This study presents a novel acoustic metasurface with a coplanar series-type Helmholtz resonator with curled necks. Using extended necks that are embedded in cavities enables one to significantly reduce the resonance frequency of the resonator. The sound absorption performance of this metasurface is investigated theoretically, numerically, and experimentally. The absorption band can be easily adjusted by altering the neck length and diameter. To achieve a wide frequency band with high absorption, two inhomogeneous units are arranged in parallel. Theoretical and finite-element predictions exhibit good agreement with the experimental measurements. The results demonstrate that the proposed acoustic metasurface with subwavelength thickness is capable of effectively absorbing low-frequency sound.
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