Patch nearfield acoustic holography combined with sound field separation technique applied to a non-free field

The conventional nearfield acoustic holography(NAH) is usually based on the assumption of free-field conditions, and it also requires that the measurement aperture should be larger than the actual source. This paper is to focus on the problem that neither of the above-mentioned requirements can be met, and to examine the feasibility of reconstructing the sound field radiated by partial source, based on double-layer pressure measurements made in a non-free field by using patch NAH combined with sound field separation technique. And also, the sensitivity of the reconstructed result to the measurement error is analyzed in detail. Two experiments involving two speakers in an exterior space and one speaker inside a car cabin are presented. The experimental results demonstrate that the patch NAH based on single-layer pressure measurement cannot obtain a satisfied result due to the influences of disturbing sources and reflections, while the patch NAH based on double-layer pressure measurements can successfully remove these influences and reconstruct the patch sound field effectively.     

变频定距法测量声速

设计了变频定距声速测量装置,发声喇叭和拾音喇叭分别安装在有机玻璃圆管的两端,保持距离不变,发声喇叭的激励信号和拾音喇叭的输出信号均接到示波器.通过连续改变喇叭的发声频率,比较发声信号和拾音信号的相位,利用相位比较法测量声速.     

A simple acoustic model to characterize the internal low frequency sound field in centrifugal pumps

Conventional centrifugal pumps with volute casing generate fluid-dynamic noise particularly at the so-called blade-passing frequency, which is attributed to the interaction of the flow exiting the pump impeller with the volute tongue. Following previous work by the authors to characterize the effect of that blade–tongue interaction on the tonal sound produced, this paper presents a simple acoustic model for centrifugal pumps that considers ideal sound sources of arbitrary position and properties. The model is to be implemented in a software code that applies it systematically to a pump previously tested at laboratory, until identifying the set of ideal sources that best reproduce the pressure fluctuation measurements. In this model, the ideal sources are assumed to radiate plane sound waves along the impeller channels, volute, and outlet diffuser. The volute was considered to be composed by a succession of slices, each of them equivalent to a linear 3-port acoustic system with individual sound transmission and reflection coefficients. A series of tests was conducted to check the validity of the acoustic model, by applying an external acoustic load onto the pump outlet duct and measuring the noise reflected. The resulting reflection coefficient was in good agreement with the predictions of the acoustic model. Finally, the model was used to investigate the pump internal sound field at the blade-passing frequency when operating at 70% of nominal flow rate. It was concluded that the sound field can be characterized reasonably by a dipole-like source located at the tongue region.     

The sound field in a hydroacoustic waveguide with an uneven hard bottom

An analytic representation is constructed for a nonaxially symmetric sound field to simulate a hydroacoustic waveguide the bottom of which is hard and has an axially symmetric relief. A numerical analytic method for finding the velocity potential is proposed, for which undetermined coefficients for normal modes are determined from a corresponding infinite system of algebraic equations. The sound fields are studied with for variations of the problem parameters.     

一种可稳定计算Pekeris波导中声场的波数积分方法

提出一种可稳定计算Pekeris波导中声场的波数积分方法,并在此基础上开发出一个数值模型,可用于提供Pekeris波导中声场的精确、稳定的数值解。在这个方法中,由于与深度有关的波动方程齐次解中所有的上行波与下行波均采用了合理的归一化表示,从而得到的系统方程是无条件稳定的。在简正波方法中,割线积分一般只对近场有显著影响。因此,传统的简正波模型一般都忽略割线积分对声场的贡献。但是,如果某号简正波离割线非常近,则割线积分对非常远距离的声场仍可能有显著影响。在这种情况下,传统的简正波模型由于忽略割线积分的贡献,从而得到的声场结果是不准确的。本文通过数值算例比较本文提出的波数积分模型与传统的简正波模型。数值结果表明,本文提出的模型可以提供精确、稳定的Pekeris波导中声场的数值解,而在某些情况下传统的简正波模型得到的声场结果是不准确的。因此,本文提出的模型可以作为Pekeris波导中声传播问题的标准模型使用。     

Correlation reception of a diffraction sound field in an oceanic waveguide

Intensity matched processing is considered for a diffracted signal when the object is detected by the main lobe of the shadow scattered sound field. The consideration is based on the few-parameter model that adequately describes the characteristics of the desired signal. It is shown that, in the absence of noise and fluctuations in the parameters of the medium, the proposed algorithm provides the determination of the exact parameters of the signal and the moving object.     

