Acoustics model in Ansys 18.1

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unitnatis posted this 3 weeks ago

For personal reasons, I used version 19.0 and downgraded it to 18.1.

Versions above 18.2 are very practical and even rudimentary users are easy to access in the course of sound analysis. Unlike the 18.2 and 19.0 versions of Ansys, there is no harmonic acoustic under 18.1 version. The versions below need to do download ACT to perform the sound analysis, and as mentioned above, I wanted to proceed with the acoustic analysis in Ansys 18.1 version after downloading ACT 18.0 version. However, the harmonic surface velocity does not working. In the latest version, if I specify the geometry, it is solved immediately. I want to know why it is not in 18.1 version.

 

 

Attached is an archive of ansys 18.1 to be analyzed.

 

 

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peteroznewman posted this 3 weeks ago

The reason why the Acoustic Surface Velocity has a ? is because your Analysis Settings show the Solution Method is Mode Superposition, but you don't have a Modal Acoustics system feeding the Harmonic Analysis. Either add a Modal Acoustics system and feed that solution into this setup, or change the Analysis Settings Solution Method to Full.  While you are there, you also need to set your Analysis Data Management, Save MAPDL db to Yes.

          

I looked at your settings for the Acoustic Surface Velocity and they look inappropriate.

You used the speed of sound as a surface velocity for a harmonic analysis. This input results in a Mach 1 shock wave sent into the air. The Acoustics solver cannot solve such a model and I doubt that is what you wanted, but correct me if I am wrong.

What sound pressure level did you want coming out of that speaker? Let's assume you want 80 dB.

 

The above is a Mass Source.

An equivalent to the mass source is the normal velocity:

 

I am also concerned about your mesh. 

But you have a Mesh Face sizing of 95 mm, which exceeds the maximum calculated above.

Also, it's not just the faces that need element size control, it is the size of the elements in the entire body of air.

Below is a slice through your current mesh. There are elements up to 250 mm edge lengths 

Here is the same slice with a uniform element size of no greater than 50 mm.

To save time, changed the analysis settings to output only two frequencies, 500 Hz and 1000 Hz.  

You don't need the speaker body to be there, just the walls, which are in the source body. Three bodies are included in the Acoustic body. I expect you want to set some particular conditions on the face of the partition, but for this model, it's all just air. but the solver runs!

The result is close to the 80 dB that was desired above, it is 73 dB due to the resonances in this chamber.

 

If I create a straight impedance tube with a Mass Source at one end and a Radiation Boundary at the other end, I get a SPL of exactly 80 dB.

 

 

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unitnatis posted this 3 weeks ago

Thank you so much for your kindly reply. It has helped a lot. I still have a lot of questions. Give me one more advice, Peter.

 

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unitnatis posted this 3 weeks ago

In addition, the noise from 200 hz to 4000 hz is mixed with white noise and emitted at once.

 

peteroznewman posted this 3 weeks ago

First, I do not think that there is any analysis that gives a pressure of 1 Mach in ansys. Why was possible it analysis in 18.2 version of harmonic acoustic? In fact, I have derived many analytical values. I gave 343m / s on surface velocity is because of the speed of the sound from the speakers coming from the air. Why is this wrong?

Just because someone made a YouTube video and entered 343 m/s as a surface velocity does not make it a reasonable input. If a surface is moving at the speed of sound, that is by definition, Mach 1.  It's a ridiculous surface velocity value for an acoustic analysis. ANSYS 18.2 doesn't check if the values you use for loads are reasonable or ridiculous.

I provided the equations to convert SPL in dB to a Surface Velocity or a Mass Source. These are equivalent and you can use either one. In an impedance tube, a surface velocity or a mass source assigned to a face give the exact same pressure when calculated from the equations I provided above.

unitnatis posted this 3 weeks ago

sir, I did your advice and the first problem I raised was solved. However, when I solve this, another error message is displayed and can not be analyzed. I do not know what this error message means. Also, the same error message appears in the file you sent. I have an ACT18.2 version on the Ansys 18.1 Student version. Is this because of this? What version of your Ansys and ACT did you use?

 

I will be waiting for your reply. thank you. 

 

 

 

 

 

 

 

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peteroznewman posted this 3 weeks ago

I used ANSYS 18.1 and Acoustics ACT 180.2

I took your archive, cleared the mesh then meshed. I deleted the Acoustic SPL plot. I solved the model then I added an Acoustic SPL plot. No error messages.

You are trying to build a model of a two-room Transmission Loss measurement, right?  And the two rooms are reverberant, right?  Because there is a Transmission Loss measurement where the second room is anechoic where every wall is lined with long foam wedges.

Why did you put an Acoustic Radiation Boundary condition on every face of your model?  No reverb will occur because the Acoustic Radiation Boundary prevents sound from reflecting off that wall. The speaker face that has the Mass Source is the only face that you might use that BC.

Also, the center partition is still Air.  Didn't you want to make it a 6 mm thick steel panel?

unitnatis posted this 3 weeks ago

The reason I set the radiation boundary is because I saw the SPL animation and the result.

If the radiation boundary is not set to ALL and only set the output plane of the speaker like you say

The animation of the sound pressure level occurs abnormally (it seems as if the sound pressure occurs simultaneously in the SOURCE ROOM and the RECEIVE ROOM).

