浅海水声信道冲激响应空变特性研究

浅海声传播对空间参数的变化十分敏感,信道冲激响应随空间的变化使水声信号处理的性能受到很大限制。文中采用射线理论模型估计信道冲激响应,用互相关方法仿真计算了信道响应对空间参数的敏感性,并对信道冲激响应空变特性的实际应用进行了讨论。     

Acoustic response accompanying stimulated Brillouin scattering in liquid

Sound waves propagating from the stimulated Brillouin scattering (SBS) region in hexane were detected. A signal correlating with the SBS process existence in liquid was detected in the direction perpendicular to propagation of waves interacting in SBS using a broadband receiver of acoustic vibrations.     

Ultrasonic studies of structural transformations and phase transitions in liquid crystal emulsions

The propagation velocity and absorption coefficient of ultrasound in the range of 0.6–150 MHz in a series of liquid crystal emulsions prepared on the basis of thermotropic liquid crystals and isotropic liquids were studied. The dispersity parameters of emulsions were controlled by polarization microscopy and dynamic light scattering. Features of the temperature dependences of the acoustic parameters of emulsions due to the structural transformations and nematic-isotropic liquid transition in disperse and dispersion phases of liquid crystal emulsions were established.     

Process monitoring using ultrasonic sensor systems

Continuous in-line measurement of substance concentration in liquid mixtures is valuable in improving industrial processes in terms of material properties, energy efficiency and process safety. Ultrasonic sensor systems meet the practical requirements of a chemical sensor quite well. Currently ultrasonic sensor systems are widely used as acoustic chemical sensors to measure concentration of selected substances or to monitor the course of polymerisation, crystallisation or fermentation processes. Useable acoustic properties for the characterisation of liquid mixtures are sound velocity, sound absorption and acoustic impedance. This contribution will give a short overview of the state of the art and several trends for the use of ultrasonic sensor systems in process applications. Novel investigations show the very promising possibility to analyse liquid multi-phase mixtures like suspensions, emulsions and dispersions.     

Effect of Mean Flow on Acoustic Wave Propagation in a Duct with a Periodic Array of Helmholtz Resonators

Sound propagation properties of a duct system with Helmholtz resonators(HRs) are affected by mean flow.Previous studies have tended to focus on the effects of mean flows on acoustic response of a duct system with a finite number of HRs. Employing an empirical impedance model, we present a modified transfer matrix method for studying the effect of mean flow on the complex band structure of an air duct system with an infinite periodic array of HRs. The efficiency of the modified transfer matrix is demonstrated by comparison between an example of transmission response calculation for a finite single HR loaded duct and the finite element simulation result calculated using the COMSOL software. Numerical results are presented to analyze the effect of mean flow on the band structure and transmission loss of the sound wave in the duct system. It is hoped that this study will provide theoretical guidance for acoustic wave propagation of HR silencer in the presence of mean flow.     

Low-frequency sound transmission through a gas-liquid interface

Typically, sound speed in gases is smaller and mass density is much smaller than in liquids, resulting in a very strong acoustic impedance contrast at a gas-liquid interface. Sound transmission through a boundary with a strong impedance contrast is normally very weak. This paper studies the power output of localized sound sources and acoustic power fluxes through a plane gas-liquid interface in a layered medium. It is shown that, for low-frequency sound, a phenomenon of anomalous transparency can occur where most of the acoustic power generated by a source in a liquid half-space can be radiated into a gas half-space. The main physical mechanism responsible for anomalous transparency is found to be an acoustic power transfer by inhomogeneous (evanescent) waves in the plane-wave decomposition of the acoustic field in the liquid. The effects of a liquid's stratification and of guided sound propagation in the liquid on the anomalous transparency of the gas-liquid interface are considered. Geophysical and biological implications of anomalous transparency of water-air interface to infrasound are indicated.     

Proper orthogonal decomposition and cluster weighted modeling for sensitivity analysis of sound propagation in the atmospheric surface layer

Outdoor sound propagation predictions are compromised by uncertainty and error in the atmosphere and terrain representations, and sometimes also by simplified or incorrect physics. A model's predictive power, i.e., its accurate representation of the sound propagation, cannot be assessed without first quantifying the ensemble sound pressure variability and sensitivity to uncertainties in the model's governing parameters. This paper describes fundamental steps toward this goal for a single-frequency point source. The atmospheric surface layer is represented through Monin-Obukhov similarity theory and the acoustic ground properties with a relaxation model. Sound propagation is predicted with the parabolic equation method. Governing parameters are modeled as independent random variables across physically reasonable ranges. Latin hypercube sampling and proper orthogonal decomposition (POD) are employed in conjunction with cluster-weighted models to develop compact representations of the sound pressure random field. Full-field sensitivity of the sound pressure field is computed via the sensitivities of the POD mode coefficients to the system parameters. Ensemble statistics of the full-field sensitivities are computed to illustrate their relative importance at every down range location. The central role of sensitivity analysis in uncertainty quantification of outdoor sound propagation is discussed and pitfalls of sampling-based sensitivity analysis for outdoor sound propagation are described.     

