A sound-absorbing metasurface with a multi-slit synergetic resonant effect was studied that can achieve broadband sound absorption in a range betwen 500 Hz and 1150 Hz by using a nested doll supercell with a thickness of only 44.1 mm (the thickness being one sixteenth of the wavelength). The basic unit of the improved micro-slit absorber (IMSA), made by designing the slit of the micro-slit absorber as a type of step and locating it at the edge of the surface panel, is proposed for sound absorption at lower frequencies. The improved step slit can not only change the structure’s impedance to match the medium of air at a lower frequency, but can also guide sound waves into the slit and increase the air velocity step by step, which results in a strong friction loss in the lower micro-slit. On this basis, a nested doll supercell with eight integrated parallel IMSAs was designed and tested, and was able to achieve
sound absorption in a range between 500 Hz and 1150 Hz. In addition, the bandwidth was further broadened (to between 330 Hz and 1150 Hz) by adding four additional IMSAs to the previous supercell, while the surface area of the panel only increased by about 20%. This work highlighted the potentially extensive application of broadband noise reduction due to its depth, i.e. subwavelength thickness, and the relatively small surface area of the panel.
Broadband sound energy enhancement is essential in practical scenarios, such as acoustic positioning and acoustic communication. In this paper, a dual anisotropic metamaterial composed of an inner Mie resonator and an outer acoustic grating is proposed, aiming to achieve enhanced broadband monopole emission and acoustic energy harvesting (AEH) via the coupling of the first and second monopole resonances. Considering thermo-viscous dissipation, numerical simulations and experimental results demonstrate that the dual anisotropic metamaterial can realize omnidirectional enhanced broadband monopole emission at 795 Hz–1511 Hz, the maximum sound pressure level (SPL) gain is 16.4 dB and the SPL gain fluctuation is 3 dB. Furthermore, simulation results reveal that the broadband AEH can be achieved by the dual anisotropic metamaterial, the fluctuation of the SPL gain at 794 Hz–1537 Hz is 3 dB and the maximum is 14.7 dB. Based on the results, the dual anisotropic metamaterial is expected to show significant potentials in acoustic positioning and acoustic communication.
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