膨胀腔类超材料的传递损失

声学超材料及结构已被广泛研究,其超结构通常利用3D打印技术实现,当结构刚度较小或者面积较大时,由声固耦合所导致的声学效果与设计不符的情况广泛存在。该文针对含有膨胀腔类的超材料,研究了声固耦合对其声学性能的影响,采用有限元计算结合阻抗管实验的方法,得到其传递损失,分析了声固耦合现象对传递损失的影响。结果表明:薄壁膨胀腔结构的作用频率范围与只考虑声学理论计算的设计不符,声固耦合现象对传递损失产生显著影响;采用增加膨胀腔壁厚、减少膨胀腔内径或选择金属材料的方式,都可以使得声固耦合现象对传递损失的影响减小;仿真结果与实验结果基本吻合。对于膨胀腔类超材料,当刚度较小或者面积较大时,对其进行声固耦合分析是完全必要的。     

声学超材料对低频噪声的消声特性

针对低频噪声较难消除的问题,设计了亥姆霍兹共振腔与声学超材料薄膜耦合的消声结构,在利用有限元软件进行屈曲分析薄膜的临界状态得知声学超材料薄膜结构临界失稳力为0.087 N·m,利用COMSOL声固耦合模块研究薄膜形态对传递损失峰值频率的影响。结果表明:薄膜扭转角度由0°增加到30°时,薄膜总体刚度增加,传递损失峰值对应频率向右偏移了30 Hz,变化并不明显。为了扩大频率偏移范围,在扭转30°的基础上,对扭矩棒施加垂直向下的压力,压力由0 kPa增加到2 kPa,薄膜预应力增大,系统刚度增加,使得传递损失峰值向右偏移了170 Hz。最后搭建实验平台,验证了薄膜在扭转时的频率偏移与仿真基本吻合,在不同压力时频率偏移一致,进而可以实现较大范围的低频率噪声控制。为声学超材料的设计和控制提供有效的依据。     

平面声波由空气经加肋板向水中传输的数值计算研究

采用有限元法和边界元法计算研究了平面声波由空气经加肋板向水中的声传输。数值计算表明：板的刚度对声传输的影响只有在板的刚度较大时才是重要的，当板的刚度较小时，空气和水之间的声阻抗失配对声传输起支配作用；板厚或肋骨惯性矩的变化会引起结构传声损失曲线上隔声低谷位置的变化；增大板厚或肋骨惯性矩可增大结构的传声损失，特别是当激励频率低于结构基频时，可通过增大板厚或肋骨惯性矩来增大结构刚度进而明显增大结构的传声损失。     

声子晶体和声学超构材料

声子晶体和声学超构材料进一步拓展了自然界中声学材料的弹性波性质．这种人工的复合结构材料,由于其周期结构的布拉格散射和局域共振特性,使得其具有奇异的色散特征,在某些频段具有负的有效弹性参数,带来了许多新颖的声学传播效应,例如声子带隙效应、负折射效应、超棱镜效应、超透镜效应、异常透射效应、异常隔声效应等．与此同时,在声子晶体和声学超构材料表面,一类具有亚波长特性的声表面倏逝波也引起了人们的关注,研究其激发、传播、耦合的过程对揭示声子晶体和声学超构材料的奇异声传播效应的物理本质具有重要意义．声子晶体和声学超构材料作为一类新型的人工声学结构材料,在隔声、防振、热控制以及新型声学器件研发等方面具有巨大的应用前景．文章综述了近十几年来国际国内关于声子晶体和声学超构材料的研究进展,并对其未来的研究发展方向做一评述．     

管道及消声器学特性的边界元法计算

从流体力学的基本方程出发，导出等截面管内具有平均流和线性温度梯度时的三维声传播方程，然后采用迭代法将其化成可用边界积分方程表示的形式，最后用边界元法求解，由边界元法计算消声器的四极参数，从而可预测传递损失等声学特性，文中计算了膨胀腔内传递损失，并与一维理论结果进行了比较。     

