Nonlocal acoustic metasurface for ultrabroadband sound absorption

Classical meta-absorber designs usually have a tradeoff between bandwidth, efficiency, and thickness. Here, we introduce the concept of a nonlocal acoustic metasurface absorber by using a bridge structure connecting resonating unit cells to improve the performances of the meta-absorber. By utilizing the coupling effect between the adjacent unit cells, ultrabroadband sound absorption is achieved with deep-wavelength thickness. The physical mechanism of the nonlocal acoustic metasurface absorber is investigated and uncovered by developing analytical models. We theoretically and numerically study the nonlocal metasurface with a connecting bridge and the traditional metasurface without a bridge. The nonlocality can introduce three specific effects: the optimization of effective acoustic impedances, the shift of Fabry-Perot resonant frequencies, and the enhancement of the coupling effects between adjacent unit cells. These effects contribute to improve the bandwidth and the efficiency of the acoustic meta-absorber. We numerically and experimentally achieve an average absorption coefficient larger than 0. 9 within the ultrabroadband bandwidth extending from about 600 to 2600 Hz, with a metasurface of 6. 8 cm, viz. , /9 for the lowest frequency. Our finding demonstrates the advantage of nonlocal acoustic metasurface to conceive a subwavelength sound meta-absorber.