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Acoustic Power/Intensity - Monopole source next to rigid sphere

Posted 5 nov 2021, 17:53 GMT-4 Acoustics & Vibrations Version 5.6 3 Replies

Michael Sonnenberg
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Hello,

I'm attempting to quantify the total acoustic power in different regions of a 1m spherical boundary that surrounds a smaller rigid sphere ("head") next to a monopole point source ("speaker"). I plan to use this as a metric of directivity/scattering.

I defined a monopole point source of strength 1W and calculated the total acoustic power by integrating Intensity over the surface of the 1m spherical boundary.

When I remove the "head" and have just the point source, I calculate the power on the surface of the 1m sphere to be 1.00W. However, when I add the rigid "head" into the model, I calculate various total power results for different frequencies ranging from ~0.75-1.25.

I'm somewhat new to COMSOL, however, I have gone over this for several days and can't figure out why I'm not consistently getting 1.00W (or close to) when I place a rigid surface next to the point source.

I have created both 3D and 2D models, and looked at various methods of surface integration that all produce similar results.

I just noted this clarification of monopole source strength in the help:

When Power is selected, specify the source’s reference RMS strength by stating the power it radiates. In a homogeneous medium, the specified power is obtained (the reference), but with other objects and boundaries present the actual power is different.

Might this explain what I'm seeing? Regardless, I don't understand how that jives with conservation of energy.

I appreciate any help. -Michael

3 Replies Last Post 16 nov 2021, 02:58 GMT-5
Acculution ApS Certified Consultant
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Posted: 3 years ago 12 nov 2021, 09:30 GMT-5

In your 2Daxi model you can see how you will get closer to 1 W as you move it away from the scattering sphere, as it modifies the radiation impedance and the resulting phase relationship between pressure and velocity. The 1W is for free space.

-------------------
René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
In your 2Daxi model you can see how you will get closer to 1 W as you move it away from the scattering sphere, as it modifies the radiation impedance and the resulting phase relationship between pressure and velocity. The 1W is for free space.
Michael Sonnenberg
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Posted: 3 years ago 15 nov 2021, 13:32 GMT-5

In your 2Daxi model you can see how you will get closer to 1 W as you move it away from the scattering sphere, as it modifies the radiation impedance and the resulting phase relationship between pressure and velocity. The 1W is for free space.

Yes, I did observe this behavior.

Can you help explain to me how this complies with conservation of energy? Am I making an incorrect assumption about the nature of the source strength?

I'd like to model the system in such a way that I have a known input (power) and can account of the dissapation/scattering of all this energy, so if I put 1W into the system I get 1W out. I'd then like to further breakdown spatially where the energy is being scattered.

>In your 2Daxi model you can see how you will get closer to 1 W as you move it away from the scattering sphere, as it modifies the radiation impedance and the resulting phase relationship between pressure and velocity. The 1W is for free space. Yes, I did observe this behavior. Can you help explain to me how this complies with conservation of energy? Am I making an incorrect assumption about the nature of the source strength? I'd like to model the system in such a way that I have a known input (power) and can account of the dissapation/scattering of all this energy, so if I put 1W into the system I get 1W out. I'd then like to further breakdown spatially where the energy is being scattered.
Acculution ApS Certified Consultant
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Posted: 3 years ago 16 nov 2021, 02:58 GMT-5

Hi Michael

I will put in on long list of questions that I will try and explain in blog post format. For now, feel free to link up with me via LinkedIn or email, and we can discuss active and reactive intensity and other fun stuff ;)

-------------------
René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
Hi Michael I will put in on long list of questions that I will try and explain in blog post format. For now, feel free to link up with me via LinkedIn or email, and we can discuss active and reactive intensity and other fun stuff ;)

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