An RCL model is intended to provide a simple model to represent the frequency-dependent mechanical properties of a system that typically has some loss, a mass, and a compliance. The option includes all combinations of a three-element circuit consisting of an acoustic damper (acoustic resistance, a resistor Rac), an acoustic mass (acoustic inertance, an inductor Lac) and an acoustic compliance (a capacitor Cac), which are the simplest resonating acoustic circuits. While full circuit models tend to be more sophisticated (see for example Ref. 26), one can typically identify a simple core circuit responsible for the basic response which consists of such three elements.

The acoustic impedance Zac is related to the specific impedance Zi implemented in the weak equations Equation 2-22 and Equation 2-23 by the area of the boundary A as

It is sometimes advantageous to use circuit models in mechanical units rather than acoustical units. In these cases, the overall structure of the impedance boundary condition equation does not change, but the numerical value of the parameters do. The mechanical impedance Zmech (the ratio of force F and velocity v) and the acoustic impedance Zac are related via the boundary surface area A by the expression