The Thermoviscous Acoustics, Boundary Mode Interface interface adds extended functionality to the abovementioned mode analysis available in 2D and 1D axisymmetric geometries. The interface can be applied on boundaries in 3D (and 2D axisymmetric) geometries and can be used to compute the propagating modes and out-of-plane wave number kn on a (flat) surface of any orientation in 3D. This makes it possible to more easily set up boundary conditions at inlets of waveguides using the mode information.

The equations solved are the same as for The Thermoviscous Acoustics, Frequency Domain Interface but using modified expressions for the gradients. The implementation assumes propagation in the out-of-plane direction (that is, in the normal direction n), and the fields can then be written as

where xip is the in-plane coordinate, xop is the out-of-plane coordinate, and kn is the wave number that this formulation computes. Because of the above assumption about the acoustic fields, the gradient operator can be split into a tangential in-plane component (||) and a normal out-of-plane component (⊥), such that

The in-plane gradient is defined in COMSOL Multiphysics by the tangential derivative operator dtang(). The divergence of the velocity field is defined as the trace of the Jacobian, that is