界面混响的耦合简正波理论研究

利用耦合简正波理论分析了粗糙界面散射引起的混响。引入了粗糙海底以及海面引起的各个模态间的耦合系数表述粗糙界面对声场的散射过程。结合风浪谱Pierson模型,仿真了不同海况下的海面混响,重点在于分析不同海况下海面起伏对海底混响的影响。利用耦合系数的求解从粗糙海面引起模态间能量耦合的角度对这种影响进行了机理性的探究。结果表明,尽管在浅海中海底混响占支配性地位,但随着海况等级的增加,海面散射对海底混响是有影响的,而这种影响可以从粗糙界面对声波模态间的耦合进行机理分析。      

浅海平均混响强度的简正波理论

本文用简正波方法导出了浅海远程混响强度的一般表式,该式对较广泛的一类声速剖面、海底反射与散射系数皆适用。由于考虑了复本征值对简正波入射场的影响,所得结果比以往的混响场简正波理论有较大的改进。所得出的平均混响强度的表式物理意义清楚,形式简单,便于进行数值计算与解析讨论。作为例子,文中分析了负跃层浅海中平均混响强度的衰减规律。计算表明,当声源与接收器皆位于跃变层之上时,远程混响强度与r^-5成正比。     

识别与提取简正波的一种简化方法

本文提出了一种识别与提取浅海简正波的简化方法。海上实验时我们利用同一个水听器,在不同深度接收固定距离上的爆炸信号,实验室回放时爆炸声信号经高频(8kHz)与低频(<0.8kHz)带通滤波,并用高频信号作为低频信号时间采样的起点。理论与实验表明,这样得到的不同接收深度的低频信号相当于一个垂直水听器阵接收的结果。对不同接收深度的低频信号分别按简正波本征函数或按本征函数的符号加权相加,较好地分离出前二号简正波。     

激冲次声信号简正波的分离

由于大气层的不均匀性,次声波分解成不同的简正方式来传播,它们有各自的相速度和群速度。当由一个声学阵接收次声波时,不同的简正波在声阵上产生不相同的时间延迟。本文根据各个简正波的主要能量分布在不同的频率区间,从理论上导出计算简正波相速度的平均数值和平均方位的方法,利用速度滤波估算出各个简正波的频谱,通过Weiner-Hopf滤波,从一个声阵接收到的数据中,估算出空间任何一点某个简正波的波形。最后给出了计算机模拟计算,其结果与理论上设定的十分接近。     

基于简正波理论的浅海低频混响时间序列仿真

混响场时域信号序列的数值实现对主动声纳目标模拟器研制具有重要意义。本文所述适用于水平变化浅海环境混响时间序列的一种实现方法,基于现有的浅海简正波相干混响理论。所述方法的思路是通过相干散射场随机序列与发射信号的离散卷积来构成混响时域序列,文中以Pekeris均匀浅海环境为例,给出了相应的混响时间序列仿真结果,并对仿真结果的频谱特性、统计特性、强度衰减以及垂直相关特性等进行了系统检验。结果表明,由仿真混响时间序列所获得的混响特性与理论结果相符,可用于浅海混响序列的仿真。     

水平不变海洋声道中的WKBZ简正波方法

本文利用本征函数的WKBZ近似和海面相移修正发展了一种新的计算海洋声场的数值方法,该方法具有形式简明、易于计算、精度满足实用要求的优点.本文给出了WKBZ方法应用于北太平洋声道中声传播的一些数值结果,这些结果表明该方法是一种计算分层海洋声道会聚区声场的快速而精确的数值方法.     

水平阵信号压缩感知用于简正波分离

针对水平阵信号简正波分离过程中常规波束形成分辨率低以及warping模态滤波不适用于复杂声信号的问题,提出水平阵信号压缩感知用于简正波分离的方法。利用压缩感知在方位估计中的高分辨特性,通过估计水平阵接收信号在频率方位角上的二维分布,分离得到各阶简正波的方位谱,并逆Fourier变换得到时域波形。仿真孔径1 km、阵元间隔10 m水平阵接收20~200 Hz伪随机声信号和脉冲声信号,所提方法分离出的各阶简正波与理论波形的相关系数在0.97~1.0。对2011年北黄海声学实验中的海底28元水平阵接收的气枪信号,在合成至1 km孔径后使用压缩感知方法分离简正波,其与warping模态滤波分离得的前5阶简正波相关系数在0.82~0.93。仿真与实验都说明了水平阵信号压缩感知简正波分离方法的有效性。     

