含空气层与多孔材料的复合结构隔声特性研究

基于高速列车减振降噪需求,本文应用Biot提出的多孔弹性介质声传播理论,采用传递矩阵法理论推导了典型分层结构的隔声量计算公式,给出了空气层与多孔材料对分层复合结构隔声特性的影响。将传递矩阵与遗传算法相结合,对特定中低频段内的复合结构隔声特性进行了优化。研究结果表明:空气层和多孔材料有助于分层复合结构隔声量的提高,特别是空气层对低频隔声有很好的促进作用,另外空气层与多孔材料的分配情况也影响着隔声效果。含有空气层的复合结构在提高隔声量的同时降低了结构的总体重量,实现了高速列车隔声材料低能耗和轻量化的设计目标。     

Numerical simulation of acoustic excitations in an imperfect 1D superlattice

Features of the propagation of acoustic excitations in an imperfect 1D superlattice have been studied within the virtual crystal approximation. The dependence of the lowest acoustic band gap width of an imperfect (disordered in the composition and layer thickness) two-sublattice 1D phonon crystal on the concentration of impurity layers has been numerically simulated.     

用非饱和三相孔弹模型研究浅层土壤中地震波的传播特性

研究浅层土壤中声波耦合的地震波的传播特性, 用于声波探雷技术的机理分析. 根据浅层土壤具有孔隙度和可压缩性的特点, 利用非饱和三相孔隙介质中的地震波模型, 研究了土壤孔隙度、含水饱和度等参数对地震波传播特性的影响. 计算结果显示: 在给定的参数条件下, 地震波的传播速度和衰减系数均随频率的增加而增加; 纵波的传播速度随孔隙度的增加而减小, 横波的传播速度随孔隙度的增加而增加; 地震波的传播特性随含水饱和度的增加变化比较复杂. 通过对计算结果与已发表实验结果的比较分析, 讨论了解析方法的可行性, 为声-地震耦合机理及其在声波探雷研究中的应用提供了一定的理论基础. 关键词： 声-地震耦合 地震波 孔隙度 声波探雷     

Effects of a multi-layered poro-elastic ground on attenuation of acoustic waves and ground vibration

Ground conditions affect the propagation of outdoor sound and vibration. This paper focuses on the interaction between air pressure and porous ground at low frequencies- where mechanisms other than the rigid porous effects used in locally reacting models may be important. A 2-D analytical model has been developed in this study for the calculation of acoustic and acousto-seismic admittances in a multi-layered poro-elastic ground. The model can be used as a prediction tool both for the ground effect on the sound and the generation of ground vibration by the sound. The modelled acoustic admittance is validated successfully against established rigid frame admittance models over a frequency range of 1 Hz-3 kHz. Moreover, the acousto-seismic impedance is verified against full scale airblast field test data measured during the Norwegian Trials full scale field test program. For certain ground types, the predicted acoustic admittance illustrates a different behaviour compared with predictions from the traditional rigid frame acoustic impedance models. This emphasises the importance of including a deformable frame in the model to obtain realistic results for these conditions.     

Acoustic streaming in an ultrasonic air pump with three-dimensional finite-difference time-domain analysis and comparison to the measurement

The direct finite-difference fluid simulation of acoustic streaming on a fine-meshed three-dimensional model using a graphics processing unit (GPU)-based calculation array is discussed. Airflows are induced by an acoustic traveling wave when an intense sound field is generated in a gap between a bending transducer and a reflector. The calculation results showed good agreement with measurements in a pressure distribution. Several flow vortices were observed near the boundary layer of the reflector and the transducer, which have often been observed near the boundary of acoustic tubes, but have not been observed in previous calculations for this type of ultrasonic air pump.     

