Utilization of coir fiber in multilayer acoustic absorption panel

Coconut is one of the most important harvests in Malaysia. Industrial prepared coir fiber is obtained from coconut husk combined with latex and other additives to enhance its structural characteristics. Unfortunately, such inevitable process diminishes the acoustical features of material. Previous studies on industrial coir fiber and fiber-air gap layers showed that low frequency absorptions needed improvements. Therefore perforated plate (PP) was added to the multilayer structure to further enhance the sound absorption in this area. Analyses were accomplished through three PP modeling approaches (Allard, Beranek and Ver, Atalla and Sgard) and Allard Transfer Function (TF) method. Experiments were conducted in impedance tube to support the analytical results. Outcomes showed that Allard TF method was generally closer to measurement values and implemented for additional analyses. Two possible conditions of putting PP in front of fiber layer or between fiber-air gap layers were investigated. Both arrangements were suitable to enhance the sound absorption. Although, when PP was backed by coir fiber and air gap, porosity of the plate had great influence in adjusting the amount of low frequency absorption. Result derived that PP might improve the low frequency absorption of coir fiber but at the same time the medium frequency absorption was reduced. This effect was noticed previously in coir fiber-air gap structures while the air gap thickness increased. The advantage of using PP was that it assisted in greatly reducing the air gap thickness under the same acoustical performance. Hence it is an efficient tool to reduce the thickness of acoustic isolators in practical purposes.     

基于声辐射模态的有源结构声传入及其辐射控制

从辐射模态的概念和角度研究利用结构误差传感方法对弹性封闭空间结构声辐射进行传感和有源控制。首先分析了辐射模态的数学和物理意义并揭示了辐射模态与声腔模态之间的内在耦合关系。通过声辐射模态建立了弹性封闭空间结构声辐射传感和有源控制模型,并提出了通过传感器阵列测量结构表面有限点的振速分布和设计特定的辐射模态空间滤波器来获得控制所需的误差信号。在此基础上对封闭空间结构声辐射有源控制和误差传感策略进行了深入的理论和数值仿真分析,重点讨论了传感器的数量和布放对辐射模态传感及其有源控制效果的影响。结果表明:辐射模态与声腔模态的耦合具有严格的选择性,各阶辐射模态的形状和与相耦合的主导声模态在耦合面上的形状非常相似;利用结构传感技术传感封闭空间的辐射模态时测点不足或空间采样不足将可能产生较严重的模态泄漏问题,使得不希望的结构模态泄露进所测的辐射模态当中来。在低频范围内,一般只需传感并最小化前三阶有效辐射模态声势能,在更低频和空间声模态频率附近,只需最小化前一阶最有效辐射模态声势能,便能和总声势能最小化策略控制效果基本一样。     

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。     

机翼颤振的有源声控制

借助于己推导出的声激励下绕三维薄翼的非定常位势流的精确解，本文进一步分析了限于弱扰动的声激励消除颤振的机理，讨论了声源参数对颤振边界的影响，并通过声一涡耦合的理论计算，解释了原有的风洞实验。结果表明，对于不同位置的声源，声致气动载荷的无环量和有环量部分起着不同的作用，而且，在声控制颤振的过程中，系统的各种参数存在一定的关系，必须合理地选取最佳参数。本文的研究为声控制技术在气动领域中的实际应用提供了理论基础。     

Interferometric control of transmission and reflection optical elements

A simple method for interferometric control is considered that permits the control of optical elements of the transmission and reflection type. A universal control instrument is proposed. The results of experimental verification of the interferometric control method, which confirm the effectiveness of its implementation in industrial conditions, are reported.     