一种低声速沉积层海底参数声学反演方法

软泥底环境下沉积层参数的声学反演是国际水声领域的一个研究热点.浅海中,当高声速基底和海水之间存在一层低声速(小于海水声速)的沉积层时,小掠射角情况下不同频率声传播损失会出现周期性增大现象.基于此现象,提出一种适用于低声速沉积层的海底参数声学反演方法.首先,推导给出小掠射角情况下传播损失周期增大的频率间隔与沉积层声速、厚度及近海底海水声速之间的解析表达式;其次,利用一次黄海实验中软泥底环境下的宽带声传播信号,提取了小掠射角下传播损失增大的频率周期;再次,把该解析表达式作为约束条件,结合Hamilton密度与声速的经验公式,采用匹配场处理反演给出沉积层的声速、密度、厚度及基底的声速、密度;然后,利用声传播损失数据反演得到泥底环境下不同频率的声衰减系数,通过拟合发现泥底声衰减系数随频率近似呈线性关系;最后,给出了双层海底模型和半无限大海底模型等效性的讨论.反演结果为低声速沉积层海底声传播规律研究与应用提供了海底声学参数.     

Illumination of the shadow zone in a two-channel oceanic waveguide with a fine structure of sound speed inhomogeneities

The experimental data on the sound field structure, which were obtained by emitting a continuous pseudonoise signal (a midfrequency of 3.2 kHz) in a two-channel oceanic waveguide, are compared with the calculations performed by the wave program with allowance for the fine structure of the sound speed inhomogeneities. A considerable increase in the intensity of the sound field with a definite angular spectrum is observed in the upper channel in the first shadow zone, and, in the experiment, the increase begins nearer to the sound source than predicted by the wave and ray calculations for a smooth sound speed profile. These features of the field structure are explained by the illumination of the shadow zone by the regular scattering of signals from highly anisotropic fine-structure inhomogeneities of the sound speed profile, which are clearly pronounced in the region of the given oceanic experiment.     

Experimental and theoretical study of density,potential, and current structures of a helium plasma in front of an radio frequency antenna tilted with respect to the magnetic field lines

Potential and density structures in the vicinity of an radio frequency (RF) electrode/antenna in a magnetized plasma are investigated using an RF-compensated cylindrical Langmuir probe. These measurements were performed in the ALINE plasma device in which only electrons can be considered well magnetized. Very precise 2-D maps of the plasma parameters are drawn thanks to a 3-D automatic manipulator on which the probe is mounted. The effect of the tilted magnetic angle between the RF-biased surface and the magnetic lines is also studied thanks to a tilting electrode. Comparison of several simplistic models with the experiments proved the reliability of simple Langmuir probe measurements in such an RF and magnetized environment (space potential vs. tilting angle of the antenna with respect to magnetic field lines and recovery of the floating potential structure using measured currents). A fluid model based on total current density and ion diffusion equations over the biased flux tube provides the same density structures in front of the electrode as the measurements. Those density structures display a “bunny ears” shape and can be explained using transverse RF and collisional current behaviour: In front of the antenna, the transverse ion currents deplete the magnetized flux tube, while at the edge of the biased flux tube, the same currents increase the density.     

电磁声换能器的辐射声场研究

分析了一种圆环形螺线圈、垂直偏置磁场结构的电磁声探头在非铁磁性介质铝中的辐射声场特性。首先根据电磁感应原理对探头进行了物理和数学建模,将表面力源近似成水平和竖直分布的两种,且水平分量远大于竖直分量,推导出该种力源产生的声场的理论解析解。分别数值计算出了横波和纵波的切向和法向指向性,并和实验结果进行对比。实验结果和理论计算符合较好,为此种结构的电磁声换能器的实际工程应用提供了理论依据。      

On the acoustic field in a Pekeris waveguide with attenuation in the bottom half-space

The acoustic field in a Pekeris channel with an attenuating basement is critically examined, based on contour integrations of the wave number integrals for the two domains. In both regions, the field consists of a finite sum of proper (square integrable) normal modes plus a branch line integral around a hyperbolic cut. For low bottom attenuation, only "trapped" modes exist but as the loss increases additional "dissipation" modes contribute to the mode sum. A Newton-Raphson iterative procedure is introduced for finding the complex eigenvalues of the modes and a new expression is derived which shows that the total number of proper (trapped plus dissipation) modes supported by the waveguide increases essentially linearly with rising bottom attenuation. Approximations for the complex shape functions of the modes in the water column and the basement are developed and compared with the exact shape functions determined from the Newton-Raphson procedure. An expression derived for the modal attenuation coefficient scales in proportion to the square of the mode number and inversely with the square of the frequency. Stationary-phase approximations for the branch line integrals for both domains are developed, which serve to illustrate the asymptotic range dependence of the lateral wave but otherwise have little utility.     

