We present a theoretical and experimental realization of a thin multi-unit metasurface with multi-order sound absorption that exhibits a continuous near-perfect absorption spectrum in the broadband range of 450 Hz–1360 Hz. The metasurface unit is a perforated composite Helmholtz-resonator (PCHR) that is constructed by inserting one or more separating plates with a small hole into the interior of a Helmholtz resonator (HR). The multi-order sound absorption mechanism can be achieved so that with the original absorption peak and the structural size unchanged, multiple near-perfect peaks are obtained in higher frequencies by a PCHR unit. This extraordinary multi-peak performance is the result of the upgraded multi-degree-of-freedom system with the separating plates, which is explained well by the equivalent acoustic circuit. The specific absorption properties of the PCHR unit are investigated thoroughly with a theoretical approach similar to the plane wave expansion method, and verified via the finite element simulations. On this basis, by precisely balancing the parameters of each unit, the absorption bandwidth of the subwavelength 8-unit metasurface is dramatically broadened about 65% by the proposed mechanism. This work would offer a new guidance for the achievement of the wider absorption band and has great potential in engineering applications.

We present an ultra-broadband acoustic metamaterial with inhomogeneous high-order Fabry–Pérot (FP) resonances that achieves near-perfect continuous absorption in the range of 400–10000 Hz. The unit is composed of multiple FP channels with inhomogeneous cross-sectional areas, allowing for flexible adjustment of the impedance characteristics of each channel. This approach prevents the high-order peaks’ resistances from increasing rapidly and ensures a smooth resistance characteristic across a broad frequency range. As a result, the absorption performance in the high-frequency range above 3000 Hz can be significantly enhanced, eliminating the need for conventional porous materials. The inhomogeneous high-order absorption mechanism is investigated thoroughly by theoretical calculations and finite element simulations. By critically coupling the FP channels, a 27-cell broadband metamaterial is obtained with an average absorption coefficient above 90% over 400–10000 Hz, which is verified by experiments in a square impedance tube and an alpha-cabin reverberation room, respectively. Characterized by its extraordinary performance and easy-fabricated structure, this metamaterial has great potential in noise control engineering applications, especially in environments with low temperature, high humidity, or significant dustiness.