Characterization of thermal, optical and carrier transport properties of porous silicon using the photoacoustic technique

In this work, the porous silicon layer was prepared by the electrochemical anodization etching process on n-type and p-type silicon wafers. The formation of the porous layer has been identified by photoluminescence and SEM measurements. The optical absorption, energy gap, carrier transport and thermal properties of n-type and p-type porous silicon layers were investigated by analyzing the experimental data from photoacoustic measurements. The values of thermal diffusivity, energy gap and carrier transport properties have been found to be porosity-dependent. The energy band gap of n-type and p-type porous silicon layers was higher than the energy band gap obtained for silicon substrate (1.11 eV). In the range of porosity (50-76%) of the studies, our results found that the optical band-gap energy of p-type porous silicon (1.80-2.00 eV) was higher than that of the n-type porous silicon layer (1.70-1.86 eV). The thermal diffusivity value of the n-type porous layer was found to be higher than that of the p-type and both were observed to increase linearly with increasing layer porosity.     

An approximate treatment of reflection coefficient in the phased beam tracing method for the simulation of enclosed sound fields at medium frequencies

The phased beam tracing method (PBTM) was suggested as a medium-frequency simulation technique for the calculation of impulse response, although main assumptions of geometric acoustics still hold. The phased method needs the reflection coefficient for characterizing the acoustic property of a surface and the complex wave number for describing the propagation characteristics. In this study, two types of approximate real reflection coefficients derived from the measured absorption coefficient were tested for a practical applicability. As a test example, pressure impulse responses and energy impulse responses computed from the PBTM were compared with those from the measurement and the ordinary beam tracing method. The PBTM employing the approximate reflection coefficients greatly increased the accuracy of the prediction compared to the ordinary beam tracing method, in particular at the medium-frequency range in octave bands above the Schroeder cutoff frequency. A comparison was made between angle-dependent and angle-independent reflection coefficients in the calculation of acoustic measures. Although the angle-dependent reflection coefficient yielded best matched results with measured data, but the simple angle-independent reflection coefficient can be also used with a reasonably good precision.     

Development of a hybrid computer model for simulating the complicated virtual sound field in enclosures

A hybrid computer model, SOFIS, has been developed for the simulation of an enclosed virtual sound field in an arbitrary shaped enclosure. It can be used to calculate the impulse response and acoustical parameters of different positions in a virtual enclosure. This paper describes the way in which SOFIS models the sound source, the receiver and the sound propagation throughout the enclosed space with curved surfaces or barriers. A phase tracing method and the calculation of acoustic indexes are also discussed in this paper.     

一种精确计算换能器空间脉冲响应的快速方法

空间脉冲响应被认为是最有效的瞬态声场计算方法,针对空间脉冲响应直接计算时需要很高的采样频率,导致数据量大、计算效率差的问题,推导了空间点与其在换能器平面的投影点脉冲响应之间的关系,提出了一种计算脉冲响应的快速算法,探讨了计算采样频率和插值采样频率对计算精度和计算效率的影响,研究发现,采用1000 MHz采样频率能够保证精度要求,采用500 MHz作为插值时的采样频率是较优的选择。该方法与直接求解相比,可提高计算效率18倍。     

The influence of boundary layer on sound propagation in optical fibers

Physical models of two-layered and three-layered fiber were built. These models were used to calculate acoustic properties of optical fibers. Acoustic properties of fibers with boundary layer and without boundary layer calculated from these model were compared. The propagation of acoustic cylindrical symmetric waves in optical fibers was considered. This problem was treated analytically and numerically in continual approximation. The possibility of simultaneous propagation of two cylindrical symmetric waves in optical fibers with a boundary layer was shown (in a three-layered model). The physical phenomenon of the presence of two waves in the fiber with boundary layer is proposed for study of the boundary layer. Formulas for the calculation of the properties of the boundary layer from the acoustic experiment, verified by numerical calculations, are represented.     

Influence of Coating Layer on Acoustic Wave Propagation in a Random Complex Medium with Resonant Scatterers

We investigate the influence of coating layer on acoustic wave propagation in a dispersed random medium consisting of coa.ted fibers.In the strong-scattering regime, the characteristics of wave scattering resonances are found to evolve regularly with the properties of the coating layer.By theoretical calculation,frequency gaps are found in acoustic excitation spectra in a random medium.The scattering cross section results present the evolution of scattering resonances with the properties of the coating layer,which offers a good explanation for the change of the frequency gaps.The velocity of the propagation quasi-mode is also shown to depend on the filling fraction of the coating layer.We use the generalized coherent potential-approximation approach to solve acoustic wave dispersion relations in a complicated random medium consisting of coating-structure scatterers.It is shown that our model reveals subtle changes in the behavior of the acoustic wave propagating quasi-modes.     

