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Simple questions using the acoustics module
Posted Nov 18, 2013, 1:57 p.m. EST Acoustics & Vibrations, Results & Visualization Version 4.1, Version 4.2, Version 4.2a, Version 4.3, Version 4.3a, Version 4.3b 6 Replies
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I have what for anyone with some experience in acoustics will probably be able to answer in seconds!
I would like to calculate the Sound Power radiated by a vibrating object (in the frequency domain). The object is in the middle of an air domain (spherical) and surrounded by a PML.
Am I correct in thinking that I need to evaluate the sound intensity at the boundary and multiply it by the surface area of the sphere? If this is so, how would I go about calculating the sound intensity at the boundary. Any help would be greatly appreciated.
My apologies if I have not given enough information here. I am just now starting to read up on basics of acoustics.
Many thanks for your help,
Rob
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Could I use the Surface Average function to calculate the average intensity on the bounding sphere and then multiply this number by the surface area of sphere?
I am assuming that this would be better than a point measurement as it will average out any fluctuations in the intensity measurement.
If so, does the shape of the radiating source have an effect?
My apologies if I am asking really dumb questions!
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No questions are stupid questions, if we don't ask, we might never learn!
As you probably know the sound power [W] is given by the integral of the intensity over the area through which it is transmitted.
Take a look at "Absorvative Muffler" model in the Comsol model library, on p.10 in the .pdf there is a description of how to use integration over an area to find the power radiated through it. This is a good way to solve your problem I would think.
A general tip is to search the model library for questions. Sometimes technical calculations are explained in the documents, and not always in the documents pertinent to your actual module. The RF module contains similar calculations in some cases (since both are wave related) and have been a useful source for me although not at all an RF-engineer.
Hope this helps
--
/RVN, Acoustical engineer.
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Thank you for your input. Always nice to know there is a community of people here looking to help where they can.
I have just had a look at the Muffler example in Comsol and it has some interesting methods in it. Their way of integration is an interesting one, I would have done it in post processing myself but it's probably better to do it as part of the simulation. Very interesting.
I will have a look at my model and let you know how I get on.
Thanks again, very much appreciated.
Rob
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To calculate the sound power radiated by a vibrating object.
1) define your boundary conditions of the vibrating object.
2) create a large spherical (other shapes work too, but spherical is easy) air domain around your object. (You may also with to create a PML around the air sphere.
3) Model 1>Definitions>Variables.
Name: power
Expression: intop1(p*conj(p)/(2*density*c)) where density is the air density and c is the speed of sound.
4) Model 1>Definitions>Model Couplings>Integration
Operator name: intop1
And now choose the boundaries of the air sphere.
5) Run your model and then plot a new 1D Plot Group>Global and then plot 'power' as a function of whatever you are looking at.
The automotive muffler example mentioned by RVN covers it nicely.
Rob
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Glad to hear I could help! Best of luck in future computations!
/RVN
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You can calculate the Sound Power in 1 of 3 ways.
1) Follow the instructions in the muffler example. This is based on performing the integration of the sound intensity during the solution.
The integral is;
(p*conj(p)/(2*acpr.rho*acpr.c))
2) This is the same as the slightly simpler method (again defined as a definition in Model 1 and is solved in the solution) of doing the integration over the function below.
(acpr.I_rms)
3) This time, we calculate the sound power in post processing.
We define a surface average, and the expression is
acpr.I_rms*area[m/s]
Answers from 1,2,3 are equivalent.
The first two solutions are preferable as you dont need to do any post-processing of the data.
Hope this helps!