Design of a broadband metasurface sound absorber based on Hilbert fractal

In this paper, a new fractal-based broadband metasurface absorber is developed, which is based on the excellent space-filling ability of the Hilbert fractal. Each unit cell in absorber consists of a micro perforated panel (MPP) and a coplanar coiled cavity. In order to gain a deep insight into the sound absorption mechanism and perform a rapid design, a theoretical model for analyzing the sound absorption characteristics of the unit cell is established, in which the Fok function is used to account for the coupling effect between holes. Afterward, the absorption mechanism as well as the effects of parameter variations on absorption characteristics are investigated. To increase the space utilization, each unit cell is arranged in space according to the Hilbert fractal curve. Consequently, a metasurface absorber with 6 detuned unit cells is constructed. The multiple resonant cavities with dissimilar lengths can provide peak absorptions at multiple frequencies, thereby broadening the attenuation frequency range. Finally, the absorption performance of the designed absorber is obtained by theoretical calculations, finite element (FEM) simulations and experimental measurements, respectively. The experimental results show that a continuous absorption spectrum is achieved in the range of 855-1359 Hz with absorption coefficient above 0.8 under a deeply sub-wavelength thickness (22.2 mm). This study provides an effective way for the design of a space-limited broadband absorber. With the advantages of ultrathin thickness, broadband, and compactness, the developed fractal-based metasurface absorber has great potential in the field of noise reduction.