基于多腔型超构材料的声场增强效应

构造了一种多腔型基本单元,由该基本单元构成的声学超构材料能够实现声场增强效应.此功能的实现是由基本单元的声腔和系统结构之间的相互耦合作用产生的单极子Mie共振引起.本文通过对多个基本单元进行不同形式的排列组合构造了对称型超构材料和非对称型超构材料,这两类超构材料可用于实现不同效果的声场增强.研究表明,由于对称型超构材料结构的高度对称,其声场增强效应的实现不受入射声波方向的影响;而非对称型超构材料的声场增强效应具有较强的方向依赖性,声波从不同侧入射时,超构材料对声场的增强效果也不同.本文关于这两类超构材料的研究将在隔声、声传感器、声通信、非对称性声学器件方面具有潜在的应用前景.     

微穿孔板复合板型声学超材料的低频吸声

针对单层微穿孔板的低频吸声问题提出了微穿孔板复合板型声学超材料结构。将板型声学超材料置入微穿孔板结构的背腔内部实现结构复合。实验结果表明:在相同背腔厚度下,复合结构的吸声性能整体优于单层微穿孔板结构,其中复合结构的吸声曲线从396~892 Hz均大于0.6,在453 Hz处吸声系数达到0.972。利用有限元方法对复合结构进行了仿真,仿真计算的吸声曲线与实验吸声曲线的趋势基本相同,同时发现低频吸声主要由板型声学超材料与声波相互作用贡献。板型声学超材料的吸声峰值的对应频率处,其等效动态质量密度从正变负。在复合结构内部的微穿孔板和板型声学超材料存在相互耦合作用,使得复合结构的第一峰值发生微小偏移。增加板型声学超材料的质量块重量可以使第一吸声峰值向低频移动;保持总背腔厚度不变,增加板型声学超材料的子腔厚度,也可以使第一吸声峰向低频移动。      

陆架斜坡海域上坡波导环境中声能量急剧下降现象及其定量分析

深度较浅的声源其辐射声波在陆架斜坡海域上坡传播时,在斜坡顶端会出现声能量急剧下降现象.利用射线声学模型分析了造成这一现象的原因,并根据抛物方程声场模型计算的深海和浅海平均传播损失定义了"声能量急剧下降距离",定量分析了声源位置对该现象的影响.结果表明:声源深度对"声能量急剧下降距离"影响较大,而声源与斜坡底端水平距离对其影响较小;当声源深度变大时,部分掠射角较小的声线最终能够达到斜坡顶端,致使"声能量急剧下降距离"增大,继续增加声源深度,将导致上坡声能量急剧下降现象消失.利用抛物方程声场模型对陆架斜坡海域上坡声传播进行数值仿真,结合"声能量急剧下降距离"的定义,计算并比较了声源位置不同时该距离的变化,数值计算结果验证了理论分析.     

具有平均流的膨胀腔声学特性的三维边界元分析

本文将边界元法应用于具有平均流的管道及膨胀腔声学特性的三维分析，获得了具的平均流介质中声传播问题的边界积分方程和基本解。采用九节点二次等参单元离散边界表面并对物理量插值，对奇异积分采用极坐标变换法和间接法联合来消除奇异性，在棱边角点处区分不同方向的质点振速。文中对有无平均流时直管的四极参数及膨胀腔的传递损失进行了计算，并与一维理论及其它方法计算结果进行了比较。     

边界条件对介电量子波导中声子输运性质的影响

运用散射矩阵方法，研究了Neumann边界条件和Dirichlet边界条件在低温下对结构不连续的纳米结构中的声学声子输运系数的影响.数值结果表明，当边界条件不同时，声学声子输运系数会有极大的不同；在一定的结构条件下，由于声子模与模的耦合作用，出现了共振透射和禁止频带.     