室内声场简正波法的数值计算方法和混响场中声强测量的误差

简正波法是求解室内声场的一种较准确的数值计算方法,准确度由所取级数的项数而定,本文详细讨论了数值计算方法、级数的收敛性,并以图线来表明在声源点级数的发散及在声源附近计算的困难,本文还讨论声强测量的理论误差,用一般声强计在混响场中测量声强,误差是很大的,以致所得结果完全不可信任,如将传声器易位、前后两次测得的声强取平均,测量误差就可大大减小,若能采用精密仪器,使两个通道的相位差小于0.05°,则测量结果的统计误差可控制在合理范围内。     

大振幅简正波

本文将简正波理论推广到大振幅声场,计及声场内的非线性效应。这涉及非线性偏微分方程的固有振动解的问题。简正波具有驻波性质,基本已反映在一阶近似解中。高阶近似解只是非线性引致参量的局部变化或修正,并不影响分布。根据这个概念解决了过去的数学困难,求得了大振幅简正函数,为线性商正函数及其谐频项,此外还有常数项,代表辐射压,与经典辐射压值作了比较。     

浅海波导中简正波相干分量的奇异值分解提取方法

浅海波导中的简正波相干特性是水声学研究的重要对象之一。提出了一种不需任何先验信息即可从一定带宽声信号中分离、提取各简正波相干分量的方法。首先利用信号功率谱数据构造一个厄米特矩阵;然后对该矩阵作奇异值分解,所得诸奇异向量即分别惟一地对应了声场中某2号简正波的相干分量。推导证明了本文方法的基本原理,通过数值仿真和海上试验验证了其有效性,最后讨论了简正波相干分量提取结果在声源被动测距方面的应用。     

Extraction of mode attenuation coefficient from low frequency reverberation signal

A method of extracting normal mode attenuation coefficient from low frequency reverberation signal has been proposed.Pseudo-inverse normal mode filtering method is implemented to get single mode reverberation field firstly.Based on the assumption of separability of modal back-scattering matrix,effective back-scattering matrix element can be calculated using single mode average reverberation intensity.Finally,mode attenuation coefficient is extracted by comparing effective back-scattering matrix elements at different ranges.The extracted mode attenuation coefficients are used to predict sound transmission loss at the same experiment area. Results show that the predicted transmission loss agrees well with the measured data.This method avoids the difficult of treating the coupling between bottom scattering attenuation and normal mode propagation attenuation.Research on extraction of mode attenuation coefficient from low frequency reverberation signal is useful for both geoacoustic inversion and rapid underwater environment assessment.     

Monotonic curvature of low frequency decay records in reverberation chambers

In the papers by Larsen [1] and Brüel [2], two interesting problems connected with reverberation room measurements are pointed out and discussed. The first problem is that the ensemble averaged decay curve reveals a monotonic curvature at low frequencies. The second phenomenon is that often systematically larger sound power output values are reported at low frequencies according to the free field method than according to the reverberation room method. In searching for an explanation of these anomalies some measurements and a classical normal mode theory analysis have been made. It is shown that it is not possible to explain fully the curvature of the low frequency decay curves by means of the normal mode theory. The measured curves are more bent than the respective theoretical ones. Most probably, it should be possible to explain this lack of agreement by the fact that the absorption characteristics of normal reverberation chambers significantly deviate from the situation of uniform wall admittance which has been assumed in the theoretical deductions. The theoretical analysis and the comparison between theory and practice indicate that the damping characteristics of the individual waves vary much more than is predicted for a uniform wall admittance. This reasoning is supported by the observation that the monotonic curvature increases when a plane concentrated absorbent is added to one of the walls. One way to decrease the curvature has also been identified. When the room surfaces are provided with randomly placed small samples of low frequency absorbents the resulting decay curves turn out to be almost perfectly linear. Furthermore, it is found that the normal mode theory does not imply significantly different sound power output values than the ISO 3741 model. This fact has been verified with a comparative test. According to the normal mode theory the average sound power output as measured in the reverberant room should equal the free field output. Therefore, one is forced to conclude that the analysis of the classical normal mode theory fails in explaining the anomalies observed.     

Accelerated Monte Carlo method to reconstruct absorption coefficient from low coherent signal

A fast and effective method for reconstruction of optical absorption property of scattering object is developed. A revised Monte Carlo forward model is set up with new detection criteria for simulation of low coherence interferometer (LCI) system. Calculation of coherent signal could be accelerated by a scaling method based on baseline data simulated by Monte Carlo method. The inverse procedure of scaling method is utilized to extract absorption coefficient by dichotomy algorithm. Fat emulsion and stain mixture of Intralipid? and India ink are taken as scattering and absorbing sample. The results of reconstruction have shown the linear relationship between absorption and concentration of India ink. The maximum absolute relative deviation is 6.21%.     

