Broadband low-frequency acoustic absorber based on metaporous composite

Broadband absorption of low-frequency sound waves via a deep subwavelength structure is of great and ongoing interest in research and engineering.Here,we numerically and experimentally present a design of a broadband lowfrequency absorber based on an acoustic metaporous composite(AMC).The AMC absorber is constructed by embedding a single metamaterial resonator into a porous layer.The finite element simulations show that a high absorption(absorptance A>0.8)can be achieved within a broad frequency range(from 290 Hz to 1074 Hz),while the thickness of AMC is 1/13of the corresponding wavelength at 290 Hz.The broadband and high-efficiency performances of the absorber are attributed to the coupling between the two resonant absorptions and the trapped mode.The numerical simulations and experimental results are obtained to be in good agreement with each other.Moreover,the high broadband absorption can be maintained under random incident acoustic waves.The proposed absorber provides potential applications in low-frequency noise reduction especially when limited space is demanded.

High-order Bragg forward scattering and frequency shift of low-frequency underwater acoustic field by moving rough sea surface

Acoustic scattering modulation caused by an undulating sea surface on the space-time dimension seriously affects underwater detection and target recognition.Herein,underwater acoustic scattering modulation from a moving rough sea surface is studied based on integral equation and parabolic equation.And with the principles of grating and constructive interference,the mechanism of this acoustic scattering modulation is explained.The periodicity of the interference of moving rough sea surface will lead to the interference of the scattering field at a series of discrete angles,which will form comb-like and frequency-shift characteristics on the intensity and the frequency spectrum of the acoustic scattering field,respectively,which is a high-order Bragg scattering phenomenon.Unlike the conventional Doppler effect,the frequency shifts of the Bragg scattering phenomenon are multiples of the undulating sea surface frequency and are independent of the incident sound wave frequency.Therefore,even if a low-frequency underwater acoustic field is incident,it will produce obvious frequency shifts.Moreover,under the action of ideal sinusoidal waves,swells,fully grown wind waves,unsteady wind waves,or mixed waves,different moving rough sea surfaces create different acoustic scattering processes and possess different frequency shift characteristics.For the swell wave,which tends to be a single harmonic wave,the moving rough sea surface produces more obvious high-order scattering and frequency shifts.The same phenomena are observed on the sea surface under fully grown wind waves,however,the frequency shift slightly offsets the multiple peak frequencies of the wind wave spectrum.Comparing with the swell and fully-grown wind waves,the acoustic scattering and frequency shift are not obvious for the sea surface under unsteady wind waves.

A joint absorbing boundary for the multiple-relaxation-time lattice Boltzmann method in seismic acoustic wavefield modeling

Conventional seismic wave forward simulation generally uses mathematical means to solve the macroscopic wave equation,and then obtains the corresponding seismic wavefield.Usually,when the subsurface structure is finely constructed and the continuity of media is poor,this strategy is difficult to meet the requirements of accurate wavefield calculation.This paper uses the multiple-relaxation-time lattice Boltzmann method(MRT-LBM)to conduct the seismic acoustic wavefield simulation and verify its computational accuracy.To cope with the problem of severe reflections at the truncated boundaries,we analogize the viscous absorbing boundary and perfectly matched layer(PML)absorbing boundary based on the single-relaxation-time lattice Boltzmann(SRT-LB)equation to the MRT-LB equation,and further,propose a joint absorbing boundary through comparative analysis.We give the specific forms of the modified MRT-LB equation loaded with the joint absorbing boundary in the two-dimensional(2D)and three-dimensional(3D)cases,respectively.Then,we verify the effects of this absorbing boundary scheme on a 2D homogeneous model,2D modified British Petroleum(BP)gas-cloud model,and 3D homogeneous model,respectively.The results reveal that by comparing with the viscous absorbing boundary and PML absorbing boundary,the joint absorbing boundary has the best absorption performance,although it is a little bit complicated.Therefore,this joint absorbing boundary better solves the problem of truncated boundary reflections of MRT-LBM in simulating seismic acoustic wavefields,which is pivotal to its wide application in the field of exploration seismology.

