The Pressure Acoustics, Boundary Mode Interface,
The Thermoviscous Acoustics, Boundary Mode Interface, and
The Linearized Potential Flow, Boundary Mode Interface are special interfaces for more advanced Mode Analysis studies on boundaries in 3D and 2D axisymmetry. Acoustic waves can propagate over large distances in ducts and pipes, with a generic name referred to as
waveguides. After some distance of propagation in a waveguide of uniform cross section, such guided waves can be described as a sum of just a few discrete
propagating modes, each with its own shape and phase speed. The equation governing these modes can be obtained as a spatial Fourier transform of the time-harmonic equation in the waveguide axial direction (here assuming propagation in the
z direction) or by inserting the assumption that the mode is harmonic in space,
and eliminating all out-of-plane z dependence. Here
p(
x,
y) is the in-plane mode shape.
Similar to the full time-harmonic equation, the transformed equation can be solved at a given frequency with a nonzero excitation for most axial wave numbers kz. But at certain discrete values the equation breaks down. These values are the propagation constants or wave numbers of the propagating or evanescent waveguide modes. The eigenvalue solver can solve for these propagation constants together with the corresponding mode shapes.
The most common use for the Mode Analysis is to define sources for a subsequent time-harmonic simulation. If there is a component with one or more waveguide connections, its behavior can be described by simulating its response to the discrete set of propagating modes on the waveguide port cross sections. The Port boundary condition in Pressure Acoustics uses this concept to define sources and absorb outgoing modes. In thermoviscous acoustics a Mode Analysis study also provides information about the absorption coefficient for the propagating modes. The complex wave number (solved for) and characteristic impedance (defined in postprocessing) define the homogenized propagation variables that can be sued in
Narrow Region Acoustics.
) is available with The Pressure Acoustics, Frequency Domain Interface, The Linearized Potential Flow, Frequency Domain Interface, and The Thermoviscous Acoustics, Frequency Domain Interface in plane 2D and axially symmetric 1D acoustics interfaces. 
