摘要
声学超材料,是一种用于操纵声波的人工声学结构,具有诸多自然界所不具备的超常物理性质.不同于早期的局域共振声学单元结构,基于希尔伯特分形曲线提出了一种具有低损耗、高折射率的分形声学超材料,给出了分形结构声学单元的进化过程,并运用等效媒质理论,提取了具有亚波长的分形超材料单元的相对折射率、相对阻抗、等效密度及等效模量等材料参数.由于分形结构具有的自相似性和空间折叠特性,所提出的分形超材料具有宽带工作频率响应,为众多应用场合比如声超分辨率成像及声隧穿效应器件提供了候选材料.为检验分形声学超材料特性,应用分形声学超材料设计了梯度折射率分布的声聚焦透镜,并由固体方块阵列仿真对比、实验测量、以及点声源入射等三种手段验证了分形声学超材料参数特性及其聚焦透镜在2~5kHz宽频带内良好的聚焦能力.
Acoustic metamaterials are artificial structures which can manipulate sound waves through their unconventional effective properties.Different from the locally resonant elements proposed in earlier studies,we utilize Hilbert fractal to realize acoustic metamaterials with both low loss and large refractive indices and explain how varying degrees of parameters expected can be engineered through the FAM（fractal acoustic metamaterials）design.To demonstrate the performance of the FAM elements,the effective acoustic parameters are calculated through a retrieval procedure based on commercial finite element analysis.Due to the self-similar properties of the proposed structure,broadband acoustic responses may arise within a broad frequency range,making it a good candidate for a number of applications,such as super-resolution imaging and acoustic tunneling.To validate the performance of the FAM,we design,fabricate,and measure a two-dimensional flat acoustic lens using FAM elements.The flat acoustic lens is simulated and experimentally verified,showing excellent focusing abilities from 2kHz and 5kHz in the measured results.As compared,we design a periodical array of square solid blocks by replacing the FAM elements in the same location yof each element in the lens.The every square solid block has the same size and material parameters as thatof the respective FAM elements at the same position.For further validation,incident sound source is replaced by a point sound source,cylindrical wave front can be converted to plane wave front through the FAM flat lens.Our new strategy may offer an alternate route to the design of novel materials and devices in acoustic engineering in the future.
作者
宋刚永
黄蓓
宋海
程强
Song Gangyong Huang Bei Song Hai Cheng Qiang(State Key Laboratory of Millimeter Wave, Southeast University, Nanjing, 210096, China North Information Control Group Co. , Ltd, Nanjing, 211106, China)
出处
《南京大学学报（自然科学版）》
CAS
CSCD
北大核心
2017年第1期61-68,共8页
Journal of Nanjing University（Natural Science）
基金
国家自然科学基金(61571117
61138001
61171024
61171026)
关键词
声学超材料
分形结构
聚焦透镜
渐变折射率
acoustic metamaterials
fractal structure
focusing lens
Gradient refractive index