Observationally constrained modeling of sound in curved ocean internal waves: examination of deep ducting and surface ducting at short range

A study of 400 Hz sound focusing and ducting effects in a packet of curved nonlinear internal waves in shallow water is presented. Sound propagation roughly along the crests of the waves is simulated with a three-dimensional parabolic equation computational code, and the results are compared to measured propagation along fixed 3 and 6 km source/receiver paths. The measurements were made on the shelf of the South China Sea northeast of Tung-Sha Island. Construction of the time-varying three-dimensional sound-speed fields used in the modeling simulations was guided by environmental data collected concurrently with the acoustic data. Computed three-dimensional propagation results compare well with field observations. The simulations allow identification of time-dependent sound forward scattering and ducting processes within the curved internal gravity waves. Strong acoustic intensity enhancement was observed during passage of high-amplitude nonlinear waves over the source/receiver paths, and is replicated in the model. The waves were typical of the region (35 m vertical displacement). Two types of ducting are found in the model, which occur asynchronously. One type is three-dimensional modal trapping in deep ducts within the wave crests (shallow thermocline zones). The second type is surface ducting within the wave troughs (deep thermocline zones).     

Filamentary coagulation of particles from water-heterogeneous systems in the field of an acoustic resonator

The coagulation of particles from water-heterogeneous systems in the field of a confocal ultrasonic resonator is studied. It is found that, at frequencies of several megahertz, when acoustic power of about 1 W is applied to the resonator, long stable filaments consisting of the material of the heterogeneous system are formed in the vicinity of the resonator axis. The filaments consist of thin disks formed by coalescent particles spaced at intervals strictly equal to half of the sound wavelength. The features of this coagulation are determined for suspensions of various nature (metal and dielectric particles, colloidal solutions, and oil emulsions). It is established that the coagulation in a standing acoustic wave occurs faster than under natural conditions (under the influence of gravity). The possibility of using this effect for cleaning liquids from impurities and separating hyperfine particles without employing filter materials is discussed.     

A low order flow/acoustics interaction method for the prediction of sound propagation using 3D adaptive hybrid grids

A low-order flow/acoustics interaction method for the prediction of sound propagation and diffraction in unsteady subsonic compressible flow using adaptive 3-D hybrid grids is investigated. The total field is decomposed into the flow field described by the Euler equations, and the acoustics part described by the Nonlinear Perturbation Equations. The method is shown capable of predicting monopole sound propagation, while employment of acoustics-guided adapted grid refinement improves the accuracy of capturing the acoustic field. Interaction of sound with solid boundaries is also examined in terms of reflection, and diffraction. Sound propagation through an unsteady flow field is examined using static and dynamic flow/acoustics coupling demonstrating the importance of the latter.     

Acoustic properties of micro-perforated panel absorber having arbitrary cross-sectional perforations

Micro-perforated panel absorber is used in many noise control applications as a next-generation absorbing material. Perforation shapes of micro-perforated panel studied are usually circular in the past. However, in practice, the perforations are often non-circular or irregular shape due to manufacturing techniques. Sound absorption coefficient and absorption bandwidth of the micro-perforated panel absorber may be further improved, when the perforations in shape are changed. In view of the existing exact solutions of sound propagation in tubes, the simple formulas of specific acoustic impedances of the tubes for triangle and square cross-sectional perforations are derived. Mass reactance end correction of the micro-perforated panel is obtained based on the sound radiation of a shaped piston. The specific acoustic impedance ratio of the micro-perforated panel absorber is calculated and analyzed, which can predict its sound absorption bandwidth. Finally, for closed perforations, the influences of the perforations in shape (including triangle, circle, square and irregular circle) on sound absorption of the MPP absorber are discussed in collaboration with FE simulations.     

An improved multimodal method for sound propagation in nonuniform lined ducts

An efficient method is proposed for modeling time harmonic acoustic propagation in a nonuniform lined duct without flow. The lining impedance is axially segmented uniform, but varies circumferentially. The sound pressure is expanded in term of rigid duct modes and an additional function that carries the information about the impedance boundary. The rigid duct modes and the additional function are known a priori so that calculations of the true liner modes, which are difficult, are avoided. By matching the pressure and axial velocity at the interface between different uniform segments, scattering matrices are obtained for each individual segment; these are then combined to construct a global scattering matrix for multiple segments. The present method is an improvement of the multimodal propagation method, developed in a previous paper [Bi et al., J. Sound Vib. 289, 1091-1111 (2006)]. The radial rate of convergence is improved from O(n(-2)), where n is the radial mode indices, to O(n(-4)). It is numerically shown that using the present method, acoustic propagation in the nonuniform lined intake of an aeroengine can be calculated by a personal computer for dimensionless frequency K up to 80, approaching the third blade passing frequency of turbofan noise.     

