Mesh convergence in in acoustic module

Hello,

I know that for acoustic models, at least 5-6 mesh per wavelength is required. I have done a parametric sweep study which changes the mesh density from 2 to 20 mesh per wavelength. The problem is that the results do not converge(Attached figure). Any idea that what can cause this issue?

I have done convergence study before and I did not have this issue in the past. My model is based on the piezoelectric transducer and I want to measure pressure at different domains (which include air, water, custom defined material).


Appreciate any help.

Thanks,