Calculation and Analysis of Acoustic Characteristics of Straight-Through Perforated Pipe Muffler Based on Multilayer Sound Absorbing Material

Using the multi-physical field simulation software COMSOL,the acoustic characteristics of the multilayer sound absorbing material straight-through perforated pipe muffler are studied by the finite element method.The results show that the finite element calculation of the multilayer sound absorbing material straight-through the perforated pipe muffler agrees well with the experimental measurement results.The reliability of the finite element method for studying the acoustic performance of the straight-through perforated pipe muffler with multilayer sound absorbing materials is shown.Furthermore,the influence of some structural parameters of porous sound absorbing material and micro-perforated plate on the acoustic performance of the multilayer sound absorbing material straight-through perforated pipe muffler is analyzed.The muffler based on multilayer sound absorbing material is different from the traditional muffler.After applying the multilayer sound absorbing material to the straight-through perforated pipe muffler,the transmission loss value greatly increases,which provides new ideas and directions for future research on the muffler.

Theory of complex-coordinate transformation acoustics for non-Hermitian metamaterials

Transformation acoustics(TA)has emerged as a powerful tool for designing several intriguing conceptual devices,which can manipulate acoustic waves in a flexible manner,yet their applications are limited in Hermitian materials.In this work,we propose the theory of complex-coordinate transformation acoustics(CCTA)and verify the effectiveness in realizing acoustic non-Hermitian metamaterials.Especially,we apply this theory for the first time to the design of acoustic parity-time(PT)and antisymmetric parity-time(APT)metamaterials and demonstrate two distinctive examples.First,we use this method to obtain the exceptional points(EPs)of the PT/APT system and observe the spontaneous phase transition of the scattering matrix in the transformation parameter space.Second,by selecting the Jacobian matrix's constitutive parameters,the PT/APT-symmetric system can also be configured to approach the zero and pole of the scattering matrix,behaving as an acoustic coherent perfect absorber and equivalent laser.We envision our proposed CCTAbased paradigm to open the way for exploring the non-Hermitian physics and finding application in the design of acoustic functional devices such as absorbers and amplifiers whose material parameters are hard to realize by using the conventional transformation method.

Software for Acoustic Design

We present the СATEС software, which implements the solution to the problems of computational acoustics. The software is based on the use of the superelement method and finite element modeling algorithms, in-cluding hydrodynamic noise. The paper presents the main possibilities of software for solving acoustic design problems. .

Design and application of functional absorbers

This paper gives an overview of the research at Institute of Acoustics, Tongji University, on functional absorbers and experience acquired in practical applications over the past three decades. Experiments and analysis of the absorption characteristics of three different geometrical forms of functional absorbers, i.e., panels, cubes and tubes, were conducted with different arrangements. The resulting esthetical effects are illustrated with pictures. Several non-fiber materials are used to compose functional absorbers with advantages both in acoustic properties and in architectural features. Cost effectiveness analysis is also given in order to provide design guidelines.

Theoretical and Experimental Study on the Acoustic Wave Energy After the Nonlinear Interaction of Acoustic Waves in Aqueous Media

Based on the Burgers equation and Manley-Rowe equation, the derivation about nonlinear interaction of the acoustic waves has been done in this paper. After nonlinear interaction among the low-frequency weak waves and the pump wave, the analytical solutions of acoustic waves＇ amplitude in the field are deduced. The relationship between normalized energy of high-frequency and the change of acoustic energy before and after the nonlinear interaction of the acoustic waves is analyzed. The experimental results about the changes of the acoustic energy are presented. The study shows that new frequencies are generated and the energies of the low-frequency are modulated in a long term by the pump waves, which leads the energies of the low-frequency acoustic waves to change in the pulse trend in the process of the nonlinear interaction of the acoustic waves. The increase and decrease of the energies of the low-frequency are observed under certain typical conditions, which lays a foundation for practical engineering applications.

