The Frequency-Transient study is a dedicated study step to use when the Acoustic Streaming Boundary Coupling or Acoustic Streaming Domain Coupling is used to couple an acoustic frequency domain interface to a time-dependent fluid flow interface. The study uses a tailored solver that solves for the acoustic fields and time dependent fields in the same study step. The solver is only valid if the timescale of the acoustics (determined by the frequency) is a lot shorter than the time-scale of the time-dependent fluid flow. The difference in time-scales is used to assume that the time-dependent flow variables are stationary (quasi stationary) in the acoustic equations.

In the Frequency-Transient study the equations related to the acoustic fields are evaluated at every time-step solved for. This makes it possible to vary the acoustic field in time (for example effects due to heating and changes in material properties), as long as the variations occur on a time-scale much longer than the timescale set by the acoustic frequency.

In the Physics and Variables Selection under the study step the equation form of the acoustic interface is designated a Automatic (Frequency Domain), the stationary interfaces is set to Automatic (Time dependent), and the two acoustic streaming multiphysics couplings are set to Automatic (Frequency-transient).