Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11804278, 11174234, 51272215) and the Fundamental Research Fund for the Central Universities, China (Grant No. G2017KY0105).
Received Date:26 October 2018
Accepted Date:02 December 2018
Available Online:01 February 2019
Published Online:05 February 2019
Abstract:Low frequency noise is always an important factor affecting people’s quality of life. At present, the most widely used sound absorbing materials include polyurethane foam, trimeric amine, mineral cotton, textiles, cotton and special sound insulation materials. However, the sizes of these materials are generally large, and the sound absorption efficiencies are often low, especially in a low frequency range (below 2000 Hz). Acoustic metamaterial is a kind of artificial composite material, which is constructed by microunits whose dimensions are much smaller than the working wavelength. The results show that if the strong coupling condition between the resonant scatter and the waveguide is satisfied, the sound energy flowing through the metamaterial will be completely offset by the internal loss of the resonant scatter. Therefore, it is believed that this kind of acoustic metamaterial can solve the absorption problem of low-frequency sound waves. In order to solve this problem, researchers have conducted a lot of exploratory researches. However, most of the structural units that are constructed with acoustic metamaterials are passive, that is, once the material is processed and shaped, its properties are fixed and cannot be changed. This defect greatly limits the development of acoustical metamaterials, so it is urgent to study acoustical metamaterials whose material properties and the working frequency bands are flexibly adjustable. Although tunable acoustic metamaterials have been studied, few people have extended this research to the field of low-frequency tunable sound absorption. In our previous work, we systematically studied the acoustic properties of two kinds of acoustic artificial " meta-atoms”, namely, open hollow sphere model with negative equivalent elastic modulus and hollow tube model with negative equivalent mass density. The research shows that these two kinds of " meta-atoms” both have obvious sound absorption effect. According to our previous studies, in this paper we couple these two kinds of " meta-atoms” into a whole, and design a new nested model of open loop. The model has the advantages of simple structure and easy preparation. Through theoretical analysis, numerical simulation and experimental testing, it is found that the strong coupling resonance effects between these " meta-atoms” can be excited by the low frequency incident acoustic wave in the nested structure, thus achieving nearly perfect sound energy absorption. In addition, the relative impedance of the metamaterial can be changed by simply rotating the inner splitting ring around the axis, therefore the position of the absorption peak can be freely controlled in a wide frequency band. Because of its deep sub-wavelength size, the metamaterial is very useful for miniaturizing and integrating the low-frequency acoustic absorption devices. What is more, this model also lays a foundation for designing the broadband absorbers. Keywords:tunable/ acoustic metamaterial/ low frequency/ absorber
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2.模型分析设计可调声学超材料模型的重点是找到其共振频率与结构参数之间的关系, 并通过改变结构参数来改变其共振频率, 进而实现频率可调的目的. 对于一个局域共振型的声学超材料, 当入射声波的频率接近其共振频率时, 流体介质的黏滞损耗、材料的摩擦损耗和阻尼损耗会使该结构对入射声波产生强烈的吸收[1]. SHS作为一种声学“超原子”是典型的亥姆霍兹共振器结构[31], 其二维模型如图1(a)所示, 图中的蓝色箭头表示入射声波的传播方向, 蓝色虚线箭头表示声波在腔体中传播的路径. 根据等效媒质理论和等效电路原理, 该结构的内部空腔部分可以被看作是一个等效电容C0, 而开口的颈部可以被看作是一个等效电感L0, 两者串联, 如图1(b)所示. 两者与SHS结构参数之间的关系为 图 1 可调声学超材料的模型设计 (a), (b)二维SHS的结构示意图和等效电路图; (c), (d)二维HT的结构示意图和等效电路图; (e), (f)SHS和HT耦合后的结构示意图和等效电路图; (g)进一步变形优化得到的可调声学超材料模型的结构示意图 Figure1. Model design of the acoustic metamaterial: (a), (b) Schematic diagram and equivalent circuit diagram of the two-dimensional SHS; (c), (d) schematic diagram and equivalent circuit diagram of the two-dimensional HT; (e), (f) schematic diagram and equivalent circuit diagram of the coupled structure of SHS and HT; (g) schematic diagram of the tunable acoustic metamaterial obtained by the deformation and optimization of the coupled structure.