Impedance
Use the Impedance node to specify a (normal) specific impedance Zn on a boundary. Add this node from the Mechanical submenu. The condition has a user defined option as well as the option to define a simple RCL impedance model. The latter is useful for modeling the interaction with, for example, a microphone represented by a simple mass-spring-damper system.
The boundary condition reads:
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The condition relates the normal stress at the boundary to the normal velocity, through the value if the specific impedance. The tangential velocity can optionally be constrained to zero.
Mechanical
Select the Impedance model as User defined (the default) or RCL. For a transient model only the User defined option exists.
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For User defined enter a value or expression for the Specific impedance Zn (SI unit: Pa⋅s/m).
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The RCL model includes all possible circuits involving a source of damping (a resistor Rac), an acoustic mass or inertance (an inductor Lac), and a source of acoustic compliance (a capacitor Cac). The circuit elements are entered in acoustic units. These can be used as a simple model of, for example, the input impedance of a microphone, a loudspeaker cone, or other electromechanical applications. Other applications include general transmission line or circuit models with applications in materials with exotic acoustic properties. More advanced circuit models can be entered manually in the User defined.
Choose an option from the list: Serial coupling RCL, Parallel coupling RCL, Parallel LC in series with R, Parallel RC in series with L, Parallel RL in series with C, Serial RC in parallel with L, Serial LC in parallel with R, or Serial RL in parallel with C.
Notice the matching diagram and Equation section information for each choice. Then enter the Equivalent acoustic resistance Rac (SI unit: kg/(m4·s)), the Equivalent acoustic compliance Cac (SI unit: m4·s2/kg) and the Equivalent acoustic inertance Lac (SI unit: kg/m4).
Select a condition for the Tangential velocity — Slip (perfect) (the default) or No slip. With the Slip (perfect) option the feature is applied outside the viscous boundary layer, as this condition mimics the behavior of a corresponding Pressure Acoustics Model with an impedance condition. The No Slip option will apply an additional constraint for the velocity in the tangential direction on the boundary. In this case, the impedance condition could represent a lumped mechanical property of a wall, including the boundary layer effects.