Learning Forum

[kasparek1]

Hello,

I am trying to perform harmonic acoustic analysis which simulates impedance tube (see picture attached). There is a speaker at one end of the tube and specimen of examined material at other end. The goal is to determine Absorption Coefficient of the specimen. I chose Delany-Bazley model for the specimen (made of PU foam) at the engineering data, so I could assign body of specimen as an acoustic body, as well as the rest of inner volume of the impedance tube.

The problem is the error which shows everytime when I try evaluate the results of Absorption Coefficient (see picture attached): The input power at the driven port is equal to zero. Please check the port definitions. Probably i didn't set boundary condition of "Port" correctly. I set Port to the face of an acoustic source (I also tried to set Port to the face of acoustics body touching the specimen) . The source is defined by setting "Pressure" at the face where the speaker is located. I tried to define source by velocity instead, but the same error occured. Could anybody help me how should I define boundary conditions to evaluate the results of absoprtion coefficient, please?

Any help will be appreciated,

Thanks

K.

[Erik Kostson]

Use the surface velocity as an excitation not pressure. Also define a port at the inlet/excitation and a radiation boundary also at the inlet. The surface velocity for a plane duct wave is just surface_velocity = pressure_source/(density*speed of sound). See below how to set the excitation as explained above.

Also make sure your mesh is connected (not sure why it is split in so many cylinders).

All the best

Erik

[kasparek1]

Hello Erik thanks a lot for your answer. I have set boundary conditions as you described and finally some results of Absorption Coefficient were evaluated. But the trend of it seems weird, it oscillates a lot (see picture). Is it normal or is there some other problem? The solution interval is 50 Hz. I see you have set second Radiation Boundary condition but I dont know where. Could this cause the problem?
To answer your note about mesh: The mesh is connected - I shared topology in spaceclaim. The reason for so many divisions into cylinders is that I wanted to evaluate SPL (dependent on frequency) at more distances from specimen, so there is Frequency Response on all front faces of every cylinder.
Have a nice day
K.

[kasparek1]

[Erik Kostson]

Hi

happy to help

"But the trend of it seems weird, it oscillates a lot (see picture). Is it normal or is there some other problem? "
This is hard to say why it does that.
"The solution interval is 50 Hz. I see you have set second Radiation Boundary condition but I dont know where. Could this cause the problem?"
As shown in the image I showed and as explained, the radiation boundary is on the same place as the surface velocity excitation (so at the inlet of the duct).

So in theory with this workflow (port, surface velocity and radiation boundary on excitation inlet) we should be able to get a reasonable curve see below:

All the best
Erik

[kasparek1]

Thanks for fast answer!
I was talking about "Radiation Boundary 2" which I cant see at the picture you posted.
Also I have one other question that you may probably know answer for. At the engineering data after I chose Delany-Bazley model, there has to be a Fluid Resistivity value filled. I suppose its a material property for every kind of materal, for example some value for PU foam, different value for mineral wool etc. But its quite hard to find some information about values of Fluid Resistivity for each material. Is there some way I can calculate it?

Thank you K.

[Erik Kostson]

No worries.
"I was talking about "Radiation Boundary 2" which I cant see at the picture you posted."

That is not needed (was just something else that we do not need) - so we only need one radiation boundary, on the same place as the surface velocity excitation (so at the inlet of the duct). See below:



The fluid resistivity is a material property and is often given by the manufacturer say of the foam or mineral wool etc. It can be measured with physical testing, but not sure if there is an easy way of calculating it - I would search the web to see if it is possible, but unfortunately that is all that I can help with since we do not have such possibility (to estimate fluid resistivity), since it is a material input from the user.

All the best

Erik

[kasparek1]

Ok I understand. Thank you so much for all the help.
Have a nice day
K.

[souvikghatak066]

[souvikghatak066]

How should I apply acoustic region in order to get absorption coefficient value