Closed-form expressions for ocean reverberation and signal excess with mode stripping and Lambert's law

Closed-form expressions for two-way propagation and reverberation in variable depth ducts are derived for isovelocity water by using ray invariants and acoustic flux. These expressions include the transition to single mode propagation at long range. Three surface scattering laws are considered: Lambert, Lommel-Seeliger, and angle independent, and these are compared with a point target to give explicit signal-to-reverberation ratios. In particular, there is interesting and sometimes surprising behavior when the propagation obeys mode-stripping (the high angles are preferentially attenuated by bottom losses) whilst the scattering obeys Lambert's law (high angles are preferentially back-scattered). There may be conditions where the signal-to-reverberation ratio is independent of range so that there is no reverberation range limit. Bottom slope dependence of both target echo and reverberation is surprisingly weak. The implications of refraction are discussed. The angle dependence for a point or surface scatterer at a given range can be translated into arrival time, so it is possible to calculate the received pulse shape for one-way or two-way paths. Because the tail is exponential with a range-independent half-life that only depends on bottom reflection properties there is scope for extracting geoacoustic information from the pulse shape alone. This environmental time spread is also of use to sonar designers.     

浅海低频本地海底混响的波导不变性结构及抑制方法研究

根据浅海混响的射线简正波相干混响理论和可分离的海底散射模型,从理论上推导了浅海低频本地海底混响信号中存在的稳定的距离-频率干涉结构(波导不变性结构),并在此基础上基于谱相减的思想提出了一种基于波导不变性的本地海底混响预测对消方法。针对某个检测距离,其对应的中心频率上的海底混响强度可以根据波导不变性通过频移的方法在另一个与之相邻的距离上估计得到,利用该估计的混响强度可以对消检测距离对应的回波信号中的海底混响强度分量,增强回波信混比,提高目标检测能力。针对典型水平分层浅海波导环境,通过计算机仿真实验分析了低频海底混响存在的稳定波导不变性结构,验证了本文提出的基于波导不变性的混响预测对消方法的有效性。     

Waveguide invariance structure for low frequency monostatic bottom reverberation mitigation in shallow water

The stable range-frequency interference structure(waveguide invariance) of the low frequency monostatic bottom reverberation in shallow water is derived theoretically in this paper by using ray-normal mode coherent reverberation model and separable bottom scattering model.And combined the concept of spectral subtraction,a reverberation forecasting and mitigating method based on the waveguide invariance is presented.According to the waveguide invariance,the reverberation intensity for a detecting range can be estimated from the reverberation returns scattered from some other range cells nearby the detecting range with frequency shift.Then the reverberation component for the detecting range in the echo can be canceled mostly by using the estimated reverberation intensity,so as to enhance the echo-to-reverberation ratio and improve the detection capability of the active sonar.Simulations in typical horizontal layered shallow water environment show the stability of waveguide invariance structure in the low frequency monostatic bottom reverberation and the efficiency of the reverberation forecasting and mitigating method based on the waveguide invariance.     

An oscillation phenomenon of low frequency reverberation in the shallow water and its physical explanation

~~An oscillation phenomenon of low frequency reverberation in the shallow water and its physical explanation1 Zhang, R.. Jin. G. Normal-mode theory of average reverberation intensity in shallow water, Journal of Sound and Vibration, 1987 19(2):215-223 2. Ellis, D. D., A shallow-water normal-mode reverberation model, J. Acoust. Soc, Am., 1995, 97(5): 2804-2814. 3. McDaniel, S. T., Seafloor reverberation fluctuations, J. Acoust. Soc. Am., 1990, 88(3): 1530-1535. 4. Abraham, …     

Azimuthal dependence of reverberation from a harmonic signal propagating in shallow water

The problem of scattering from a rough sea surface is considered for a harmonic low-frequency (200–400 Hz) acoustic signal propagating in a shallow water area. An experimental study of the azimuthal reverberation dependence is carried out on the basis of the bistatic location scheme with the use of linear phased arrays. By comparison with calculations, the formation of the frequency-angular characteristics of reverberation is investigated for various wind directions with respect to the path of acoustic signal propagation.     

Method of coupled mode for long-range bottom reverberation

The theory of coupled mode is used for modeling the long-range bottom reverberation in shallow water caused by bottom roughness. The distant bottom reverberation level and spatial coherence of impulsive source are both derived. The results agree with those from the classical reverberation model, and are compared with the experimental data. The influence of source bandwidth and the distance between sources and receivers on the intensity of bottom reverberation are particularly discussed. The method is shown to be available for both the monoand the bi-static cases.