Eigen-frequency pressure reading

thachmonkeythachmonkey
Dear Victor,

I just bought the commercial license and right now I'm working in the acoustic 2D module. I would like to ask you about the pressure reading in each of the eigen modes. Is the pressure indicator (reading) really absolute or it is just relative?

To be clearer, can I compare between the pressure readings of two different modes to understand which mode is more irritating in a room?

Thank you very much Victor for your help!

Comments

  • VictorVictor
    Hello thachmonkey

    It's relative - the pressures are scaled by an arbitrary factor that's different for each mode. The actual amplitude would depend on the source of sound and the damping in the room, neither of which are modeled by Mecway.
  • h0inzh0inz
    Hello,

    is it possible to change those arbitray factors of all modes to the same value, so they become compareable?
  • thachmonkeythachmonkey
    Thank you Victor for your answer. I have quite the same question as h0inz which is if it is relative, why would the maximum pressure value for each eigenfrequency not the same (like 1 for example) so that we can compare?
  • VictorVictor
    The modes are mass normalized where the mass matrix is that of a material of 1kg/m^3 density. That's just the default that the eigenvalue solver produces and there's no compelling reason for it. Though I know or someone who used that fact to do their own additional processing of the output.

    I agree that normalizing to the maximum pressure or magnitude of pressure would seem less arbitrary but I'm not sure what that really helps. What would you be comparing it to?
  • thachmonkeythachmonkey
    You are right. There's nothing much to compare to but if I was to print out a whole bunch of eigen-mode-shape, the indicator is just somewhat confusing for the reader. Anyway, I was just being curious on the way the program came up with the value on the pressure indicator which had just been answered by you so thanks a lot.
  • h0inzh0inz
    "What would you be comparing it to? "
    Well, I would like to know which eigenmode has the strongest influence to the room.
    For example, a low frequency room response shows if one mode has a higher/lower pressure level than the others.
    Do you have any idea or tips, of how processing the output could accomplish something similiar?
  • VictorVictor
    A modal analysis without damping doesn't provide that - there's no non-arbitrary way to scale the modes that would preserve any information. You would need a steady state frequency response where you impose a driving frequency and see what the sound pressure reaches after transients have been damped away. I'm not sure what software does that but I'm sure it exists.

    Maybe there's some hint of it in "mode participation factor" or "effective modal mass". For structures, these quantities give some idea about how easily a mode is excited. Here's a description of how to calculate them:

    http://www.vibrationdata.com/tutorials2/ModalMass.pdf

    - m-hat, the generalized mass matrix is the identity matrix
    - phi is the vector of nodal pressures.
    - M, the mass matrix can be obtained from Analysis Settings -> Output Files -> Write mass matrix to file

    I'm not confident that will really give much insight but it could be worth a quick look.
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