formulation with the Acoustic FEM–BEM Boundary, the Impedance (subfeature) can be added to the multiphysics coupling to model an interior impedance.