In most cases, the acoustic wave pattern that is to be simulated is not contained in a closed cavity. That is, there are boundaries in the model that do not represent a physical wall or limit of any kind. Instead, the boundary condition has to represent the interaction between the wave pattern inside the model and everything outside. Conditions of this kind are generically referred to as artificial boundary conditions (ABCs).

One particular case that occurs frequently in acoustics concerns boundaries that can be assumed to let wave energy propagate out from the domain without reflections. This leads to the introduction of a particular group of artificial boundary conditions known as nonreflecting boundary conditions (NRBCs), of which three kinds are available in this module: port conditions, matched boundary conditions, and radiation boundary conditions.

Another way to model an open nonreflecting boundary is to add a so-called perfectly matched layer (PML) domain or an absorbing layer domain. These domains use two different techniques to dampens all outgoing waves with no or minimal reflections. See, for example, Perfectly Matched Layers (PMLs) for more information. In pressure acoustics a special version of the PML exists as a boundary condition, the Perfectly Matched Boundary.