Solutions to acoustic problems are wavelike. The waves are characterized by a wavelength λ in space, whose value depends on the frequency and speed of sound c in the medium according to λ = c/f. This wavelength has to be resolved by the mesh.

To represent a wave on a discrete grid (the mesh), it is obvious that the mesh elements must be smaller than the wavelength in order to resolve the wave. That is, there needs to be several degrees of freedom (DOFs) per wavelength in the direction of propagation. In reality, the lower limit for a fully reliable solution lies at about ten to twelve degrees of freedom per wavelength.

Before starting a new model, try to estimate the required number of DOFs using these guidelines. The maximum number of DOFs that can be solved for differs between computer systems. See Solving Large Acoustics Problems Using Iterative Solvers for solver suggestions.

When creating an unstructured mesh for use with the default 2nd-order Lagrange elements or 2nd-order serendipity elements (see Lagrange and Serendipity Shape Functions), set the maximum element size hmax to about  λ/5 or smaller. Because all elements in the constructed mesh are smaller than hmax, the limit is set larger than the actual required element size. The mesh should also resolve important geometric features and possible gradients in the material parameters and model inputs. After meshing the model, check the total number of DOFs against the model volume and the above guidelines. If the mesh turns out, on average, to be too coarse or too fine, try to change hmax accordingly.

When using a perfectly matched layer (PML) to truncate the computational domain it is good practice to use a structured mesh inside the PML region. In 3D models use a Swept mesh inside the PML and in 2D models use a Mapped mesh. Use at least 5 elements in the thickness when using rational PML scaling and 8 elements when using the default polynomial scaling in the PML. Again make sure the check for mesh convergence by adding more layers.

When creating the geometry for your model use the Layers option to create the geometry of your PML layer/domain. This will ensure that it is suited for proper meshing using a structured mesh.