The elastic microstructure of various tissues

Previous work has indicated that a modified Quate-Lemons scanning acoustic microscope (SAM) is capable of measuring the acoustic propagation properties of sections of biological tissue. The lens is excited by an impulse, rather than a tone burst, and the undemodulated returning signal from the tissue is recorded. The variations in received signal with time are used to deduce the sound speed, attenuation, impedance, and section thickness. In this article, the technique is applied to various types of tissue, and the variations in acoustic propagation properties are computed. Conventional tone burst SAM images at 425 MHz are compared with the time resolved data in order to elucidate the contrast mechanisms. The effects of varying the frequency and position of the focal plane on the tone burst images are interpreted in the light of the broadband results.     

A hybrid model for the acoustic response of a two-dimensional rough surface to an impulse incident from a refracting medium

A numerical model to calculate the impulse response of a two-dimensional, impenetrable, rough surface directly in the time domain has been recently introduced [J. Acoust. Soc. Am. (2000) 107, 27]. This model is based on wedge diffraction theory and assumes that the half-space containing the source and receiver is homogeneous. In this work, the model is extended to handle media where the index of refraction varies with the distance to the surface by merging the scattering model with a ray-based propagation model. The resulting hybrid model is tested against a Finite-Difference Time-Domain (FDTD) method for the problem of backscattering from a corrugated surface in the presence of a refractive layer. This new model can be applied, for example, to calculate acoustic reverberation from the sea surface in cases where the water mass is inhomogeneous and in the presence of a subsurface bubble layer at low frequencies where dispersion is negligible. It can also be used for atmospheric propagation problems where there is a sound speed gradient overlying rough terrain.     

Sound transmission of an active acoustic structure with porous materials based on the(u,p) formulation

<正>A method based on the combination of the(u.p) formulation and finite element method was applied to calculating the acoustical performance of a double-wall active acoustic structure with porous materials.The(u,p) formulation based on the displacement in solid phase and the pressure in fluid phase was developed to investigate the sound propagation in porous materials.The acoustic performance of the double-wall active acoustic structure having porous materials was calculated and the measurement was taken.The numerical results matched well with the measured data.More than 10 dB transmission loss of the double-wall active acoustic structure can be improved in the resonance frequency with active control,and its absorption coefficient is up to 0.6 over 500 Hz.The relative error between the prediction and measurement is less than 5%at the resonance frequency of the porous materials.     

Dissolution behaviour of the barrier layer of porous oxide films on aluminum formed in phosphoric acid studied by a re-anodizing technique

Chemical dissolution of the barrier layer of porous oxide films formed on aluminum foil (99.5% purity) in the 4% phosphoric acid after immersion in 2 mol dm⁻³ sulphuric acid at 50 °C has been studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. A digital voltmeter with a computer system was used to record the change in the anode potential with re-anodizing time. It has been found that the barrier layer material may consist of two or three regions according to the dissolution rate. The barrier oxide contains two layers at 35 V: the outer layer with the highest dissolution rate and the inner layer with low dissolution rate. The barrier oxide contains three layers at 40 V and above it: the outer layer with high dissolution rate, the middle layer with the highest dissolution rate and the inner layer with low dissolution rate. It has been shown that there is a dependence of the dissolution rate on the surface charge of anodic oxide film. Annealing of porous alumina films for 1 h at 200 °C leads to disappearance of layers with different dissolution rates in the barrier oxide. We explained this phenomenon by the absence of the space charge in the barrier oxide of such films.     

