Analytical modeling of evanescent coupling in metasurface absorbers for enhanced low-frequency sound control

This study presents an analytical approach to model evanescent coupling in planar metasurface absorbers, specifically designed for broadband low-frequency sound absorption. While traditional absorbers rely on thick, wavelength-comparable porous materials, metasurface absorbers with deeply sub-wavelength thickness typically achieve low-frequency absorption using arrays of resonators, such as Helmholtz resonators, folded quarter-wavelength resonators, or backed micro-perforated panels. Standard surface-impedance-based models of metasurface absorbers often ignore inter-resonator coupling effects, leading to inaccuracies in frequency response predictions. Our method incorporates evanescent wave interactions between resonators, whether rectangular or circular in cross-section, arranged in regular super-cells that can repeat periodically or with mirror symmetry, which also corresponds to one super-cell placed in a rigid-walled rectangular waveguide (impedance tube). This approach reduces computational complexity significantly compared to finite element simulations, while still enabling accurate predictions of metasurface absorbing performance. Validated through comparison with two numerical finite element models, this analytical method proves effective for optimizing metasurface absorbers for low-frequency sound control. Numerical experiments further illustrate performance degradation from neglecting evanescent coupling or mismatched super-cell periodicity. Implementation MATLAB code is available on https://github.com/MilanCervenka/Evanescent.