composed of an elastic membrane and rigid disks, can absorb nearly the whole incident sound energy at certain frequencies and their thicknesses are even less than the peak absorption wavelength by two orders of magnitude. Nevertheless, it is susceptible to the mechanical damage because of the soft membrane. The coiled-up space metamaterials [9][10][11][12][13][14][15][16][17][18], which can achieve extreme acoustic absorption performance by increasing the sound path, are another significant type of the acoustic metamaterials. The structure presented by Li et al can absorb the sound with a thickness of 1/223 of the wavelength [9]. However, most of these metamaterials can only obtain good absorption performance within a narrow frequency band AbstractWe 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.

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We propose a multi-order Helmholtz metamaterial with deep-subwavelength thickness in which perfect continuous acoustic absorption is achieved within 400 Hz ∼ 2800 Hz. The metamaterial is composed of multiple detuned cells, each of which is constructed by several perforated plates inserting into the cavity of a Helmholtz resonator (HR) and hence gains multiple individually-tuned high-order peaks besides the original HR peak. By precisely designing each peak of the cells, the extra-broadband perfect absorption can be obtained. This kind of metamaterials could possess broad applications in noise control engineering owing to the extraordinary absorption performance and high flexural stiffness.

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