Properties of the acoustic intensity vector field in a shallow water waveguide

Acoustic intensity is a vector quantity described by collocated measurements of acoustic pressure and particle velocity. In an ocean waveguide, the interaction among multipath arrivals of propagating wavefronts manifests unique behavior in the acoustic intensity. The instantaneous intensity, or energy flux, contains two components: a propagating and non-propagating energy flux. The instantaneous intensity is described by the time-dependent complex intensity, where the propagating and non-propagating energy fluxes are modulated by the active and reactive intensity envelopes, respectively. Properties of complex intensity are observed in data collected on a vertical line array during the transverse acoustic variability experiment (TAVEX) that took place in August of 2008, 17 km northeast of the Ieodo ocean research station in the East China Sea, 63 m depth. Parabolic equation (PE) simulations of the TAVEX waveguide supplement the experimental data set and provide a detailed analysis of the spatial structure of the complex intensity. A normalized intensity quantity, the pressure-intensity index, is used to describe features of the complex intensity which have a functional relationship between range and frequency, related to the waveguide invariant. The waveguide invariant is used to describe the spatial structure of intensity in the TAVEX waveguide using data taken at discrete ranges.     

Sound propagation and scattering in media with random inhomogeneities of sound speed,density and medium velocity

Abstract

This review presents both classical and new results of the theory of sound propagation in media with random inhomogeneities of sound speed, density and medium velocity (mainly in the atmosphere and ocean). An equation for a sound wave in a moving inhomogeneous medium is presented, which has a wider range of applicability than those used before. Starting from this equation, the statistical characteristics of the sound field in a moving random medium are calculated using Born-approximation, ray, Rytov and parabolic-equation methods, and the theory of multiple scattering. The results obtained show, in particular, that certain equations previously widely used in the theory of sound propagation in moving random media must now be revised. The theory presented can be used not only to calculate the statistical characteristics of sound waves in the turbulent atmosphere or ocean but also to solve inverse problems and develop new remote-sensing methods. A number of practical problems of sound propagation in moving random media are listed and the further development of this field of acoustics is considered.     

Sound propagation and scattering in media with random inhomogeneities of sound speed, density and medium velocity

This review presents both classical and new results of the theory of sound propagation in media with random inhomogeneities of sound speed, density and medium velocity (mainly in the atmosphere and ocean). An equation for a sound wave in a moving inhomogeneous medium is presented, which has a wider range of applicability than those used before. Starting from this equation, the statistical characteristics of the sound field in a moving random medium are calculated using Born-approximation, ray, Rytov and parabolic-equation methods, and the theory of multiple scattering. The results obtained show, in particular, that certain equations previously widely used in the theory of sound propagation in moving random media must now be revised. The theory presented can be used not only to calculate the statistical characteristics of sound waves in the turbulent atmosphere or ocean but also to solve inverse problems and develop new remote-sensing methods. A number of practical problems of sound propagation in moving random media are listed and the further development of this field of acoustics is considered.     

Antenna array in the sound field of an arctic waveguide

An antenna array placed in an arctic-type acoustic waveguide is considered. The guided sound field consists of a signal generated by a point source and an isotropic interfering noise produced by the ice cover. The array is operated in a specific regime: the output signals correspond to individual modes of the sound field. The signal received by the array is subjected to correlation processing with a finite averaging time. It is shown that, depending on the processing method, the signal-to-noise ratio can reach 40–60 dB for a realization duration of 1 min, and the quality of the signal detection is increased.     

声速不均匀介质热声成像的声场仿真

为了精确表示声速不均匀介质热声成像的声场,提出了一种时域有限差分(FDTD)仿真方法。先用FDTD将热声成像的基本方程离散化,给出描述热声成像的离散差分方程;再对电磁波脉冲进行两次修正,以消除高频电磁波引入的计算误差。计算机仿真研究的结果表明:当介质声速不均匀时,FDTD仿真方法其准确度高于目前常用的飞行时间法。在仿真实验条件下,当介质的声速差异大于50%时,FDTD法的误差比飞行时间法小十倍以上。可见,FDTD方法是一种有效的热声成像声场仿真方法,可以为声速不均匀介质热声成像算法的研究提供理论基础。     

Efficiency of sound field focusing in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning the focal point in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically considered. The localized fields are controlled by varying the transmission frequency without a change in the distribution of the reversed field over the array aperture. The effect of the periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path relative to the propagation direction of internal waves.     

Tracking-positioning of sound speed profiles and moving acoustic source in shallow water

An ensemble Kalman filter(EnKF) approach is proposed to perform sequential tracking of water column sound speed profile(SSP) using a moving acoustic source. First,the SSPs are discretized in depth and range, and are expressed by the empirical orthogonal functions(EOFs). Second, the acoustic source state information and the first three orders of EOF coefficients are expressed as the state variable, and the acoustic field information received by the vertical line array are the measured values. Successively, the state variables and measured values are used to establish the state-measure model. Last, the EnKF is utilized to track the state variables. The simulation results show that the root mean square error of SSP and the absolute error of source are all small, and thus the acoustic source tracking-positioning has high accuracy. Moreover, increasing the number of sample collection, the signal-to-noise ratio and the number of receiving elements can improve the tracking-positioning results. The method is verified using the experimental data of the East China Sea.