聚焦学术前沿之“声学超材料”最新进展！

英文摘要：Acoustic absorbers based on single-order resonant mode of the resonators have been extensively investigated for low-frequency broadband absorption. However, they are usually limited to a certain range of working frequency bands and often do not work effectively in the high frequency range. Here, we present an acoustic metamaterial, perforated panel with tube bundles (PPTB) combined with coiled-up cavity, which effectively expands the operating bandwidth from low to high frequencies by utilizing the multi-order resonances. The PPTB ensures the efficient low-frequency absorption effect though the adjustment of tube diameter and length. The coiled-up prolongs the propagation path of sound waves, thereby facilitating the excitation of higher-order resonance modes at high frequencies. The multi-order resonances mechanism of the metamaterial is revealed thoroughly by theoretical calculations and finite element simulations. The results show that at the first-order peak, the energy dissipation mainly occurs in the tubes, while for the high-order peaks, the energy dissipation of coiled-up cavity gradually increases. Moreover, this work introduces the coupled-mode theory to determine the reasonably structural parameters, which enables to achieve the desired leakage and loss factors simultaneously, maintaining the higher muti-order absorption peaks in a wide frequency band and providing the wider single-order absorption peak in high frequency range. Utilizing multi-order resonances, a broadband metamaterial supporting an average absorption coefficient above 0.93 within 300 -3600 Hz and eliminating the tangible absorption dips is obtained, which is mutually verified by theory, simulation and experimentation. Owing to its lightweight, lesscomplicated structure, terrific acoustic and mechanical performance, this kind of metamaterial may have a broad application prospect in noise control engineering.

英文摘要：In this paper, a kind of laminated acoustic metamaterials (LAMS), which can realize the bandwidth widening and amplitude improvement through the parallel coupling of in-plane gradient parameters and the series laminated design in the thickness direction of the local resonance units, is proposed. The designed LAMS consists of two different layers of single-layer acoustic metamaterials (SLAMs) laminated in series in the thickness direction. During the design process of the two layers of SLAMs, non-uniform mass distribution and non-consistent frame coupling layouts are respectively introduced to broaden the sound insulation frequency band of the acoustic metamaterials. Through careful design, the two layers of SLAMs have distinct working frequency bands, effectively preventing the occurrence of sound insulation valleys with low amplitude. Based on numerical analysis of two layers of SLAMs, this study discusses the results of transmission loss (TL) and the working mechanism of interlayer superposition coupling sound insulation. The analysis of TL for LAMS reveals its capability to achieve ultra-strong sound insulation effects across a low-frequency broadband range. The accuracy of the numerical analysis in this study is verified through semi-analytical methods and small-scale impedance tube experiments. The LAMS proposed in this paper holds significant potential for wide-band noise reduction control in various high-noise mechanical equipment or workplaces.

英文摘要：The design concept of integrating locally resonant metamaterials with sandwich plates has demonstrated promising prospects in the development of lightweight, load-bearing structures endowed with excellent capabilities for noise and vibration attenuation. However, achieving low-frequency vibration attenuation in the locally resonant metamaterial sandwich plates remains a challenging task that frequently requires the inclusion of additional centralized mass or heavy local resonators. This study proposes a novel multi-bandgap metamaterial sandwich plate with the lever-type inertial amplification mechanism (LIA-MMSP) for achieving the lowfrequency vibration attenuation. Compared with the metamaterial sandwich plates incorporating multifrequency local resonators (LR-MMSP) with equivalent additional mass, the LIA-MMSP exhibits the ability to achieve lower-frequency multiple bandgaps. The theoretical dynamic model is employed to elucidate the underlying mechanism behind the generation of multiple bandgaps at lower frequencies in the LIA-MMSP. The vibration attenuation performances of the LIA-MMSP are analyzed through both the finite element method and experiment study. The effect of various parameters on the vibration transmission characteristics of the LIA-MMSP is studied. The results show that the boundary frequencies of the LIA-MMSP are precisely one of the lever ratios of the LR-MMSP. By altering the lever ratio within the LIA-MMSP, precise fine-tuning and optimization of the low-frequency multiple bandgaps are achievable. When the attached mass is constrained, increasing the lever ratio enables the achievement of lower bandgaps. In addition, as the eigenfrequency of the primary lever-type IA resonator fp and secondary lever-type IA resonator fs decrease, both the first attenuation zone (AZ1) and the second attenuation zone (AZ2) of the LIA-MMSP shift towards lower frequencies. However, as fp decreases, the width of AZ1 expands, and the minimum accelerations within the AZs decrease even further. Moreover, a normalized comparison provides validation of the exceptional performance of the proposed LIA-MMSP in terms of lightweight design, as well as its ability to achieve low-frequency broadband vibration attenuation.

英文摘要：The suppression of vibration and sound radiation of thin plates has always been a concern in engineering fields such as aerospace, transportation, etc. The rainbow effect refers to that the waves with different frequencies can be stopped in different positions, where the wave energy is also enhanced, as the result of the spatial slowing-down of wave velocities. This effect exhibits great potential in suppressing structural vibration and sound radiation, but corresponding studies have not been reported yet. In this paper, a meta-plate composed of ring-type unit-cells with gradient heights is proposed to generate the radial rainbow reflection effect. Upon analyzing the dispersion relationship of unit-cells and revealing the rainbow reflection mechanism, the broadband vibration and sound radiation performances of a square meta-plate with free boundaries are studied. Numerical and experimental results demonstrate that the proposed metaplate can suppress the vibration and sound radiation in broadband only with the extreme-low structural inherent loss. The wavenumber spectra of vibration velocity and supersonic intensity at typical frequencies including local and global resonant ones are analyzed to reveal the suppression mechanism of sound radiation, which is the vibration redistribution and systematic vibration reduction resulting from the rainbow reflection effect. Our design enhances the stiffness of plate structures and requires no additional damping, providing a novel approach for structural vibration and sound radiation suppression.

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