An extremely-thin acoustic metasurface for low-frequency sound attenuation with a tunable absorption bandwidth

This study proposes an ultra-thin sound-absorbing metasurface for low-frequency sound absorption, which is composed of resonators with an embedded spiral neck and a coiling-up backing cavity. The analytical, numerical and experimental results show that the proposed metasurface can achieve excellent absorption (absorption coefficient being 0.98) at 180 Hz with an extremely thin thickness of 13 mm, i.e., about 1/145th of the operating wavelength. To broaden the narrow effective absorption bandwidth of the uniform absorber, multiple units with different geometric parameters are assembled in parallel. These units are designed to share the front covering plate for space-saving in the lateral direction. With the help of multiple resonances, the coupled units show promising absorption properties with multiple frequency bands. It is numerically and experimentally demonstrated that a dual-band low-frequency absorber and a wide-band one are achieved, and the absorption bands can be flexibly tuned by changing the designs of units. The features of excellent low-frequency noise absorption in an extreme deep-subwavelength scale and tunable broadband absorption characteristics make the proposed metasurface a promising potential for realistic broadband especially low-frequency sound attenuation applications.