The Pressure Acoustics, Transient (actd) interface (), found under the Acoustics>Pressure Acoustics branch () when adding a physics interface, is used to compute the pressure variation when modeling the propagation of acoustic waves in fluids at quiescent background conditions. It is suited for time-dependent simulations with arbitrary time-dependent fields and sources.

The physics interface can be used to model linear and nonlinear acoustics that can be well described by the scalar pressure variable. Domain conditions include the Nonlinear Acoustics (Westervelt) Contributions to include nonlinear effects, the Anisotropic Acoustics feature to model fluids with an effective anisotropic density, and the Background Pressure Field (for Transient Models) for defining a background acoustic field to model scattering problems or defining incident waves. User-defined sources can also be added via the Monopole Domain Source or the Dipole Domain Source. For open problems, Perfectly Matched Layers (PMLs) can be applied, also in the time domain for Pressure Acoustics, as efficient nonreflecting boundary conditions.

When this physics interface is added, these default nodes are also added to the Model Builder — Transient Pressure Acoustics Model, Sound Hard Boundary (Wall), and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and source. You can also right-click Pressure Acoustics, Transient to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is actd.

Enter a value or expression for the Typical wave speed for perfectly matched layers cref (SI unit m/s). The default is actd.c and the value is automatically taken from the material model. If several materials or material models are used it is best practice to add one PML for each. This will ensure that the typical wavelength is continuous within each PML feature.

It is important to set this speed correctly as the performance of the Perfectly Matched Layers (PMLs) in the time domain depends only on the wave speed and not the wavelength. For further details see the Time Domain Perfectly Matched Layers section.

Enter the Maximum frequency to resolve in the model. The value is used for setting up the solver as well as the physics-controlled mesh. The default frequency is set to 1000[Hz] but should be changed to reflect the frequency content of the sources used in the model. Select the Time stepping (method) as Fixed (preferred) the default and recommended or Free. The Free option is in general not recommended for wave problems. The generated solver will be adequate in most situations if the computational mesh also resolves the frequency content in the model. Note that any changes made to these settings (after the model is solved the first time) will only be reflected in the solver if Show Default Solver or Reset Solver to Defaults is selected in the study. It is also important to Reset Solver to Defaults if the Nonlinear Acoustics (Westervelt) Contributions feature is added as special handling of the nonlinear term is enabled. For highly nonlinear problems set up with user-defined terms, manual tuning of the solver may be necessary. In nonlinear models the maximum frequency to resolve should be selected based on the number of harmonics to be resolved.