Theoretical investigation of the sound attenuation of membrane-type acoustic metamaterials

Membrane-type acoustic metamaterials have been recently shown to exhibit good performance of sound attenuation in a low frequency range. An analytical approach for the fast calculation of sound transmission loss of the membrane-type acoustic metamaterials is presented here. The discussion indicate that the first transmission loss valley and the transmission loss peak depend strongly on the attaching mass, while the second transmission loss valley is mainly influenced by the membrane properties. The effects of membrane tension and mass position on the transmission loss and characteristic frequencies are also discussed in detail.     

Simulations of the scattering of sound waves at a sudden area expansion

The scattering of acoustic plane waves at a sudden area expansion in a flow duct is simulated using the linearized Navier–Stokes equations. The aim is to validate the numerical methodology for the flow duct area expansion, and to investigate the influence of the downstream mean flow on the acoustic scattering properties. A comparison of results from numerical simulations, analytical theory and experiments is presented. It is shown that the results for the acoustic scattering obtained by the different methods gives excellent agreement. For the end correction, the numerical approach is found superior to the analytical model at frequencies where coupling of acoustic and hydrodynamic waves is significant. A study with two additional flow profiles, representing a non-expanding jet with an infinitely thin shear layer, and an immediate expansion, shows that a realistic jet is needed to accurately capture the acoustic–hydrodynamic interaction. A study with several different artificial jet expansions concluded that the acoustic scattering is not significantly dependent on the mean flow profile below the area expansion. The constructed flow profiles give reasonable results although the reflection and transmission coefficients are underestimated, and this deviation seems to be rather independent of frequency for the parameter regime studied. The prediction of the end correction for the constructed mean flow profiles deviates significantly from that for the realistic profile in a Strouhal number regime representing strong coupling between acoustic and hydrodynamic waves. It is concluded that the constructed flow profiles lack the ability to predict the loss of energy to hydrodynamic waves, and that this effect increases with increasing Mach number.     

A finite element method for damped acoustic systems: An application to evaluate the performance of reactive mufflers

The approximate equations governing the forced harmonic motion of a damped acoustic system are set up by using a variational principle. Acoustic finite elements are then used in a computer program to study the transmission loss and insertion loss performance of some expansion chamber mufflers. The manner in which the equations are set up allows a number of input and output nodes, and two-dimensional effects involving the influence of transverse acoustic modes to be taken into account. Although only the simplest of elements and coarse mesh sizes are used the resulting accuracy of the solutions is extremely good; thus the method should be a viable one for studying the performance of more complicated mufflers, having variable cross-sections and internal energy dissipation.     

Experimental and calculated research on a large band gap constituting of tubes with periodic narrow slits

A new band gap structure composed of a square array of parallel steel tubes with narrow slits is presented. The propagation of acoustic waves in a two-dimensional composite medium constituted of slit tubes in air is investigated both theoretically and experimentally. The band gap is calculated with the finite element method in which the acoustic-solid coupling is taken into account. The transmissions of the band system with both different single-width narrow slits and multi-width narrow slits are analyzed. Experimental measurements show that the transmission through an array of slit tubes with periodic narrow slits drops to noise level throughout frequency interval in good agreement with the calculated forbidden band. The large band gap and low starting frequency is obtained by arranging different width of slits embedded in the tubes. The experiments and theoretical results show that this new band gap structure has an especial character based on the resonant cavity playing an important role on the band gap besides the traditional Bragg interference.     

Theoretical model and analytical approach for a circular membrane–ring structure of locally resonant acoustic metamaterial

Recently developed locally resonant acoustic metamaterials (LRAM) display useful sound attenuation properties over narrow frequency bands. In this study, we present a theoretical model and analytical approach to investigate transmission loss of a circular membrane–ring structure of LRAM. As a degeneration of the ring inner radius being zero, the present model and approach is also suited for acoustic response analysis of the membrane–central-mass structure. Analytical results are compared with the ones from the finite element method to show a quite good agreement. The transmission loss characteristics dependence on the material and geometrical properties of the membrane–ring structure are obtained and discussed. It is indicated that multi-peak transmission loss profile can occur in the membrane–ring structure of LRAM, while the peak transmission loss frequency and bandwidth can be tuned by varying the ring mass location, surface density and ring numbers on the membrane.     

