General Mechanical

General Mechanical

Acoustics Analysis changing materials

    • acbof


      I'm trying to analyze the influence of material change in sound transmission loss. For that, I tried to follow the orientation on this discussion, changing only the final step on Harmonic Acoustics analysis. That is, I used Modal analysis to get the vibration modes of a simple retangular board made of wood, then steel and then polyethylene. Once I had the inicial vibration modes, I used Harmonic Response to get the frequency response. Finally, I used these results as input to a Harmonic Acoustics analysis, with a Mass Source emiting sound over a board.

      My idea was to do this for all materials and see the changes in sound pressure level behind the board, and with this define the most efficient material to isolate sound. However, the results are pratically the same, so I don't think it actually worked. First, I thought it could be because the sound can go around the barrier, therefore it would be very similar for any material, but even on points right next to the board the results are the same. I don't know what could be the problem.

      (My version is the 2021 R2 Student)

    • Ashish Khemka
      Ansys Employee
    • Bill Bulat
      Ansys Employee
      Hi I'm not sure any of the following will make the results obtained by using different plate materials differ as you expect them to, but...
      I can't tell if you've imbedded the modeled computational domain within another solid. In open radiation problems, we immerse the computational domain within another domain meshed with PML (perfectly matched layers). This absorbs outgoing radiation without reflection back into the computational domain (to represent the surrounding environment which is assumed to extend infinitely away from the boundaries of the model). Without PML, the boundaries will be perfectly reflective, and things will get noisy in there.
      So, are you saying you calculated a frequency response in System B (your MSUP - mode superposition - harmonic analysis of the plate)? It sounds like you're exciting the plate (in-vacuo) then feeding the calculated surface velocities into the acoustic analysis (without consideration of how the presence of the acoustic medium might affect plate vibration) then adding a monopole pressure source in the acoustic medium. It isn't clear to me what that represents physically... I'm not sure what kind of scenario you're trying to posit.
      My guess is that what you want to do is posit radiation emanating from a monopole onto a plate that would otherwise not be subject to any excitation (the incoming radiation from the monopole source is the only source of excitation). For that, I would do a FULL (not MSUP) harmonic vibroacoustic analysis (use a single analysis system). FULL harmonic response analyses are a little more computationally demanding than MSUP ones.
      If different materials transmit sound differently, it may be due more to mechanical damping than stiffness. Maybe you can assign relevant material damping to steel/wood/polyethylene plates. In all three cases the calculated sound pressure level might be similar near the plate surface of the plate that faces the monopole (due to reflection). Where I'd look for differences in your three cases is near the plate surface that faces away from the monopole. Don't forget the PML!
      And have fun Bill

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