Mismatch Between Bandgap Calculated by Eigenfrequency Study and Transmission Loss Spectrum from Frequency-Domain Study in the Solid Mechanics Module

Bir Bahadur Chaudhary

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I am using the Solid Mechanics module in COMSOL Multiphysics 6.3 to study in-plane elastic wave propagation in a periodic acrylic plate with holes. From the eigenfrequency study with Floquet periodic boundary conditions, the dispersion curve shows a bandgap between the 3rd and 4th bands, approximately from 133 kHz to 154 kHz.

To verify this bandgap, I performed a frequency-domain study on a finite structure with PML layers, I gave prescribed displacement of 1mm in x direction and calculated the transmission loss spectrum. However, the transmission results do not fully agree with the eigenfrequency analysis. The transmission loss is not consistently high throughout the predicted bandgap range, and noticeable attenuation also appears near 118 kHz, where no bandgap is observed in the dispersion curve.

Could you please explain the possible reason for this discrepancy? How can I determine whether the bandgap given by the eigenfrequency study is a true complete bandgap for in-plane elastic waves, and what is the recommended COMSOL procedure for validating it using a frequency-domain transmission loss analysis?



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