Features of the motion of particles suspended in liquid under an applied field

It is experimentally shown that a tenfold-hundredfold increase in the effective diffusion coefficient along the direction perpendicular to an applied electric field takes place in various emulsions and suspensions. It is caused by electrophoretic motion of particles. In inverse emulsions and many suspensions, only a certain fraction of dispersed material is contained in particles whose average radius is determined by the dynamic light scattering method. It is shown that the average coherence time of light scattered by silicon particle suspensions at a constant laser power depends on the beamdiameter. This means that semiconductor particles are characterized by strong nonlinearity.     

一种水平变化可穿透波导中声传播问题的耦合简正波方法

通过利用标准简正波程序KRAKEN计算本地简正波解及耦合矩阵, 进一步发展了求解水平变化波导中声场的全局矩阵耦合简正波方法(Luo et al., "A numerically stable coupled-mode formulation for acoustic propagation in range-dependent waveguides," Sci. China-Phys. Mech. Astron. 55, 572 (2012)), 使得该方法可以处理具有可穿透海底及随深度变化声速剖面等实际问题, 并提供声场的完全双向解. 本文还给出了双层波导中耦合矩阵的解析表达式, 并利用其验证了本方法中耦合矩阵数值算法的精度. 最后, 利用改善后的全局矩阵耦合简正波模型(DGMCM)计算了美国声学学会(ASA)提出的可穿透楔形波导标准问题, 将所得数值解与参考解比较, 结果表明DGMCM方法可以精确处理水平变化波导中声传播实际问题. 关键词： 耦合简正波理论 全局矩阵方法 可穿透楔形波导     

Attenuation of noise by windbreaks

Sound attenuation by narrow forest belts, under quasi-line source conditions has been investigated. Experiments were conducted on windbreaks of Casuarina and Eucalyptus belts, along three sites at Nobria.Windbreaks of Casuarina were found to act as sound barriers, which reduce the highway noise resulting from trucks, cars and other traffic. Reduced or even negative attenuation is, however, recorded in some locations behind mixed windbreaks of Eucalyptus and Casuarina as a result of downward scattering of acoustic propagation.     

On the scattering of sound by a magnetic field in a MHD fluid

We investigate the effect of a localized magnetic field on the propagation of sound in an infinite fluid described by the magnetohydrodynamic equations (MHD). An externally imposed magnetic field will scatter an acoustic wave, and the scattered wave is related to the spatial structure of the magnetic field. Measuring it is thus a non-intrusive probe for the magnetic field. Simple examples likely to be encountered in practice are worked out, and estimates are given that suggest the practical feasability of this diagnostic tool in current MHD experiments.Received: 1 May 2003, Published online: 15 October 2003PACS: 42.25.Fx Diffraction and scattering - 47.65.+a Magnetohydrodynamics and electrohydrodynamics - 52.35.Dm Sound waves     

Sound velocity determination in gel-based emulsions

Sound velocity is a main parameter in non destructive characterization, closely related to the elastic properties and to the microstructure of heterogeneous materials.The accurate determination of the sound velocity using pulse-echo technique relies on the ability to reduce pulse distortion and to measure specimen dimensions with a high precision. In the field of bio-mimetic materials and biological tissues, the nature of the specimen makes this last requirement highly difficult or inappropriate.The present work, using a through-transmission configuration, allows, in a stress free environment, to access the sound velocity in soft, low acoustic contrast materials without requiring the specimen dimensions. The specimen sound velocity is obtained from the echo time-of-flights through a Z-scan process providing the absolute medium sound velocity as reference.The technique uses an excitation burst at a frequency below the transducer resonance to ensure a significantly reduction in pulse distortions and improve signal-to-noise ratio. The accurate determination of the echo time-of-flight relies on a highly efficient cross-correlation/Hilbert transform signal processing.The method has been applied to gel-based emulsions of different microstructures considered as biomimetic phantoms, as well as to their constituents: pure gelatin and vegetable oil.     

沉积层声学特性对浅海低频声场分布的影响

已有对浅海低频水声场的讨论多是关注声能量在水体中的分布特性,对水体下沉积层、基底中低频声传播的同步研究相对较少。本文基于波动方程,在柱坐标系下推导了一种浅海水体/海底统一波导下低频声能流的计算模型,在此基础上结合具体仿真算例与波动理论阐述了不同沉积层声学参数对声场能量分布的影响规律及机理。仿真结果表明,在沉积层纵波声速>水中声速>沉积层横波声速的前提下,沉积层中密度与纵波声速数值越大,声能量越趋于保留在水体中而不向海底泄漏,横波声速的影响正好相反;沉积层厚度增加到一定量后,基底对流体层中声传播的影响可近似忽略不计。