基于声学超材料的低频可调吸收器

在当今社会,噪声污染已经成为人类健康的一大威胁,如何有效地控制和消除噪声污染一直是科研领域的一个重要话题.本文以开口环嵌套结构为模型,设计并制备了一种声学超材料.通过理论分析、数值模拟和实验测试,发现由于模型内部空腔的强烈耦合共振效应,该超材料可以在低频区域实现接近完美的吸声效应.此外,通过简单地绕轴旋转其内腔开口方向,即可改变该超材料的相对阻抗值,进而在较宽的频带范围内实现对吸收峰位置的可调控制.由于该超材料具有深亚波长的尺寸,因此非常有利于低频吸声器件的小型化和集成化,同时该模型也为宽带吸收器的设计奠定了基础.     

Collision enhanced hyper-damping in nonlinear elastic metamaterial

Nonlinear elastic metamaterial, a topic which has attracted extensive attention in recent years, can enable broadband vibration reduction under relatively large amplitude. The combination of damping and strong nonlinearity in metamaterials may entail extraordinary effects and offer the capability for low-frequency and broadband vibration reduction. However, there exists a clear lack of proper design methods as well as the deficiency in understanding properties arising from this concept. To tackle this problem, this paper numerically demonstrates that the nonlinear elastic metamaterials, consisting of sandwich damping layers and collision resonators, can generate very robust hyper-damping effect, conducive to efficient and broadband vibration suppression. The collision-enhanced hyper damping is persistently presented in a large parameter space, ranging from small to large amplitudes, and for small and large damping coefficients. The achieved robust effects greatly enlarge the application scope of nonlinear metamaterials. We report the design concept, properties and mechanisms of the hyper-damping and its effect on vibration transmission. This paper reveals new properties offered by nonlinear elastic metamaterials, and offers a robust method for achieving efficient low-frequency and broadband vibration suppression.     

Physically Realizable Broadband Acoustic Metamaterials with Anisotropic Density

Transformation acoustics are concentrated for the purpose of designing novel acoustic devices to tailor acoustic waves to achieve desirable characteristics. However, these devices require fluid or fluid-like materials with an anisotropic density that generally does not exist in nature. Therefore, we introduce pentamode metamaterials into an alternating multilayer isotropic medium model to build fluid-like metamaterials with an anisotropic density. A 2D acoustic bending based on transformation acoustics is established and investigated to verify our method. This idea provides a method to design broadband and physically realizable acoustic metamaterials with an anisotropic density and is meaningful for the design of acoustic metamaterials.     

Scattering and absorption of sound at flow duct expansions

The scattering of plane acoustic waves at area expansions in flow ducts is analysed using the vortex sheet model where the flow at the expansion is modelled as a jet, with a vortex sheet emanating from the edge. Of particular interest is the influence of the flow field on acoustic scattering and absorption.First, it is demonstrated that the stability properties of the shear layer can be simulated by modifying the edge condition within the vortex sheet model. To this end the accuracy for the region where the shear layer is changing from unstable to stable is improved by introducing a gradually relaxed Kutta edge condition with empirically derived coefficients. For low Strouhal numbers the vortex sheet model applies and for higher Strouhal numbers the two models converge.Second, it is demonstrated that the acoustic transmission through the jet expansion region can be determined by neglecting the scattering there. Incorporating this assumption, the vortex sheet model reproduces well the experimental results on transmission and absorption for an area expansion. This result supports the assumption that the main part of the scattering occurs at the area expansion at least for the low-frequency range. Furthermore, the influence of the flow field is particularly strong for small Strouhal numbers.