Harmonic Analysis for Acoustics

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• Last Post 23 September 2018
216dd11 posted this 25 April 2018

Hi, I am trying to find the acoustic characterization of a perforated plate, for this, I have created an impedance tube and put a perforated plate in that with some air cavity. I am using ACT module and boundary conditions, but not getting any result.

My objective is to find the harmonic response of perforated plate, ie how much sound is absorbed if the sound is passed through it, (to get a graph between the absorption coefficient and frequency.)

I have attached some pics of my Boundary condition, load, and steps followed.

Kindly help me.

216dd11 posted this 25 April 2018

Hi peteroznewman i need a help in the Harmonic analysis of a perforated plate and porous absorber.  i am attaching it how it works experimentally and what is the graph i need to obtain.

The sound is passed through from a speaker and the plate is placed which absorbs the sound.

peteroznewman posted this 25 April 2018

Kindly follow these directions to create a Workbench Project Archive .wbpz file, then Attach it to your reply.

Please describe in detail the sound from the speaker, is it a single frequency that gets swept over time, or is it a broad band noise. If it is broad band, what is the spectral content of the sound.

The absorber is on a piston. Is the piston always in the same location or does it move during the test?  What is the piston material and its physical and acoustic properties?  Same question for the cylinder walls.

I am learning acoustics myself,  so I may be asking you to teach me acoustic measurement science so I can help you with ANSYS.

216dd11 posted this 26 April 2018

It is the plane wave that is passed in an impedance tube. The absorber is not on the piston, it is kept in between the tube so that some air cavity is created behind the tube which will increase the absorption.

Properties of plate and cylinder walls are- brass( impedance tube) and aluminium ( perforated plate).

perforated plate - no of holes -21, the diameter of the hole is  - 1.2mm, the distance between holes- 8.7mm. The space between the perforated plate and back end of the tube is 15mm,

Attached Files

216dd11 posted this 26 April 2018

Usually, the acoustical sample is put at one end of a tube and a loudspeaker is mounted at the other end. The loudspeaker generates sound and this results in a forward travelling sound wave. A part of the sound is reflected, causing a backward travelling sound wave. The reflection coefficient is determined by measuring sound is travelling in the forward and backward direction.The test sample is mounted before the end of a straight, rigid, smooth and airtight impedance tube. Plane waves are generated in the tube with the help of a loudspeaker (random, pseudo-random sequence, or chirp) fixed at the other end.

peteroznewman posted this 28 April 2018

I have made some progress working on this project.

Here are the three things that are not air in your model, note that I renamed the bodies from Solid to meaningful names.
I noticed that the two ends are solid bodies with a thickness. These are not needed. Only the perforated plate is needed.

Here is the last body that is assigned the property of Air.

The problem is that the Air body does not fill the tube!
To fix this, I suppressed the unnecessary end bodies and the tube. You only need one body, the air. I created a new body that is the actual air volume that is in the shape of a tube, but has the perforated plate subtracted from it, which looks like this:

Attached is the ANSYS 17.0 archive that has the geometry repaired. I couldn't delete the old acoustic items, but I put in my best guess for what you need. I don't know how to use ports and how to use the sound power calculation. What can you do with this?

Attached Files

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216dd11 posted this 28 April 2018

Sir, I saw the model, I need to plot the graph between absorption coefficient and frequency in "acoustic power result plot" option. for choosing acoustic mass source u had selected outer face of the speaker, is selecting outer face will let sound pass from the inner face of speaker also.?? and as the plate and impedance tube is of aluminium alloy material and the air is passed through them. I didn't get the concept that you modelled the all geometry with air, can u help.I tried to assign the plate and tube material as air, but in engineering data of air, elasticity and other terms are required and elasticity and E are defined for air so, I was getting the errors. I am attaching ACT 180.2 acoustic extension so that u will not need to suppress.Can u create a fresh model with using 180.2 extension. And did u made geometry in SolidWorks or ANSYS..??

216dd11 posted this 28 April 2018

ACT 180.2

File extensions are not allowed here. So can u share ur mail where I can send u extension.

peteroznewman posted this 28 April 2018

Your screen shot showed ANSYS 17.0.  ACT Acoustics V4 is compatible with 17.0, which is why I choose that version.  What version of ANSYS do you have installed?

