Reconfigurable modular acoustic metamaterial for broadband sound absorption

This study presents a reconfigurable acoustic metamaterial consisting of stacked multiple absorption modules, which can achieve efficient broadband sound absorption in both enclosed space and ventilation system. Each absorption module is composed of four Helmholtz resonators with embedded tubes, forming a square-shape structure with an air channel in the middle of the module. A theoretical model based on the impedance transfer method and a simulation model based on the finite element method were developed to investigate the sound absorption performance and the underlying working mechanisms of the metamaterial. Through the coupled resonance effects, each module exhibits a wide operating frequency band, and the absorption frequency range can be adjusted by modifying the module's geometrical parameters. And by stacking multiple modules, the acoustic metamaterial achieves an absorption performance that covers the operating frequency range of each module. Additionally, it was found that the interlayer coupling effects of modules significantly enhance overall absorption performance due to stepped well design in the center of the metamaterial. Moreover, by reconfigure the absorption modules, the acoustic metamaterial can be easily customized to achieve the desired absorption spectra. To illustrate the design concept, a metamaterial consisted of five-layer absorption modular is presented, with an average absorption coefficient of 0.92 within 450-2000 Hz range. Furthermore, the proposed metamaterial can be applied in different scenarios, i.e., with open and cloesed end. In the case where a rigid backing is placed at the bottom, the metamaterial can be applied as a sound absorber and its absorption bandwidth can be broadened to 430-2600 Hz by incorporating a porous material liner within the stepped well. In another case where the bottom of the stepped well is left open, the metamaterial can be used as a ventilation barrier, allowing airflow circulation and suppressing up to 90 % of sound energy within the 650-2400 Hz range. Therefore, not only does the proposed acoustics metamaterial design realize structural reconfigurability, but it also suggests a robust solution to low frequency noise-control that caters to diverse noise reduction requirements.