Tunable Sound Absorption via Evanescent‐Wave Coupling in Asymmetric Bilayer Metasurfaces

This study explores tunable sound absorption using a bilayer configuration of phase-gradient acoustic metasurfaces. By carefully adjusting the cavity length between two metasurface layers, the proposed system can modulate its acoustic response between highly reflective and perfectly absorptive states without changing the internal geometry of the unit cells. The underlying mechanism results from evanescent-wave coupling, which becomes significant at sub-wavelength cavity length and is strongly influenced by the phase gradient and integer parity of each metasurface. To analyze the scattering behavior of the bilayer system, an analytical model based on coupled-mode theory is developed, identifying the conditions that ensure both reflection and transmission are effectively suppressed. Theoretical predictions are validated by full-wave simulations using bilayer metasurfaces realized with space-coiling structures. The results demonstrate broadband tunability in sound absorption, with optimal configurations achieving an absorption coefficient exceeding 95%. Owing to its structural simplicity and high tunability, the proposed approach offers an effective solution for dynamic sound control in applications such as tunable noise barriers and reconfigurable sound-absorbing devices.