Broadband sound absorption using hybrid resonators with embedded necks and micro-perforations in parallel

This study proposes an acoustic metasurface composed of Helmholtz resonators coupled with micro-perforations to achieve perfect sound absorption with a low Q-factor. A key feature of the metasurface is the parallel arrangement of an embedded neck and a micro-perforated panel within a single unit cell. This geometrical configuration facilitates the generation of hybrid resonance, leading to perfect absorption at the target frequency, in which the metasurface dimensions are significantly smaller than the corresponding wavelengths. To guide design choices, a theoretical model is introduced to outline the frequency-dependent effective impedance of the resonators, thereby aiding in specifying the geometric parameters required for impedance matching with the ambient air. Experimental validation for a unit cell fabricated using threedimensional printing confirms the efficacy of the metasurface at a target frequency of 480 Hz. Supercells comprising multiple resonators are explored to extend the absorption bandwidth, and absorption coefficient exceeding 0.9 over a two-thirds octave range (472-739 Hz) is achieved with a supercell of subwavelength thickness lambda/12. A machine learning algorithm based on artificial neural network is applied to further optimize the design. Using the theoretical model as a foundation, these techniques facilitate the identification of an optimal metasurface design, achieving a 90 % absorption band over an octave range (380-790 Hz) with a thickness of lambda/15. The metasurfaces introduced herein offer space-efficient alternatives to conventional soundabsorbing materials and hold promise for applications in noise reduction and architectural acoustic design.