1.State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China 2.State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University, Xi’an 710049, China 3.Department of Function and Structure, Shanghai Aircraft Design and Research Institute, Shanghai 201210, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11502110, 11972185) and the Open Fund of the State Key Laboratory for Strength and Vibration of Mechanical Structures, China (Grant No. SV2018-KF-01).
Received Date:28 June 2021
Accepted Date:10 August 2021
Available Online:30 August 2021
Published Online:20 December 2021
Abstract:For realizing the effective broadband insulation of sound at low frequencies, a novel local resonant acoustic metamaterial plate having quasi-zero stiffness is proposed. Based on the classical mass-spring local resonance model, a metastructure is constructed by introducing additional inclined springs with negative stiffness. First, the normalized equivalent stiffness of the quasi-zero stiffness structure is derived from the perspective of dynamics. Then, by employing the method of equivalent medium, a sound insulation model of the metastructure is established theoretically. For validation, numerical simulations as well as experimental measurements are carried out. It is demonstrated that in the positive (equivalent) stiffness regime, increasing either the stiffness ratio or pre-compression can significantly reduce the local resonance frequency of the metastructure, which exhibits the great insulation performance around the local resonance frequency. For a typical example, the proposed metastructure can achieve a transmission loss of 30 dB around 10 Hz. In contrast, within the negative stiffness regime, the metastructure does not exhibit local resonance, thus avoiding sound insulation valley caused by the “coincidence effect”. Compared with traditional materials or similar metamaterials, the proposed metastructure has significant advantages in sound insulation (e.g. more than 30 dB drop over a wide frequency band of 53-1500 Hz). By analyzing the equivalent mass density, reflection coefficient, and acoustic impedance ratio of the metastructure, the physical mechanism behind its superior insulation performance is further explored. The equivalent mass density changes from positive to negative and tends to infinity at the insulation peak. The insulation peak is attributed to a nearly perfect total reflection of sound wave caused by impedance mismatch, while the insulation valley is caused by low-frequency “coincidence effect” originating from the local resonance band gap. The using of the quasi-zero stiffness local resonance to achieve low-frequency broadband sound insulation overcomes the disadvantages of traditional metamaterials such as reduced stiffness or additional mass, thus becoming vastly attractive for constructing low-frequency broad band sound insulation structures. Keywords:metamaterial/ local resonance/ quasi-zero stiffness/ low frequency range/ sound insulation
全文HTML
--> --> --> -->
2.1.准零刚度局域共振结构
如图1所示, 本文提出一种准零刚度局域共振型声学超材料板, 由方形金属薄板(铝板)及周期分布其上的准零刚度谐振单元组合而成. 其中, A为铝板, 厚度h = 4 mm, 元胞尺寸a = 25 mm; B为准零刚度谐振单元的外围框架, 材质为环氧树脂, 尺寸H = 8 mm, l 2 = 2 mm, 壁厚t = 1 mm; C为质量单元, 质量m = 0.1 kg, 材质为铅, 尺寸l 1 = 3 mm; D为弹簧(包括竖直弹簧和倾斜弹簧, 刚度分别为k1 = 10000 N/m和k2 = 8000 N/m), 弹簧端部与质量块和框架、框架与基板均采用胶接的方式连接. 无外力作用下, 取系统的平衡态位置为初始静平衡位置, 此时, 倾斜弹簧处于水平位置, 其压缩后的长度为d, 原长为l, 竖直弹簧的压缩力与质量块重力相平衡. 声学超材料板的相关材料参数(密度、杨氏模量和泊松比)列于表1. 图 1 准零刚度局域共振结构[25] (a)结构示意图; (b)单胞结构; (c)主要几何参数; (d)准零刚度单元俯视图 Figure1. Quasi-zero stiffness local resonance structure[25]: (a) Schematic of whole structure; (b) unit cell; (c) main geometric parameters; (d) top view of quasi-zero stiffness element.