You say ACT 180.2, does that mean Acoustics for ANSYS 18.0 V2?  I hope your answer to the first question is ANSYS 18.0.  If you installed this ACT on ANSYS 17.0, that is not recommended.  If I open your model in ANSYS 18.0, you won't be able to open that model after I save it if you are on ANSYS 17.0.

Acoustics is the physics of waves travelling through air.  If you thought that ANSYS would automatically create air between the solid bodies, you were wrong. You have to create a body that is the air. When you have a body that is just air, ANSYS can transmit sound waves through that body. The simplest model is a body of air, in a Modal Acoustics system with no boundary conditions. ANSYS assumes all outside surfaces of the body are perfectly reflecting walls. When it solves, the result is the modes of all the standing wave patterns which form in that shaped body of air.

I built the model of the air in DesignModeler, but you could do that in SolidWorks.

The flat end of the tube of air, far from the perforated plate, was assigned an acoustic mass to represent a speaker that can send acoustic pressure into the air. Because it is flat and the adjacent surface is perpendicular, this generates a plane wave that travels down the tube. If the value of the acoustic mass is Q and the speed of sound is c then the pressure p created by the acoustic mass is p = Qc.  I could generate that same plane wave using a surface velocity boundary condition on that same face instead of using an acoustic mass. This is somewhat easier for me to understand.

I will post this now, but I await your reply on which version of ANSYS you are on. I will be learning how to get Ports working and will reply when I have figured it out.

Regards

216dd11 posted this 28 April 2018

Sorry, I mistyped sir. I am using ANSYS 17.0 and ACT module 18.0.  And this Version is working fine with 17.0

peteroznewman posted this 28 April 2018

216dd11 posted this 29 April 2018

Sir, I tried some modifications in boundary conditions and I am getting a graph but the graph is showing linear response ie one value of absorption coefficient at all frequency and that value is in powers of 10^30. The value should be between 0 to 1. Did u got some clue.??

peteroznewman posted this 29 April 2018

I am going to do some testing of ACT V4 for 17.0 as I suspect that ACT 180.2 installed on ANSYS 17.0 may have some problem. That will include trying to do the absorption coefficient. I will let you know.

216dd11 posted this 30 April 2018

Okay sir

peteroznewman posted this 01 May 2018

I took the geometry for the air into ANSYS 19.0 which includes support for Acoustic Harmonics without the need to install an ACT extension.

I requested 600 results between 3 kHz and 12 kHz and got these results:

I refined the mesh and the drop-outs have tightened up. Is there a requirement on element size at different frequencies?

The wavelength of a 12 kHz tone in air is about 28 mm, so I used a 3 mm maximum mesh size to get 10 elements along one wavelength of the highest frequency.

Regards,

Peter

216dd11 posted this 01 May 2018

Thanks sir for the help. I have ansys 17 version only, Can u please send me the geometry of air  and the ansys file. The graph between absorption coefficient and frequency should start from 0 or 0.1 as in my most analytical cases like i have posted on the top.

peteroznewman posted this 01 May 2018

The attachment above is an ANSYS 17.0 archive. You can download that to get the geometry of air. I wanted to use ANSYS 19.0 as a point of reference for what the results should look like when you go back to 17.0 since the software gets smarter with each new revision.

The graph of absorption coefficient depends on the material and air gaps between the two ports. If the material is a perfectly reflecting wall, which is the ANSYS default for an exterior face, and there is no air gap between two acoustic domains, then the absorption coefficient is 1.0 over all frequencies.

Here is a new perforated plate, it has one big hole. I expect this will have an absorption coefficient near 0.0 so let's see what ANSYS predicts. I will cut back on the number of points plotted to save some time.

The result looks the same as the small hole perforated plate, so I don't believe I am building these models properly and will need to study acoustic modeling in more detail and reply when I have mastered this type of model.

If anyone knows how to build these "muffler" type problems, please post.  I believe there is a muffler tutorial somewhere that uses some advanced technique to do a perforated plate without drawing the holes.