However, if it is set as ALL, it will form a normal wave from the speaker

This can be confirmed by your ANSYS.

 

radiation boundary set on output plane 

 

all face set spl animation 

 

Above pictures is simulated by ansys 19.0 

peteroznewman posted this 3 weeks ago

I thought you were doing a Transmission Loss calculation for a panel between two reverberant rooms.

If all you want to see is the spherical wave coming from the speaker, why do you have these weird shapes for the air domain?

Please explain what is your objective for this simulation.

unitnatis posted this 3 weeks ago

My goal is just the way you think.

However, even if it is a reverberation room, if i do not set the radiation area to all, i can not even start a proper analysis. In addition, is transmission loss analysis using reverberation time impossible in the Ansys?

So I modeled it like the shape of a mini-chamber used in a real experiment,

i wanted to find the transmission loss by subtracting the spl value with and without the partition.

 

To summarize, I wanted to find the transmission loss in an environment similar to the actual experiment, so I created the shape of this air.

 

 I wanted to get a specific spl value at six points in the receive room and use it to average the transmission losses.

If I was just trying to get a simple transmission loss, i would have already get the transmission loss and completed the task using the rectangular model you showed above.(Of course, in this case, the radiation area is just the outport plane and inport plane.)

 

Would it be better to just get the transmission loss to the rectangular model?

The results for this model were almost identical with experimental and theoretical values.

I wonder what your idea is

 

I'll be waiting for your reply, sir.

peteroznewman posted this 3 weeks ago

Would it be better to just get the transmission loss to the rectangular model?

It would not be better to use a rectangular tube if your goal was to reproduce the testing conditions you did the experiments on. You took the time to construct the room geometry since that was your goal. When you created the shape of the room that enclosed the air, you were constructing walls that reflect sound. You don't want to set those reflective walls to be perfectly absorbing surfaces, that defeats the whole point of having a reverberant room. 

if i do not set the radiation area to all, i can not even start a proper analysis. 

I don't understand what you mean here. The definition of Transmission loss is the ratio of incident acoustic power on the panel to the transmitted acoustic power.

If I was just trying to get a simple transmission loss, i would have already get the transmission loss and completed the task using the rectangular model you showed above.

While this is different from your original goal, I recommend you build and solve that simple model so you can see how to get a result.

peteroznewman posted this 2 weeks ago

I changed the title of your post to add the word Acoustics.

You deleted your last post. Perhaps you learned some new knowledge about acoustics making that post obsolete. I prepared this post as I thought it might be helpful as you don't seem to have a firm understanding of what ANSYS is calculating for you when you request an Acoustic Harmonic Response analysis. It's all about standing waves.

Imagine an irregular shaped pond that has a mirror flat surface on the water. A pipe is suspended over the water and the faucet is turned on so that drips start falling. The first few drops send out perfect circular waves that grow in size. The dripping continues, and the first wave reaches the pond wall, reflects off it and heads back into the center of the pond, interfering with the circular waves coming from the dripping source. Come back 5 minutes later and the whole surface of the pond is just choppy water. The surface pattern of circular waves travelling out from the source is hidden in the interference of all the waves bouncing off all the walls. However, a stable pattern of the chop develops. Those are called standing waves. Some areas have high chop while other areas have low chop. You could color the highest chop area red and the lowest chop area blue.

If you take away the walls of the pond and do this in an infinite surface of water, there is nothing for the waves to reflect off and when you come back in 5 minutes, the wave pattern is just a set of concentric circles.

The waves on the surface of the water is a 2D analog to the 3D standing sound waves in a reverberant room. Where the waves add, there is a high SPL, where the waves cancel, there is a low SPL.  

Standing wave examples

 

 

peteroznewman posted this 2 weeks ago

You mentioned having a model with and without the panel. That measurement would be called insertion loss, while you started this discussion wanting to simulate transmission loss.  I took a short course in acoustics given by a professor from Michigan Tech and this is one of his slides...

unitnatis posted this 2 weeks ago

That's right sir. The reason I deleted the last post is what you think. I wanted to find the insertion loss you mentioned, and I turned to a more basic direction in putting results into it. It is to find the transmission loss as the ratio of the sound power you said. I installed ACT and checked the result, but the analytical value is not good. What is the problem? I considered all possible parameters for the port configuration but it did not work. Because it is for students, it is understandable that the result is strange at 800hz or more, but the result is strange even if it is less than 800hz. and acoustic face result solution is not working

 

In addition, I removed the receive room. 

 

Please. I would like to hear your advice.

 

 

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peteroznewman posted this 2 weeks ago

I haven't opened your model yet, but I can see from the results that you have a mistake in your model. You highlighted unimportant rows in the table above. The most important result is the average transmission loss is 9999 dB.  That means you have no transmission. None. Zero. I'm sure that is wrong. I will take a look at your model and try to find the mistake, then post another reply.

unitnatis posted this 2 weeks ago

I made the emphasis on the power level because I felt it was impossible to get transmission losses where the speakers are in the air area. I've actually done a lot of simulations, but it is impossible. I was going to get the power level and convert it to power to derive the transmission loss.

 

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