# Acoustic Analysis

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• Last Post 05 June 2019
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hhusker posted this 15 May 2019

Hi,

I am conducting an experiment using the impedance tube,and would like to simulate for transmission loss.I am quite new to ansys.I used harmonic response for the transmission loss. My problem is i have to simulate three conditions,where two of them have pre-stressed or moving components while I conduct transmission loss experiment. I was wondering how I can simulate this as combining both systems doesn't seem to work as I cant give the air gap inside as acoustic body. Any help would be much appreciated.

Hhusker

peteroznewman posted this 15 May 2019

Please reply with a sketch or screen snapshot of the components in this system. Identify which are the pre-stressed or moving components.  Identify the air gap. Which version of ANSYS are you using?  One of the members here jj77, has some background in acoustics and might have a comment.

hhusker posted this 16 May 2019

I am using Ansys 19.1,I also have the extension. I initially planned to excite the structure by air pressure ,but will soon turn to other means. I wanted to know how I can prestress it and run analysis. I was planning to apply pressure force on the second image just for a general idea. Your Input is much appreciated.

peteroznewman posted this 17 May 2019

You say you want to simulate having these solids and airspace (green) in an impedance tube.

Please annotate a screen image to show the face that receives the input plane wave and I assume the opposite face emits the sound that can be measured to calculate the transmission loss.

Do you have an impedance tube with a circular cross-section? What exactly is in the center of the tube? It's still not clear how you are arranging that rectangular block in the impedance tube.

hhusker posted this 18 May 2019

Yes the tube is circular in cross section,the sample in the screen shot 2 is tested in the tube. I had made two cylinders that corresponds to Interior and exterior ports. I already conducted the harmonic analysis for the sample. But i want to know how to conduct it by pre-stressing the the structure in the second screenshot. I am sorry if I am not being very articulate. Btw I do have doubt on the test i have conducted too.

peteroznewman posted this 18 May 2019

If you put the structure shown above  that has two large disks and a small block of material between the two disks into an impedance tube, then the Transmission Loss through that structure will have a significant amount of energy that bypasses the complex body in the center and goes straight through the air from one disk to the other.

This method of holding a material sample in an impedance tube is not valid.  The material sample should fill the entire cross-section of the impedance tube.  If the sample is naturally square and not easily made to fit in a tube, then a square duct should be used for the impedance tube.

hhusker posted this 20 May 2019

The design is in such a way that it the prototype covers the whole tube. The 'Disks' you referred to are actually flanges that was modeled from the tube itself.there are no air gaps between the tube and the model except the one i showed you. I usually use harmonic response for analysis of impedance tube experiment. I have a 4 microphone setup. I was wondering if there are some material that shows how to do simulations for this experiment.

peteroznewman posted this 21 May 2019

It seems that the complex material covers only a small portion of the cross-sectional area of the impedance tube.  I sketched in the impedance tube walls as orange lines and show the air that is outside the complex material but is between the disks, allowing sound energy to go from disk to disk and bypass the complex material that only occupies the center.

hhusker posted this 21 May 2019

The complex material covers the whole tube of the impedance the disk is used to just fasted it. The apparatus has an diameter of 35mm and the structure in the middle is as big as that. Hence the sound is normally incident only on the structure. I have done the experimental part,just need clarifications and help in conducting the same in simulation.

what is the difference in harmonic acoustic and harmonic response using the extension? which one should i use? since I used white noise as the incident source,is the surface velocity or mass source?

peteroznewman posted this 21 May 2019

Sorry, I still don't understand how the geometry you show relates to the impedance tube. You say that the disk is just used to fasten it.  Don't put geometry in the model that is not in the cylindrical air column of the impedance tube.

What direction is the incident sound coming from that hits the sample? X, Y or Z?  In my figure above, I assumed the impedance tube was along Y, but now I am guessing the Y axis is a cross tube to insert the sample and the impedance tube is along the blue Z axis.

If the impedance tube has a diameter of 35 mm, please show that diameter of material sample for the acoustic model with two 35 mm diameter solids that represents the air in the impedance tube before and after the sample.

The point I'm making is the geometry that goes into a simulation is different from the geometry used to make fixtures to hold samples to put in a physical experiment. This is just as important to understand as your other questions.

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hhusker posted this 21 May 2019

Oh thank you. I am sorry. You are right in your axis assumption,the sound is incident in the y axis. I haven't shown in the previous images,but I have made two 35 mm cylinders that is assigned as air in the model,which I had given as the inlet and outlet ports respectively. I shall remove the two flanges while I run the simulation. I have attached a figure like you asked without the holder and with the inlet and outlet. I have removed the lid so that you can better understand what I am trying to do.

I would like to know the applictaion of static acoustic module.

Thank you

peteroznewman posted this 21 May 2019

Okay, now it looks like you have geometry for an acoustic simulation. I saw above that you had a solid for the air inside the sample too.

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PDF file

I am going on vacation for a few weeks starting tomorrow so I hope you will get some help while I am gone.

Good luck,
Peter

hhusker posted this 21 May 2019

I have gone through most of this beforehand. My issue is I want to run a  simulation for transmission loss,simultaneously while applying pressure to the design i showed initially. How can I go about that?

Hope you have a great vacation and Thank you.

peteroznewman posted this 22 May 2019

If you couple a Static Acoustics system with a Harmonic Acoustics system, you can have geometry that has interference in the Static system and contact resolves the interference and creates a pretensioned structure.

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baijns posted this 05 June 2019

Hi peteroznewman,

I am using Ansys 19.2 to simulate to sound radiation of vibrating rectangular plates. I have used modal analysis, followed by harmonic response analysis and finally harmonic acoustic analysis. My methodology and results are correct as I have imported the velocity b.c and solved it. The problem is how to validate the results with theory or experiment. What is the mathematical equation that ansys solves in harmonic acoustic analysis. Is it the rayleigh integral or someother equation. Please help.

Thanks & Regards

Baij