A mosquito-coil-like acoustic artificial structure consisting of a spiral channel and a perforated plate with excellent impedance matching is proposed, which can realize strong sound absorption within a certain frequency range. Due to the difficulty in matching the impedance of the single-hole structure with that of the sound propagation medium, the sound absorption should be poor. To overcome this shortcoming caused by the mismatched impedance, some multi-hole microstructures are designed. Moreover, since single-chamber labyrinth can only achieve single-frequency perfect sound absorption, a labyrinthine channel is divided into several chambers with each length distributing by an arithmetic progression gradient. The sound absorption bandwidth can be extended by synergetic coupling resonance among multiple chambers. By selecting different structural parameters including the number of holes, the width of the labyrinthine channel, and the depth of labyrinthine channel, sound absorption of these mosquito-coil-like structures is investigated. The results suggest that the multi-hole structures are helpful in improving the impedance matching, while the synergetic coupling resonance among multiple chambers ensures that the sound absorption coefficient of the structure can be maintained at a high level within a certain frequency range. In addition, some mosquito-coil-like sound absorption structures are fabricated by 3D printing, then the sound absorptions under vertical sound incident conditions are measured, and the strong sound absorption ability in a wide band is experimentally demonstrated. Finally, a method is proposed for adjusting the sound absorptions by proportionally zooming in or out the structure, by which the sound absorptions of the acoustic structure can be effectively shifted to lower or higher frequencies.
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.
An extreme anisotropic metamaterial consisting of one central cavity, eight zigzag and straight channels is proposed, aiming to achieve acoustic emission enhancement and self-centering effect. By placing a monopole source in the center of the metamaterial, acoustic emission enhancement can be achieved through the resonance in the zigzag channels and the monopole resonances. Theory and simulation confirm the self-centering effect of the proposed metamaterial, that is, when monopole sources are placed away from the center of the metamaterial, the external sound field can still be regarded as a uniform sound field generated by a monopole source placed in the center.
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