On the reflection and transmission of light through thin transparent films

A formula is deduced for the reflection and transmission of light through one thin film. The direction of the passing rays of light (right and left) is taken into account whereby a formula is obtained which is more general than that usually used. The former was used as the starting point for the deduction of formulae for the reflection and transmission of light through two and more thin films which were published by the author some years ago. An explanation is given of the complication of the formulae when the reflection of light from the upper film, adjacent to the air, is computed. The auxiliary phase deduced by the author in a previous paper is also explained as a necessary consequence.     

Investigations on the acoustic transmission characteristics of underwater perforated panel structures

Perforated panel structures have a wide potential in underwater applications. However, up to now there has been little related research. The acoustic impedance of an underwater perforated panel is obtained based on the theories for air perforated panel sound absorption. In this paper sound transmission characteristics of underwater perforated panel structures are theoretically analyzed by the transfer matrix method. A formula for normal incidence sound transmission coefficients is given. The main factors that have effects on the acoustic transmission coefficient are analyzed by numerical simulations. The perforated panel structures made by ourselves are tested in a standing-wave tube by the four-sensor transfer-function method. The experimental results are well in accord with the results obtained by the numerical method, which proves that the theoretical analysis is correct. This paper has provided theoretical and experimental bases for the design of underwater perforated panel structures.     

Wave reflection and transmission from a thin film with one-dimensional disorder

This paper deals with a TE plane wave reflection and transmission from a thin film with one-dimensional disorder by means of the stochastic functional approach. The relative permittivity of the thin film is written by a Gaussian random field in the horizontal direction with infinite extent, and is uniform in the vertical direction with finite thickness. Arandomwavefield is obtained in terms of a Wiener-Hermite expansion representation with approximate expansion coefficients (Wiener kernels) under a small fluctuation case. For a SiC thin film and a glass thin film having one-dimensional disorder with Gaussian correlation or an exponential correlation, numerical examples of the first-order incoherent scattering cross section and the optical theorem are illustrated in the figures. It is then found that ripples and four major peaks appear in angular distributions of the incoherent scattering. Such four peaks may occur in the directions of forward scattering, specular reflection, backscattering and in the symmetrical direction of forward scattering with respect to the normal to surface of the thin film. Physical processes that yield such ripples and peaks are discussed.     

声表面波在圆弧处反射及透射的数值研究*

为了研究直达表面波在圆弧过渡面处传播性能的变化,采用有限元方法模拟了热弹机制下,线性脉冲激光辐照金属铝块表面时激发的表面波在近表面传播过程中,在不同曲率半径的圆弧处发生的反射及透射现象,建立了圆弧半径与反射表面波以及透射表面波时域信号特征之间的联系。计算结果表明:曲率半径与表面波中心波长的数值关系对表面波在圆弧处的传播有显著的影响;同时证明了根据透射表面波信号的到达时间可以反演圆弧半径的大小,为之后利用表面波信号定量检测材料表面圆弧凹痕的深度提供了理论依据。     

Active control by using optical sensors on the acoustic radiation from square plates

This study is reported of an optical approach to reduce the acoustic radiation from some uniformly square plates of different sizes. The content of this study includes the dynamical analysis of uniform plates, the theoretical consideration dealing with both the optical-error path and neural network control system, and the laboratory demonstration on the acoustic-radiation reduction from plates by using optical approach.Results of dynamical analysis on three different plates obviously reveal low odd-odd modes such as (1, 3) and (3, 3) mode must respond to the most acoustic radiation from plates. Laboratory demonstration claims that much more reduction of acoustic radiation from plates within greater frequency range can be reached when using an optical-error path to replace acoustic-error path. The corresponding experimental results obviously show that the greater control effectiveness on (3, 3) mode for smaller plate, single frequency, the greater effectiveness by 26 dB of acoustic radiation reduction for single point control than that for multi points control. Regarding to the double frequencies, the acoustic attenuation by 14.6 dB for compose of (3, 3) and (3, 4) in 60 cm plate are obtained.     

