The Impedance node adds an impedance boundary condition with the option to select between several built-in impedance models that allow modeling frequency dependent impedance conditions in the time domain. The impedance condition is a generalization of the sound-hard and sound-soft boundary conditions, and in the time domain reads:

Here, Zn is the (normal) specific input impedance of the external domain and it has the SI unit Pa·s/m — a pressure divided by a velocity. From a physical point of view, the acoustic input impedance is the ratio between the local pressure and local normal particle velocity. The Impedance boundary condition is a good approximation of a locally reacting surface — a surface for which the normal velocity at any point depends only on the pressure at that exact point.

In many practical applications the surface impedance is known (measured or simulated) in term of its real and imaginary part as function of frequency. To capture this behavior in the time domain the Impedance condition has built-in functionality for setting up a General local reacting (rational approximation) impedance through a rational approximation. Fitting of the frequency domain data can be performed using the Partial Fraction Fit function.

See the details for the Impedance condition in The Pressure Acoustics, Time Explicit Interface.