Attached Files

216dd11 posted this 01 May 2018

Yes, there is a muffler tutorial which finds the transmission loss across two ports. I am attaching the link from where you can get that.

https://www.simutechgroup.com/tips-and-tricks/fea-articles/379-acoustic-simulation-of-a-silencer

sir the above graphs you are getting resembles like the graph of reflection coefficient vs frequency. below is the link to a paper which will telll how a graph looks like. I am also attaching a pic and dimension of that.

https://www.sciencedirect.com/science/article/pii/S0003682X14002746?via%3Dihub

The dimensions of the plate are-

hole diameter- 1.6mm, thickness 3mm and distance between two hole is 5mm and distance between plate and other end of tube is 70mm.

Attached Files

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peteroznewman posted this 01 May 2018

Okay, did you build the model written about in the SimuTech Group article and reproduce the results?

I can only read the sciencedirect abstract. I would have to purchase that to read the whole article.

216dd11 posted this 02 May 2018

This the whole article from that journal,pls find the attachment..

and no sir i didn't reproduce it. even ansys have a tutorial on that and i am attaching the ansys tutorial of muffler.

Attached Files

216dd11 posted this 02 May 2018

Ansys tutorial of muffler.

Attached Files

peteroznewman posted this 02 May 2018

Thank you for the article, I will read that later.

Thanks for the muffler example, that was the one I was thinking of. I will try to follow that.

I used your geometry to make the air, so it is your dimensions.

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216dd11 posted this 02 May 2018

Sir, I made some progress but somewhat i am lacking with the boundary conditions, pls take a look. i tried some manipulations and graph now starting to rise from zero. Kindly comment.

Attached Files

216dd11 posted this 03 May 2018

Sir i am not able to see any example.

peteroznewman posted this 05 May 2018

to find the harmonic response of perforated plate, ie how much sound is absorbed if the sound is passed through it, (to get a graph between the absorption coefficient and frequency.)

and your approach to obtaining this information is to build a model in ANSYS using simulation methods you don't sufficiently understand. That doesn't seem like the most straightforward approach.

If that was my objective, I would go into the lab and do a measurement on a physical sample using an actual impedance tube, a pair of microphones  and a data acquisition system. I am fortunate to have the microphones and data acquisition system, and I could have an impedance tube made or purchase or rent one.

My objective is to learn how to build acoustic models in ANSYS. Maybe that is your objective too, since you are posting on the ANSYS Student Community site and not a sound measurement site.

When I am learning simulation software, I like to find textbook problems where I know the answer, and see if I can build a model to give the same answer. I have been doing this in acoustics and was able to predict the natural frequencies of columns of air that match equations in books. That is a good way to build confidence in my ability to model new physics. I recommend you take a similar approach .

It seems you are just adding things to your model to "improve the fit" without understanding what those things do. Why does your model have a Mass Source AND an Incident Wave at port 1? These are both adding waves at the speaker end of the tube.  For the Incident Wave, why for the Wave Type did you choose Circular Duct and not Plane Wave? An impedance tube is intended to have a plane wave.  Why did you add a Monopole Acoustic Wave Source at the center of your impedance tube, which generates spherical waves and not plane waves. There is an expression called Garbage in, garbage out. What does it matter if the output from the model looks something like what you saw in a paper if the model does not simulate the fundamental physics of the problem? The model will have no value.

I have some more notes in the attached zip file of a Word document.

Attached Files

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peteroznewman posted this 06 May 2018

In the paper, there is a simple model of a muffler and the computed TL.

Here is my ANSYS model:

Hey... how about that, my model matches a published paper!

I read that a Radiation Boundary is needed on the faces used at the inlet and outlet ports. Let's see what the TL looks like if I suppress the Radiation Boundary and it is not in the model...

This confirms what I read, that the Radiation Boundary prevents acoustic reflections, so when the paper says the test uses an anechoic chamber at the output end, that tells me to apply a Radiation Boundary condition.

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iza hamdan posted this 14 September 2018

Hi,

I also trying to find the acoustic characterization of a perforated plate similar with this thread. Just want to know have you solved the problem yet? or you're still working on this project?

peteroznewman posted this 23 September 2018

Hello iza hamdan,

If you are interested in the acoustic characterization of perforated plates, you will find the attached research paper very useful.

Regards,

Peter

Attached Files