Active structural acoustic control of repetitive impact noise

This paper discusses the effectiveness of an Active Structural Acoustic Control (ASAC) system for the reduction of repetitive impact noise, radiated by structures with a high modal density in the controlled frequency range. Although there is a significant difference in nature between periodic and transient noise, up till now no specific research on ASAC of transient noise was reported. The development of the ASAC system is divided into two phases: the definition of the control configuration and the design of a suitable control algorithm. The optimal control configuration as well as the implemented control algorithm for the reduction of impact noise differ significantly from the common solutions in periodic noise control. In the first part of the paper, a practical methodology is presented to define a good control arrangement for transient noise control. The second part of the paper focuses on the design of control algorithms, adapted to the specific properties of impact noise. Since many industrial impact noise problems involve successive impacts with a repetitive behaviour, control algorithms with a learning behaviour are discussed. The efficiency of these Iterative Learning Control (ILC) algorithms is extensively demonstrated in this paper. The developed ASAC strategy has been verified on a thick steel plate, which is excited by successive impacts. The obtained results show that ASAC can be a very efficient transient noise control technique in certain industrial applications (e.g. presses, punching machines, etc.).     

Cathodoluminescence of large-area PLD grown ZnO thin films measured in transmission and reflection

Epitaxial ZnO thin films on sapphire substrates can be used as fast and laterally homogeneous scintillators to convert electrons into photons, for example for imaging purpose. We report on the improvement of the cathodoluminescence intensity of epitaxial pulsed laser deposited ZnO films on a-plane sapphire substrates with diameter up to 33 mm. The lateral homogeneity of the integral cathodoluminescence intensity was inspected using a modified RHEED setup. Cathodoluminescence spectra were excited at the ZnO side of the samples and detected both in reflection and in transmission geometry. The redshift of the excitonic cathodoluminescence peak in transmission relative to reflection and the peak shift with the excitation depth can be explained by a model based on self absorption of the photons in the ZnO film. PACS 78.60.Hk; 78.20.Bh; 81.05.Dz; 81.15.Fg     

Active sound transmission control of a double-panel module using decoupled analog feedback control: experimental results

Low-frequency sound transmission through passive lightweight partitions often renders them ineffective as means of sound isolation. As a result, researchers have investigated actively controlled lightweight partitions in an effort to remedy this problem. One promising approach involves active segmented partitions (ASPs), in which partitions are segmented into several distinctly controlled modules. This paper provides an experimental analysis of a double-panel ASP module wherein the source- and transmitting-side panels are independently controlled by an analog feedback controller. Experimental results, including plant frequency response functions, acoustic coupling strengths, frequency response functions, and transmission losses (TLs) of single- and double-panel modules, are presented and compared to numerical predictions. Over the bandwidth of 20 Hz to 1 kHz, the average measured TL for an actively controlled single-panel module was 29 dB, compared to 14 dB for the passive case. The average measured TL over the same bandwidth for the actively controlled double-panel module was 57 dB, compared to 31 dB for the passive case.     

Active and passive structural acoustic control of the smart beam

Structural noise and vibrations control can be achieved using two strategies: active - with feed-forward controller, a sensor and an actuator [1-3] or passive, by piezoelectric shunt damping [4-8], when a piezoelectric transducer will act as a sensor and an actuator. Potential applications of these structures are investigated. Two numerical (FEM) models based on the active and passive damping strategies are compared. The numerical solutions were confirmed experimentally.     

