An acoustic metaliner for ultra-broadband sound absorption

Emerging artificial acoustically soft boundaries (ASBs) have shown great potential for developing compact sound absorptive devices with excellent ventilation performance. However, current realizations of ASBs suffer from narrow-band limitations, which necessitate a stringent matching of resonant characteristics between dissipated meta-atoms and ASBs to achieve perfect absorption. In this study, we propose a paradigm to construct a broadband ASB (BASB) by coupling two multi-band ASBs with the help of coiled space resonators (CSRs) that possess multiple harmonically resonant states. We explore the modulation mechanism of CSRs using coupled mode theory. By elaborately hybridizing dissipated meta-atoms and the proposed BASB, we numerically and experimentally realize a sound metaliner that exhibits ultra-broadband absorption (267-1430 Hz) with high efficiency ( A > 90 %). Our design strategy overcomes the narrow-band limitation of conventional ASBs and enables more flexible and robust ventilated sound absorbing devices.