# Discussion Forum

## Acoustics and electric circuit coupling

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I have a model that combine pinna simulator in pressure acoustic physics with 3D geometry and microphone in analog circuit(not a simple RCL circuit but a complex circuit).

Then I want to couple them by adding a acoustic impedance on the boundary which represente microphone, the acoustic impedance is defined as voltage of circuit source devided by current of circuit. On the circuit side, I put the pressure as the source of the circuit. The study is in frequency domain.

However, when I press compute, I always got the error :

Division by zero. - Function: / Failed to evaluate variable. - Variable: comp1.acpr.Zi - Defined as: (((comp1.cir.v_1unit_Pa_cf)unit_s_cf)unit_A_cf)/((unit_m_cfunit_V_cf)comp1.cir.L4_i) Failed to evaluate expression. - Expression: dvol_spatial((((-comp1.acpr.delta)comp1.acpr.iomega)comp1.acpr.p_t)(1/comp1.acpr.Zi)) Failed to evaluate Jacobian of expression. - Expression: (-comp1.acpr.deltacomp1.acpr.iomegacomp1.acpr.p_ttest(comp1.p)/comp1.acpr.Zi)*(dvol_spatial)

which may mean that impedance is zero.

I want to know how to fix the model to couple analog circuit and pressure acoustics.

6 Replies Last Post Jan 17, 2022, 9:37 p.m. EST

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Posted: 2 weeks ago

I just spent a few minutes on this, and it seems to work if you put a velocity boundary condition instead of an impedance boundary condition; make the volume velocities match similar to what you have already done.

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Posted: 2 weeks ago

I just spent a few minutes on this, and it seems to work if you put a velocity boundary condition instead of an impedance boundary condition; make the volume velocities match similar to what you have already done.

Thank you very much. It does works! But I wondering that why impedance boundary wasn't work, if I disabled impedance boundary and calculated impedance manually after study, it was not NaN or zero; however if I enabled it, the impedance became zero, this is very strange.

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Posted: 2 weeks ago

The impedance is a ratio between pressure and velocity, but you need a more strict coupling than that. What you could do is first calculate the microphone impedance (apply a current, find the voltage, or the other way around), and use the found values in tabular form as the input for the impedance boundary conditions.

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Posted: 2 weeks ago
Updated: 2 weeks ago

OK, thank you very much.

In other words, The coupling of electric lumped model of acoustics and pressure acoustics need to be seperated into two steps.

But what's the differents between my model and lumped_loudspeaker_driver which is in application library?

It seems that lumped_loudspeaker_drive model just change the impedance boundary to interior normal acceleration in pressure acoustics, and add a additional voltage source in electrical circuit and change the first voltage source into constant voltage. I'm wondering why this is ok but not ok for my model.

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Posted: 2 weeks ago

Which version of COMSOL are looking at when it comes to the lumped_loudspeaker_driver file? (There are some changes in 6.0, but I suspect that you are looking at an older model?)

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Posted: 2 weeks ago

Oh sorry, I didn't notice that. My version is 5.3a, so maybe in COMSOL 6.0 the model setting had already changed something. Anyway, thank you very much.