周期镶嵌亥姆霍兹共鸣腔平板的声全背向反射

研究斜入射声波在周期镶嵌亥姆霍兹共鸣器的刚性界面上的反射行为。共鸣器间的空间周期间隔d和入射声波的波长λ相当,且大于共振腔尺寸,因此可被视为具有集总参数的声学元件。理论分析和数值计算均表明,当入射波频率和方向既满足布拉格共振条件又同时满足亥姆霍兹共振条件时,在通常的反射方向上反射声波消失,反射波逆入射方向传播,引起与常规反射全然不同的全背向反射。     

均匀磁化等离子体与雷达波相互作用的数值分析

采用平板几何对不同磁场强度下等离子体对电磁波的吸收、反射和透射特性进行了数值分析.结果表明,不同磁场可以显著改变等离子体对不同频率的雷达波的吸收和反射特性.当雷达波频率接近等离子体高混杂频率时,磁化等离子体将对该波产生强的共振吸收,带宽在2GHz左右,吸收比可达90%以上.因此,通过适当调整磁场强度、等离子体密度和碰撞频率,可实现较宽雷达波段的等离子体隐身 关键词： 电磁波 磁化等离子体 共振吸收 等离子体隐身     

Narrow band filters in both transmission and reflection with metal/dielectric thin films

We present a novel narrow band filter operating in both transmission and reflection for the first time to our knowledge. This proposed structure consists of one unsymmetrical dielectric Fabry-Perot cavity and an ultrathin metal film with n ≈ k. Theoretical analysis shows that both the reflectance and transmittance at the central wavelength are maximums. Due to the high absorption induced by the metal, a good rejection level can be obtained for a wide spectral range. In addition, the changes of peak value ratio R_max/T_max is also investigated by adjusting the amount of dielectric stacks. We finally demonstrate the experimental results to verify these designs.     

Material property estimation in thin plates using focused,synthetic-aperture acoustic beams

The method developed here exploits the wide angular range of focused acoustic probes and the large synthetic aperture of scanned transducers to permit a rapid and reliable estimation of material properties in thin plates. It is found in several tests with various materials that estimates of elastic behavior using this method agree with contact measurements to within less than 5%. The method utilizes transmission (or reflection) coefficient reconstruction for an infinite thin plate, across a wide range of frequency and wave number, from which elastic property estimates are made. Data collected over a large synthetic acoustic aperture are processed with temporal and spatial Fourier transforms applied to change the acquired data from the coordinate and time domains to the wave number and frequency domains. Extrinsic real-beam effects on the data are accounted for with a complex transducer point analysis. Transmission measurements yield reconstructed data extending to the mode cutoffs, permitting easy and nearly unambiguous estimation of a subset of the elastic stiffnesses. For anisotropic plates, elastic stiffnesses are estimated with an inversion procedure that uses only limited data carefully selected from different portions of the measured scattering coefficient. Estimates are made by reconstructing in a stepwise fashion, based on sensitivity studies, where only one stiffness is estimated from the data at any one time, restricting the optimization to a robust one-dimensional search.     

Effects of core position of locally resonant scatterers on low-frequency acoustic absorption in viscoelastic panel

Locally resonant sonic materials, due to their ability to control the propagation of low-frequency elastic waves, have become a promising option for underwater sound absorption materials. In this paper, the finite element method is used to investigate the absorption characteristics of a viscoelastic panel periodically embedded with a type of infinite-long noncoaxially cylindrical locally resonant scatterers(LRSs). The effect of the core position in the coating layer of the LRS on the low-frequency(500 Hz–3000 Hz) sound absorption property is investigated. With increasing the longitudinal core eccentricity e, there occur few changes in the absorptance at the frequencies below 1500 Hz, however, the absorptance above 1500 Hz becomes gradually better and the valid absorption(with absorptance above 0.8) frequency band(VAFB)of the viscoelastic panel becomes accordingly broader. The absorption mechanism is revealed by using the displacement field maps of the viscoelastic panel and the steel slab. The results show two typical resonance modes. One is the overall resonance mode(ORM) caused by steel backing, and the other is the core resonance mode(CRM) caused by LRS. The absorptance of the viscoelastic panel by ORM is induced mainly by the vibration of the steel slab and affected little by core position. On the contrary, with increasing the core eccentricity, the CRM shifts toward high frequency band and decouples with the ORM, leading to two separate absorption peaks and the broadened VAFB of the panel.