The Convected Wave Equation, Time Explicit Interface includes so-called absorbing layers, also often referred to as sponge layers. The layers work by combining three techniques: a scaling system, filtering, and simple nonreflecting conditions. For a review of the method see, for example, Ref. 5.

The layers are set up by adding the Absorbing Layer under the Definitions node. This adds a special scaled system. The scaling effectively slows down the propagating waves and ensures that they “align up” (normal) toward the outer boundary. This means that the waves will hit the outer boundary in a closer to normal direction. Filtering attenuates and filters out high frequency components of the wave. The filter parameters are controlled under the Filter Parameters for Absorbing Layers section at the main physics level. Finally, at the outer boundary of the layer add a simple Acoustic Impedance condition, which will work well to remove all remaining waves, as normal incidence has been ensured.

The physical thickness of the layer should be such that it contains 2 to 3 mesh elements. This in practice means that it should roughly have a thickness equal to the largest wavelength resolved in the model. This is due to the meshing requirements described in the Meshing, Discretization, and Solvers section. You want to avoid to create small mesh elements in the absorbing layers. These can potentially slow down the solution procedure since the internal time stepping is controlled by the mesh size.

For the Absorbing Layers select the Type (Cartesian, cylindrical, spherical, or user defined) under the Geometry section. Enter values for the Physical Width and Pole Distance under the Scaling section.

For the layers to work optimally the filter should not be too aggressive. Moreover, the scaled coordinates in the layer domain should also vary smoothly. To inspect the scaled system you can, for example, plot the coordinate variables x_absorb_ab1, y_absorb_ab1, and z_absorb_ab1. In 2D axisymmetric they are r_absorb_ab1 and z_absorb_ab1. Note that the variables are not in the plot menu under the Replace Expression.