as it gives me acoustic pressure but I want noise absorption coefficient
Hi
To get sound absorption, you must simulate a case the way you would measure it, e.g. an absorber placed in Kundt's tube. You compute your acoustic response and work out the sound absorption from this data.
Sincerely
Claes
Sound absorption factor simply is a damping measure that is not independent from its testing environment. You attribute absorption to an area, not to the material or the volume it encompasses.
To calculate absorption you must therefore input material damping, then simulate the experiment and from this extract the absorption factor.
If you want an excellent exposé over these issue, I suggest finding the first or second (cannot remember which) 1929 JASA issue, in which Firestone writes a very good editorial on the matter. It was the issue in which Beranek and Mah published their papers on modal density. You only find the editorial on a hardcopy of the issue.
Hi Fredo,
The calculation for sound absorption gets quite complex if I get ACV data from my simulation. Then calculate impedences and then sound absorption coefficient.
Do you know any other way i can approach the problem of calculating the absorption or where can I find that in the literature ?
Hi Claes Richard Fredö
Thanks for your help. I am using impedance tube method in ABAQUS to get the sound absorption of an intricate structure.
As you mentioned earlier "To calculate absorption you must therefore input material damping, then simulate the experiment and from this extract the absorption factor. ". I input complex (real+imag) impedance of the material as an interaction property in ABAQUS. My absorption results doesn't seem correct. Do you have any idea about this?
Hi
Complex impedance should add damping as well but it works only for normal direction wave incidence.
Try using a complex wave speed velocity as a model of the sound absorber.
Also, please observe that the absorption factor is a surface property. This is rather peculiar, in particular when modelling a volume absorber.
The world's first acoustician, Wallace Sabine, did things this way and the concept is still around. However, the absorption factor does not respect that a perfectly working absorber that is placed in one location may provide negligible absorption and work beatifully when placed elsewhere.
That said, Kundt's tube is a good case for cutting one's teeth on. I suggest that you make a simple analytic model to compare your FE data with.
Have fun
Claes
Hi Claes Richard Fredö ,
Thanks for your response.
If you can have a look in this approach mentioned below.
https://studentcommunity.ansys.com/thread/harmonic-analysis-4/?order=all#comment-b261cb14-b53d-4404-a9f5-a8d400477f94
I am trying to follow the same approach for my sample. I am following a similar approach and using ABAQUS.
1) Making an acoustic domain for my part before and after the part.
2) Getting pressure values at two different points before the part. Then using transfer function method to get absorption factor and surface impedance of the part.
https://studentcommunity.ansys.com/thread/harmonic-analysis-4/?order=all#comment-b261cb14-b53d-4404-a9f5-a8d400477f94
What do you think about this approach?
Regards,
Zaryab
Hello Zaryab,
I am trying to do a similar thing on Abaqus. Find the absorption coefficient and transmission loss with the impedance tube model using Abaqus.
Have you gotten a breakthrough in the model?
Thanks
Hi Zaryab Shahid and Oluwafemi Akinmolayan
The approach of simulating an absorber works within the limits that it ignores and structural effects from the absorber (for such you need, e.g. Biot material models).
The simplest way to physically model the absorber domain is to use a complex wave speed in this domain, within the above mentioned limitations.
Hope this helps
Claes
The best models for sound absorption in porous materials are probaly semiempirical: e.g. Mechel. The model give complex impedances related to the airflow resistance and thickness. If this is implemented in Abaqus or not I do not know. But it is typical to enter the loss information in the imaginary part of the amplitude functions. For the absorption from a vibrating plate with a void behind it, a diffrent model is needed. No simple formulation exists for any type of sound absorber.
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