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No circumferential modes in simulation result

Posted 30 oct. 2017, 04:13 UTC−4 Acoustics & Vibrations Version 5.1 6 Replies

June Fu
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Hello,

I am simulating the vibration of a speaker membrane driven by a coil. However, I cannot get any circumferential modes of vibration in my simulation result.

I have set the properties of the membrane correctly (according to a real speaker membrane). And circumferential modes of vibration is obvious in the real speaker testing. The speaker is a circular structure and therefore my model in comsol is axial symmetric. Also I adopted a 3D structure in order to get the vibration modes. Is it that circumferential vibration modes cannot be obtained for a axial symmetric structure in simulation? Or my settings are not correct?

By the way, the vibration modes obtained in eigen frequency analysis agree well with the real speaker testing. In addition, I tried to add a small part to break the symmetry but the result had little change. Anybody can help?

Thanks a lot in advance for your help. Have a nice day.

Best, June

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6 Replies Last Post 1 nov. 2017, 08:39 UTC−4
Edgar J. Kaiser Certified Consultant
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Posted: 7 years ago 30 oct. 2017, 06:43 UTC−4
Updated: 7 years ago 30 oct. 2017, 06:43 UTC−4

June,

everything in the model needs to respect the symmetry, not only the geometry.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
June, everything in the model needs to respect the symmetry, not only the geometry. Cheers Edgar
June Fu
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Posted: 7 years ago 30 oct. 2017, 10:05 UTC−4

June,

everything in the model needs to respect the symmetry, not only the geometry.

Cheers Edgar

Hi, Edgar. Thanks for your kind reply.

Sorry that I don't quite follow you. Besides the geometry, where else shall I respect the symmetry? Although my model is axial symmetric, I have to the 3D model so that I can get the circumferential modes. What do you think leads to the lack of circumferential modes in the simulation result?

Your kind help is much appreciated.

Best, June

>June, > >everything in the model needs to respect the symmetry, not only the geometry. > >Cheers >Edgar Hi, Edgar. Thanks for your kind reply. Sorry that I don't quite follow you. Besides the geometry, where else shall I respect the symmetry? Although my model is axial symmetric, I have to the 3D model so that I can get the circumferential modes. What do you think leads to the lack of circumferential modes in the simulation result? Your kind help is much appreciated. Best, June
June Fu
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Posted: 7 years ago 31 oct. 2017, 03:54 UTC−4

everything in the model needs to respect the symmetry, not only the geometry.

Do you mean that the vibration analysis cannot obtain the circumferential modes of an axial symmetric model as the eigen-frequency analysis does? Thanks in advance.

Cheers, June

>everything in the model needs to respect the symmetry, not only the geometry. Do you mean that the vibration analysis cannot obtain the circumferential modes of an axial symmetric model as the eigen-frequency analysis does? Thanks in advance. Cheers, June
Edgar J. Kaiser Certified Consultant
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Posted: 7 years ago 31 oct. 2017, 08:29 UTC−4

What I mean is that the 2Daxial symmetry will not show any modes that are not axially symmetric.

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
What I mean is that the 2Daxial symmetry will not show any modes that are not axially symmetric.
June Fu
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Posted: 7 years ago 31 oct. 2017, 23:52 UTC−4
Updated: 7 years ago 1 nov. 2017, 03:37 UTC−4

What I mean is that the 2Daxial symmetry will not show any modes that are not axially symmetric.

Hi, Edgar. Thanks for your kind reply.

I tried to add a small part on the membrane to break the symmetry but the result showed little change. Is there any possible solution to this kind of problem? After all, speakers are normally with a circular structure (although real speakers are not prefectly axial symmetric). So this problem is quite typical. If the vibration simulation does not show circumferential modes, the sound simulation of the speaker will, in turn, be incorrect.

If this problem exists intrinsically, it would be very interesting, and important as well, to come up with a possible solution.

Best, June

>What I mean is that the 2Daxial symmetry will not show any modes that are not axially symmetric. Hi, Edgar. Thanks for your kind reply. I tried to add a small part on the membrane to break the symmetry but the result showed little change. Is there any possible solution to this kind of problem? After all, speakers are normally with a circular structure (although real speakers are not prefectly axial symmetric). So this problem is quite typical. If the vibration simulation does not show circumferential modes, the sound simulation of the speaker will, in turn, be incorrect. If this problem exists intrinsically, it would be very interesting, and important as well, to come up with a possible solution. Best, June
Jeff Hiller COMSOL Employee

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Posted: 7 years ago 1 nov. 2017, 08:39 UTC−4
Updated: 7 years ago 1 nov. 2017, 08:55 UTC−4

Hello June,

If I understand your posts correctly, you are performing an eigenmode analysis and carrying it out as a full 3D analysis of an axisymmetric structure, not as a 2D axisymmetric analysis of an axisymmetric structure. If that understanding is correct, then yes, such a model is capable of finding all eigenmodes, not just those that are axisymmetric.

If your eigenmode analysis model is not showing you certain modes that you expect, the most likely scenario is that those modes are at frequencies past the ones where you are telling COMSOL to look. By default COMSOL returns the 6 modes with the lowest eigenfrequencies. It could be that for your structure the circumferential modes are not among the bottom 6, in which case you would need to set up your solver to ask COMSOL to return more than 6 eigenmodes.

If, on the other hand, what you are saying is that the eigenfrequency analysis model does find all the expected modes, including the circumferential ones, but that those modes do not appear to be involved in a subsequent frequency-response analysis, then one likely scenario is that your loading simply does not excite those circumferential modes or excites them much less than it excites the other modes.

Best,

Jeff

PS: If Edgar and I misunderstood your work, please post the mph file so we (and other Discussion Forum users) can took a look at it for ourselves.

-------------------
Jeff Hiller
Hello June, If I understand your posts correctly, you are performing an eigenmode analysis and carrying it out as a full 3D analysis of an axisymmetric structure, not as a 2D axisymmetric analysis of an axisymmetric structure. If that understanding is correct, then yes, such a model is capable of finding all eigenmodes, not just those that are axisymmetric. If your eigenmode analysis model is not showing you certain modes that you expect, the most likely scenario is that those modes are at frequencies past the ones where you are telling COMSOL to look. By default COMSOL returns the 6 modes with the lowest eigenfrequencies. It could be that for your structure the circumferential modes are not among the bottom 6, in which case you would need to set up your solver to ask COMSOL to return more than 6 eigenmodes. If, on the other hand, what you are saying is that the eigenfrequency analysis model does find all the expected modes, including the circumferential ones, but that those modes do not appear to be involved in a subsequent frequency-response analysis, then one likely scenario is that your loading simply does not excite those circumferential modes or excites them much less than it excites the other modes. Best, Jeff PS: If Edgar and I misunderstood your work, please post the mph file so we (and other Discussion Forum users) can took a look at it for ourselves.

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