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Thermoviscous boundary layer impedance vs Narrow region acoustics
Posted 10 feb 2024, 15:35 GMT-5 Structural & Acoustics, Acoustics & Vibrations Version 5.3a 3 Replies
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Hi all,
Now I'm evaluating a loudspeaker with a bass-reflected tube since I noticed that without adding the losses, which are induced by the boundary layer, the result seemed to be too nice ( the boosted level is too exaggerated). Hence, I tried to add the losses to the tube, but here comes the problem:
the two ways to add the losses, which are thermoviscous boundary layer impedance and narrow region acoustics, gave different results, the thermoviscous boundary layer impedance showed a nice boosted SPL while the narrow region acoustics showed no boosting.
According to the tutorial from the COMSOL, I've checked that the boundary layer is thinner than the geometry dimensions, which means that the thermoviscous boundary is not overlapping together, thus the thermovsicous boundary layer impedance is usable, hence the thermoviscous boundary layer impedance and the narrow region acoustics should both be feasible.
Could someone explain the reason? I appreciate any help you can provide.
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Without seeing the model, it is difficult to say, as there could be some underlying issues. The short answer is to use Narrow Region as it is applicable in a more general sense, whereas for Bli you have an underlying assumption about the effects taking place on the surface (and there might also be some issues when curvature is present).
-------------------René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
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Hi René,
Thank you for the reply, I always learn a lot from the discussions with you no matter whether here or on Linkedin.
Indeed, I noticed the results of narrow region acoustics fit better than BLI through the experiments but still underestimated the damping effect, so the SPL was boosted in the simulation while the experiment did not.
But I am also curious that in my opinion, I thought the boundary layer of this port is much thinner than the geometry, I thought the narrow region acoustics and the BLI should give similar results at least, but the results seemed to don't.
So I wonder is there any theoretical explanation for the discrepancies?
Thank you for the kind reply.
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Without seeing the file, it is difficult to say. What I would suggest is that you do a study independent of your loudspeaker case, and see when the Bli breaks down, and whether it under- or overestimates compared to narrow region. There is analytical expression to compare to also, if the geometry is simple. Also, the Bli calculates a tangential derivative, and there could something numerical issue depending on your mesh.
-------------------René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com