An Absorbing Layer node ()adds an absorbing layer to a time-explicit simulation. Absorbing layers are often referred to as sponge layers. The absorbing layers work by combining three techniques: a scaling system, filtering, and simple nonreflecting conditions. The Absorbing Layer node adds a special scaled system, where the scaling effectively slows down the propagating waves. Filtering attenuates and filters out high frequency components of the wave. In the Convected Wave Equation, Time Explicit interface (in the Acoustics Module), you can specify filter parameters in the Filter Parameters in Absorbing Layers section of its Settings window. See also Filter Parameters for the Wave Form PDE.

To add an Absorbing Layer go to any Component, then in the Definitions toolbar, click Absorbing Layer, or right-click the Definitions node under the Component and choose Absorbing Layer. If the nodes under the Component node are grouped by type, you can instead right-click Artificial Domains under Definitions.

The Label is the default absorbing layer name.

The default Name (for the first absorbing layer in the model) is ab. The Name provides a namespace for variables created by the Absorbing Layer node. For example, the scaled x coordinate can typically be accessed in equations and postprocessing as ab1.x. See the Equation View subnode for a complete list of available variables.

Select a set of domains conforming to the selected geometry type. See Standard Geometry Configurations.

Select a Type: Cartesian (the default), Spherical, Cylindrical, or User defined. If User defined is selected, first choose the Number of stretching directions appropriate for the geometrical configuration. Then for each stretching direction specify a Distance function, evaluating to the distance from the inner boundary of the absorbing layer measured in the stretching direction, and the Thickness of the domain in the same direction.

Enter expressions for the Physical width (SI unit: m) and the Pole distance (SI unit: m). The default values are 2.0*dGeomChar and 0.25*dGeomChar, respectively, 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 plot the coordinate variables x_absorb_ab1, y_absorb_ab1, and z_absorb_ab1 (for an Absorbing Layer node ab1). Using the absorbing layers with the three combined techniques will enable the reduction of spurious reflections by a factor between 100 and 1000 compared to the incident amplitude.