由(2)式和(3)式可见, 准零刚度系统归一化恢复力和等效刚度均与结构力学参数密切相关, 图2给出了归一化恢复力和等效刚度随预压缩量和刚度比的影响规律曲线. 具体讨论如下: 图 2 弹簧刚度比保持不变($\gamma = 0.4$), (a)归一化恢复力和(b)等效刚度随弹簧预压缩量的变化趋势; 弹簧预压缩量保持不变($\bar \delta = 0.2$), (c)归一化恢复力和(d)等效刚度随弹簧刚度比的变化趋势 Figure2. (a) Normalized reacting force and (b) equivalent stiffness plotted as functions of pre-compression of springs for $\gamma = 0.4$; (c) normalized reacting force and (d) equivalent stiffness plotted as functions of spring stiffness ratio for $\bar \delta = 0.2$.
表2三种声学板结构的结构参数 Table2.Structural parameters of the three structures.
图4给出三种板结构的隔声曲线, 可见本文理论模型(equivalent medium method, EMM)结果与数值仿真(finite element method, FEM)结果非常符合, 验证了理论模型的正确性. 对比三种结构的隔声效果发现, 基体参数保持一致的情况下, 准零刚度超材料板和传统弹簧振子板在低频段的隔声效果更好, 前者还可显著降低局域共振频率. 预压缩量调节为$\bar \delta = 0.2$时, 准零刚度超材料板的局域共振频率为22.5 Hz, 较传统弹簧振子板(50 Hz)减小了一半, 故更具优势. 图 4 三种声学板结构的传输损失: 理论预测与数值模拟对比 Figure4. Comparison of sound transmission loss among three different plate structures: Analytical model prediction versus numerical simulation.
为进一步分析准零刚度超材料板的隔声机理, 图5(a)给出三种声学板结构的等效质量面密度. 特定频率下, 相较于无振子板, 准零刚度超材料板和传统弹簧振子板均可显著增加系统的等效动态质量面密度, 进而实现低频段的超常隔声效果. 图5(a)中蓝实线显示, 在22.5 Hz附近(图4中的隔声峰处), 准零刚度超材料板的等效质量面密度趋于无穷. 结合(6)式可知, 此时入射声激励频率与准零刚度谐振单元的固有频率一致(即$1 - {\omega ^2}/\omega _0^2 \to 0$), 局域共振单元发生共振. 振子在平衡位置上下往复剧烈振动, 导致声能集中于谐振单元. 此时, 结合图5(b)三种结构的反射系数曲线可知, 准零刚度超材料板的反射系数趋近于1, 声波呈现近乎完美的全反射, 和图6(a)的声压云图反映一致. 在局域共振处, 图5结果还显示等效质量面密度趋于无穷, 且由正变负. 负的等效质量密度表明其具有偶极子振动模式(见图5(a)), 此时弹簧振子相当于一质量块, 局域共振时, 质量块与薄板反相位振动, 系统总的外力与总的加速度反相, 故产生负的等效质量密度. 从振幅上看, 图5(a)表明在局域共振附近, 振子的振幅相比薄板更大, 吸收了基板的能量, 从而表现出超乎寻常的隔声效果. 图 5 三种板结构对比 (a)等效质量面密度和超材料板的偶极子振动模式; (b)反射系数 Figure5. Comparison among three different plate structures: (a) Equivalent mass surface density and dipole vibration mode of QZS; (b) reflection coefficient.
表3准零刚度单元在正等效刚度区的调节参数 Table3.Selected values of spring stiffness and pre-compression in the regime of positive equivalent stiffness.
图 7 正刚度区传输损失对比 (a)刚度比的影响($\bar \delta = $$ 0.2$); (b)预压缩量的影响($\gamma = 0.4$) Figure7. Comparison of transmission loss versus frequency curves in positive equivalent stiffness zone: (a) Influence of stiffness ratio at $\bar \delta = 0.2$; (b) influence of pre-compression at $\gamma = 0.4$.