流体/准饱和多孔介质中伪Scholte波的传播特性

研究流体/多孔介质界面Scholte波的传播特性对于水下勘探、地震工程等领域具有重要意义.本文基于Biot理论和等效流体模型,采用势函数方法,推导了描述有限厚度流体/准饱和多孔半空间远场界面波的特征方程和位移、孔压计算公式.在此基础上,分别以砂岩和松散沉积土为例,研究了流体/硬多孔介质和流体/软多孔介质两种情况下,可压缩流体层厚度和多孔介质饱和度对伪Scholte波传播特性的影响.结果表明：多孔介质软硬程度显著影响界面波的种类、相速度、位移和水压力分布;有限厚度流体/饱和多孔半空间界面处伪Scholte波相速度与界面波波长和流体厚度的比值有关;孔隙水中溶解的少量气体对剪切波的相速度的影响不大,对压缩波相速度、伪Scholte波相速度和孔隙水压力分布影响显著.     

Characterization of various aluminium oxide layers by means of spectroscopic ellipsometry

This paper reports the characterization of both barrier type and porous type anodic oxide films on aluminium by means of spectroscopic ellipsometry (SE). In order to show the capabilities of the technique for quantitative determination of the layer characteristics, results based on ellipsometric data are correlated with complementary information from the analytical techniques transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). It is concluded that ellipsometry yields an accurate characterization for the thicknesses and the interfacial properties of both the barrier layer and the porous layer. The porosity of the porous layer, determined with SE, is found to be in good agreement with the results obtained from TEM.     

A new empirical model for the acoustic properties of loose granular media

This paper examines physical parameters of loose granular mixes and their empirical relations to the acoustic performance of these mixes. In this work a new classification of granular media has been proposed which is related to the characteristic particle dimension and the specific density of the grain base. It has been shown that this classification is a useful characteristic for rapid evaluation of the acoustic performance of loose granular mixes. The characteristic impedance and propagation constant have been measured for a representative selection of grain mixes and used to develop a new empirical model. This model relates the above acoustic characteristics to the characteristic particle dimension, porosity, tortuosity and specific density of the grain base, which are routinely measurable parameters. A very good agreement with the experimental data is illustrated in the frequency range of 250-4000 Hz for materials with the grain base of 0.4-3.5 mm and specific densities between 200 and 1200 kg/m³. Unlike many theoretical models for the prediction of the acoustic properties of porous media, the proposed expressions do not involve any special functions of complex argument, empirical shape factors or sophisticated characteristics of porous structure. These are practical enough to be of interest to acoustic and noise control engineers and material manufacturers.     

Boundary layer effects on sound in a circular duct

Boundary layer effects on an acoustic field in a unidirectional flow with transverse shear are studied. The acoustic pressure variation in the direction normal to that of the flow is governed in the boundary layer by a second order differential equation. The problem in the boundary layer is reduced from a two point boundary value problem to a one point boundary value problem by transforming the governing equation into the Riccati equation. The Riccati equation is easily integrated with standard numerical procedures. The integration process yields the effective admittance of the wall-boundary layer combination. The acoustic field in the uniform flow is then determined for this effective admittance. Further complications imposed by the boundary layer are thus eliminated. The simplicity of the technique allows calculation of the propagation and decay constants in a circular duct over a wide range of parameters and duct modes.     

Analysis of chemical dissolution of the barrier layer of porous oxide on aluminum thin films using a re-anodizing technique

Chemical dissolution of the barrier layer of porous oxide formed on thin aluminum films (99.9% purity) in the 4% oxalic acid after immersion in 2 mol dm⁻³ sulphuric acid at 50 °C has been studied. The barrier layer thickness before and after dissolution was calculated using a re-anodizing technique. It has been shown that above 57 V the change in the growth mechanism of porous alumina films takes place. As a result, the change in the amount of regions in the barrier oxide with different dissolution rates is observed. The barrier oxide contains two layers at 50 V: the outer layer with the highest dissolution rate and the inner layer with a low dissolution rate. Above 60 V the barrier oxide contains three layers: the outer layer with a high dissolution rate, the middle layer with the highest dissolution rate and the inner layer with a low dissolution rate. We suggest that the formation of the outer layer of barrier oxide with a high dissolution rate is linked with the injection of protons or H₃O⁺ ions from the electrolyte into the oxide film at the anodizing voltages above 57 V.