Rapid Prototyping of Pyramidal Structured Absorbers for Ultrasound

Acoustic measurements or ultrasonic testing can be strongly affected by reflections or echoes from test tank walls. In order to create a non-reflecting environment equivalent to infinite medium, a pyramidal structured absorber (PSA) can be used to coat the walls of an ultrasonic tank. In this work, we model an array of tetragonal pyramid ultrasonic wave absorbers. This model is based on two coupled first-order equations describing the stress and particle velocity within an isotropic medium. For absorbing media, the Kelvin-Voigt model of viscoelasticity is used. The equations are discretized in 2D using an efficient time-stepping pseudo-spectral scheme that takes in consideration both, the acoustic properties and attenuation characteristics of the composite materials. We then built a 3D printed PSA using a Stratasys Objet500 Connex 3D printer, which allows to combine photopolymers in specific concentrations and microstructures. We designed PSA covering the frequency ranges from 0.5 MHz to 5 MHz and from 1 MHz to 10 MHz, with double homogeneous layer: a core of rubber material with a skin of a variety of elastomers by combining rigid and flexible materials. Each single pyramid contains two major parts: the ground of the pyramid (9.4 mm base × 4.7 mm height, for 0.5 MHz and 4.7 mm base × 2.35 mm height, for 1 MHz) and the body of the pyramid (23.5 mm height, for 0.5 MHZ and 11.75 mm height, for 1 MHz). The measured echo-reduction was greater than 35 dB at the covering frequency range and the transmission loss was estimated by 20 dB. Echoes increase rapidly for frequencies below the minimum frequency of the covering range. The modeling and 3D printing of PSA with different sizes, in a wide range of frequencies, is a cost-effective custom solution for a wide range of applications including for example, radiation force balances, hydrophone mounts and medical ultrasound equipment.

木质材料对室内声学调控的研究进展

木材为多孔性材料,在室内空间中,使用木质材料除了具有饰面作用外,还可以有效吸收声能,控制室内混响时间,改善音质,提高语言清晰度。本文概述了木质材料的吸声和隔声性能两方面的研究现状,并对未来的研究提出了建议。在此基础之上,总结了前人在大型建筑空间、小型办公空间、家居室内空间中声环境调控的不足之处。主要有以下3点:1)研究者多选取某一特定的室内空间类型进行声环境改善,尚缺乏对不同使用功能的室内空间声环境的规律性研究,以形成较为系统的室内声环境调控体系。2)成片的家具几何结构改变室内声环境的程度有待进一步研究验证。3)对于改变空间的形状、比例、布局的方法有待进一步研究其内在规律,为设计师、建筑师和科研人员提供参考依据。未来研究重点包括:1)通过使用木质材料对空间进行合理化布局与设计;2)处理好不均匀的声频或室内声场驻波的简并现象,需合理选择吸声构件。3)建立智能化声学仿真模型。

基于声学黑洞波动控制技术的槽型轨动力吸振器减振降噪特性研究

轨道交通引起的环境振动噪声问题持续增加,即使目前具有多种控制效果良好的减振降噪措施,但仍有望做进一步的提升。在该研究中提出了一种新型的槽型轨道动力吸振器,将声学黑洞波动控制技术与动力吸振原理相结合。该吸振器设计的目标是保证主结构强度与刚度的前提下,采用附加的声学黑洞阻尼振子作为吸能单元,对主结构的振动能量进行传递、吸收与耗散。为了研究声学黑洞型动力吸振器对槽型轨道振动特性和声辐射特性的影响,利用仿真分析对不同类型的动力吸振器下槽型轨道的位移导纳和振动衰减率进行了评估;采用滚动噪声预测模型计算分析了声学黑洞型动力吸振器的降噪效果并探究了其参数对轮轨振动噪声的影响规律。结果表明:槽型轨在800~1000 Hz频段内的一阶pinned-pinned在未采取措施的情况下振动响应显著,振动衰减率仅为0.68 dB/m,在安装了声学黑洞型动力吸振器之后轨道结构的振动衰减率上升到1.80 dB/m,提高率可达265%。

