Discussion Forum

Hi all,

I am trying to use the modal basis of a very simple acoustic 3D problem for a frequency sweep (room acoustics). First, entirely in Comsol, and second, by exporting the reduced modal system matrices/vectors to Matlab and work with it separately.

My room is a 3D cavity with absorbing materials (real impedance) on the walls. I have set an acoustic flow point-source somewhere in the room with amplitude "linper(1e-3m^3/s)".

Several issues :
1) When I look at the frequency responses in several points of the room (tranfer function p/u, w.r.t. the flow u of the source), there are peaks and dips, as expected. But the peaks are not at the frequencies identified in the "eigenfrequency" step (I'm talking about the complex modulus of these complex eigenfrequencies).

2) When I export the system matrices (Er, Dr, Kr) and the load vector (Lr), I am not sure exactly how to use the resulting system. According to the documentation (Studies and Solvers > Solution Operation Nodes and Solvers > The Modal Solver Algorithm), I should solve:
[((W-W0)/(2Pi))^2.Er - (W-W0)/(2Pi).Dr - Dratio - Kr] . c = Lr
for the vetor of modal coefficients "c". (I have used here 'W' for 'Omega', and Dratio is null for me as I already have damping in my system due to the boundary conditions)

a) It is said in that same doc that W0 is "the first frequency given". I don't get what this means, but I figured that it should be null for me (actually, the eigenfrequency analysis does give me a null eigen frequency in the list). Am I correct, so far ?
b) To reconstruct the frequency response at a given point, I use the complex value of the presure field given for each mode at this point. Is this correct as well ? I am not so confident about how these modes are normalized, though.
c) If all of the above is right (although, independantly of 2b), then I get a result different from COMSOL, but with the same type of inconsistency: the absissa of the peaks in the frequency response do not match the complex modulus of the eigen frequencies given by the first step of the study.

Any idea why ?

Thanks,
Sami