基于高斯展开法的碟形声学黑洞宽频调谐减振机理研究

近年来,嵌入式声学黑洞(acoustic black holes,ABH)以其优异的性能,在结构减振降噪、声波调控、能量回收等领域展示了广阔的应用前景,但其局部结构强度弱化会影响其工程实用性。提出一种碟形声学黑洞(dish-shaped acoustic black hole,DABH)结构,将其附加在主体结构上,以实现对主体结构的宽频减振。在Rayleigh-Ritz法框架下,选择高斯函数作为基函数,根据声学黑洞板的形状确定基函数的分布,避免质量和刚度矩阵的奇异化,建立了其耦合系统半解析模型。通过与有限元模态分析结果的对比,验证了半解析建模方法的正确性。研究了碟形声学黑洞结构参数以及连接位置对主体结构振动响应特性的影响规律,分析了碟形声学黑洞的ABH效应以及与主体结构的耦合效应,揭示了其宽频调谐减振的机理,为拓展声学黑洞在宽频结构振动控制上的应用提供了新的思路。

对称折叠通道结构的低频通风吸声体

为了消除或减少低频噪声,该文提出了一种低频通风超材料吸声体。该吸声体由对称的折叠通道结构组成,具有深度亚波长、高通风空间占比和低频高效吸声的特性。通过传递矩阵方法、有限元模拟和四传声器实验法,揭示了对称折叠通道结构通风吸声的物理机制。首先在理论上分析单个吸声体的通风吸声性能并进行了仿真模拟,在共振频率423 Hz附近,吸声系数大于0.9,通风空间占比高达40%。其次,吸声单体的共振频率可通过改变折叠通道的长度来灵活调控,组合多个不同共振频率的吸声单体可以拓宽吸声体的有效吸声带宽。由4个吸声单体组合的通风吸声体可实现314~366 Hz频率范围内的高效声吸收(吸声系数大于0.8),且通风空间占比达到35%,而结构厚度仅为314 Hz时波长的1/10。该低频通风吸声体具有结构简单、结构强度高和容易制造等特点,在低频通风降噪领域有着潜在的应用前景。

基于超微孔吸声体的钢结构教室声环境优化研究

依托H型钢梁侧面空腔构造,提出了融合装饰与声学设计的超微孔吸声体,将超微孔吸声体置于钢结构教室内部,通过声学仿真和现场实验测量改善前后的声环境状况,探究了超微孔吸声结构的吸声性能。结果表明,空腔深度150mm、内填50mm吸音棉的超微孔吸声结构吸声性能较好,综合式布置方案能在实现钢梁装饰效果的同时,显著改善钢结构教室的混响时间及语言清晰度,提高声环境品质。本研究可为相关工程提供参考。

多胞复合型声学结构吸声性能研究

针对一种多胞复合型吸声结构进行声学性能研究,其具有吸声频带宽、声学性能突出、设计自由度高的特点。利用马氏理论和Delany-Bazley模型分别构建微穿孔板和多孔性吸声材料层,通过传递矩阵法和声电类比等效电路法建立平行排列的多胞复合型声学结构理论模型,利用有限元法构建阻抗管仿真模型并对多胞复合型声学结构进行声学分析。对比理论计算结果和仿真试验数据,发现两者具有较好的拟合度。研究表明:多胞复合型吸声结构在全频率段内具有较好的吸声性能,通过调整各吸声单元的设计参数和变量,例如排列组合方式、微穿孔板的穿孔率以及微穿孔板后空气层厚度等,可以对不同频率段内的噪声进行有效吸收。

A mixed method for measuring low-frequency acoustic properties of macromolecular materials

A mixed method for measuring low-frequency acoustic properties of macro-molecular materials is presented. The dynamic mechanical parameters of materials are first measured by using Dynamic Mechanical Thermal Apparatus(DMTA) at low frequen-cies,usually less than 100 Hz; then based on the Principles of Time-Temperature Super-position (TTS),these parameters are extended to the frequency range that acousticians are concerned about,usually from hundreds to thousands of hertz; finally the extended dynamic mechanical parameters are transformed into acoustic parameters with the help of acoustic measurement and inverse analysis. To test the feasibility and accuracy,we measure a kind of rubber sample in DMTA and acquire the basic acoustic parameters of the sample by using this method. While applying the basic parameters to calculating characteristics of the sample in acoustic pipe,a reasonable agreement of sound absorp-tion coefficients is obtained between the calculations and measurements in the acoustic pipe.

On the certain semi-analytical models of low-frequency acoustic fields in terms of scalar-vector description

Behaviour of scalar and vector characteristics of steady-state acoustical field is modeled based on analytical-numerical approach. This field is radiated by low-frequency monochromatic point-like source in the deterministic layered shallow sea, which has various hydrologic and bottom conditions. Approach being developed is free of any mathematical approximations and without the difficulties it enables to calculate sound field vector characteristics for various

3D Printed Ultra-thin Acoustic Metamaterials with Adaptable Low-frequency Absorption Performance

The inherent absorption frequency of traditional sound absorbers makes it difficult to solve the problem of acoustic wave removal in a changeable acoustic environment.In this study,acoustic absorption metamaterials(AAMs)with adaptable sound absorption performance were innovatively designed using the structural combination concept and fabricated via 3D printing.Accordingly,two coiled-up channels were combined in a single cell,which could effectively broaden the absorption bandwidth in a limited space.The longitudinal movement of the coiled-up channels endowed the tunable entire depth and internal cavity of the AAMs;thus,the sound absorption performance could be tailored accordingly.Through computational analysis and experimental verification,it was demonstrated that the depth of the AAM could be adjusted from 10 mm to 20 mm,and the corresponding absorption frequencies of the two channels ranged from 206 Hz to 179 Hz and 379 Hz to 298 Hz,respectively.In addition,the finite element results also indicate that the sound absorption bandwidth of AAMs could be further improved by the periodic arrangement of the units.This work opens a promising structural design approach for presenting a route toward acoustic devices with adaptable absorption performances.

An intracellular study on low-frequency acoustic signal processing in locust——Structure and function of the cercus-to-giant interneuron system

The structure and function of the cercus-to-giant interneuron system,relevant to the receptionof low-frequency sound,within the terminal abdominal ganglion of the locust Locusta migratoria were revealedby using intracellular electrophysiological recording and dye labeling technique.This system consists of 4 bilater-al pairs of the giant interneurons(GIs 1—4).Each GI has distinct dendritic branching fields,position of thesoma,and location and orientation of its major axon.The characteristics of the system in responseto low-frequency sound,such as discharge patterns,the relationships between response threshold-frequency,in-tensity curves,and encoding of stimulus frequency,were also studied.The role of the system in low-frequencysound communication was discussed.

The dynamics of low-frequency signal acoustic intensity vector vortex structure in shallow sea

Spatiotemporal vector and phase properties of interference field of low-frequency signalling tone between three local vortices in a real shallow sea wave-guide have been studied.It has been demonstrated that in the field of constructive interference,components of particle velocity field and acoustic pressure are coherent.As a consequence the transfer of signal energy alog the axis of a shallow sea wave-guide is accomplished with plane wave.Physical objects are detected in the field of destructive interference,which,according to known deterministic signs,can be defined as local vortices of the intensity vector.A large-scale vorticity with acoustic intensity vector curl,components different from zero originates in the vicinity of local vortices.Regular particle displacements of local vortices have been detected against combined receiving device phase centre along the axis of a wave-guide.It has been demonstrated that the structure of vortices depends on signal/noise ratio.Local vortices and vorticity form vortex structure of vector acoustic field.Signalling tone with frequency of 88 ± 1 Hz from near-surface moving sound source was taken into consideration.Introduced results of full-scale experiment expand our concepts of real fundamental properties of shallow sea acoustic field and are to be considered in theoretical models.