南海夏季海洋环境噪声与海面风速相关特性分析

海洋环境噪声是影响潜艇作战及远程目标探测和定位的重要参数,由于海洋环境噪声具有很强的时空变异性,因此只能通过试验数据测量和处理了解环境噪声的统计特性。本文以2013年4月30日至8月1日,南海典型海域布放海洋环境噪声测量潜标系统所收录的环境噪声为研究对象,数据处理结果表明,在整个频率范围内,海洋环境噪声与对数风速具有很好的线性关系,但环境噪声随风速变化的斜率与频率相关,在1kHz附近达到最大值。本文通过波束形成分析了海洋环境噪声的垂直指向性,并给出了“垂直方向噪声”、“水平方向噪声”和“全向噪声”与风速相关性分析结果,可以看出,在风浪噪声与远处航船噪声都有贡献的中频段,“垂直方向噪声”与海面风速的相关性好于“全向噪声”和“水平方向噪声”与海面风速的相关性,且“垂直方向噪声”随风速变化的斜率略大于“全向噪声”随风速变化的斜率。     

海面随机起伏对噪声场空间特性的影响规律

对于1 k Hz以上声波,海面起伏会对浅海声传播产生显著影响,现有的噪声预报模型在建模过程中基本没有考虑海面起伏的影响.针对这一问题,本文基于传输理论建立了随机起伏界面下噪声场垂直相关性和指向性模型,仿真分析了海面起伏对噪声强度、垂直相关性与指向性的影响.结果表明,对于表面噪声,海面随机起伏使声波能量从中间阶简正波向低阶和高阶简正波转移,而对噪声强度起主要贡献的一般是中间阶简正波,所以海面起伏使得噪声强度减弱;简正波之间能量的耦合导致垂直平面上不同掠射角方向上到达的声波响应发生变化,经由海面反射大掠射角到达的声波响应以及中小角度到达的声波响应变弱,而经由海底反射大掠射角到达的声波响应变强;海面随机起伏还会扰动各阶简正波相位,使不同阶简正波互相关性变弱,致使噪声场的空间相关性也变弱.     

涡旋对深海风成噪声垂直空间特性的影响

涡旋是深海环境中频繁出现的海洋现象,它会引起上层海水的声速扰动,改变海面风成噪声的传播过程,最终导致噪声场特性异常.本文采用高斯涡模型描述涡旋引起的声速扰动,分别使用射线和抛物方程模型描述近场和远场噪声信号的传播,研究了涡旋对其水平中心位置不同深度上的风成噪声垂直空间特性(包括噪声垂直方向性和垂直相关性)的影响.研究表明:1)在涡心深度上,涡旋对噪声垂直空间特性的影响最大,其中冷涡导致噪声垂直方向性中水平凹槽的宽度增加,凹槽下边缘峰值的高度降低,噪声垂直相关性减弱,暖涡的影响反之; 2)在远离涡心的深度上,涡旋对噪声垂直空间特性的影响减小,冷涡和暖涡分别仅引起噪声垂直方向性中水平凹槽下边缘峰值的高度升高和降低,对噪声垂直相关性几乎没有影响; 3)涡旋对噪声垂直空间特性的影响均随其绝对强度增大而增强.针对以上现象,使用射线逆推方法分析了涡旋影响噪声垂直空间特性的机理.该方法由噪声接收点发射声线,利用声场互易性分析噪声沿声线反向到达接收点的俯仰角和能量大小.分析表明,存在涡旋时,噪声沿海面反射声线反向到达接收点的俯仰角和能量变化,是引起噪声垂直空间特性变化的主要原因.此外,仿真表明,当接收点偏离涡旋水平中心但两者距离较近时,研究中的分析和结论仍是近似成立的.     

岛礁海域海洋环境噪声垂直分布和垂直指向性

依据岛礁海域复杂海底地形、海试期间航船分布和实测风速数据,应用射线声传播理论,建立岛礁海域海洋环境噪声三维模型。在海试岛礁海域深海声道条件下,采用射线3D算法,仿真计算了32元垂直测量阵所处265~885 m负声速梯度深度范围内1 kHz风关和50 Hz远处航船海洋环境噪声级垂直分布,以及50 Hz航船海洋环境噪声垂直指向性,并与实测分析进行比较。结果表明,仿真结果与海试实测数据一致性良好。在本例海底起伏、接收点周边存在众多岛礁和海底山的三维环境中,1 kHz风关海洋环境噪声级随深度分布较近于均匀;西南方向较远处航道区海域海底较平坦,航道区至接收阵为缓斜坡海底,50 Hz远处航船海洋环境噪声级随深度有所增加,其噪声垂直指向性无明显水平凹槽。文中建立的岛礁海域海洋环境噪声三维模型,可较好地表征本例岛礁复杂地形海底起伏海域的风关和航船海洋环境噪声级的垂直分布、及航船环境噪声的垂直指向性,实测和仿真的岛礁海域海洋环境噪声相关数据,可供实际应用及相关研究参考。      

利用海洋环境噪声直接提取海底反射系数

提出了一种利用海洋环境噪声直接(非迭代)提取海底反射系数的方法,基于海洋环境噪声垂直相关函数与方向密度函数的相似性,结合海洋环境噪声射线理论,由两个垂直排列的水听器实现海底反射系数的被动获取.方法适应性的仿真分析说明较高海况下且近场无航船干扰的海洋环境噪声数据有利于方法的实现;对于两层海底,海洋环境噪声垂直相干函数经高...     

浅海舰船噪声声强流的垂直方向性

浅海环境中,确定性声源的多途声信号干涉使得接收点处声强流的方向发生改变,不再与声源位置处的声强流方向一致。只测量声场的标量声强时,无法得到接收点处声强流的垂直方向性,而基于简正波矢量场建模和仿真,可获得理想条件下宽带点声源激发声场声强流的垂直方向性。本文采用单矢量水听器进行海上实验,获得了海洋环境噪声和干扰条件下舰船噪声声强流的垂直方向性。仿真和实验结果表明:远场条件下,浅海干涉现象引起接收点处声强流的方向(极角)随频率和距离变化,其时间-频率分布呈现与LOFAR谱干涉条纹相似的条纹,声强流的极角值主要分布在70?～110?范围内。     

海面波浪对海洋环境噪声垂直空间相关性的影响

研究了由海浪谱导得的噪声源的相关函数对海洋环境噪声垂直空间相关性的影响。常用的海洋环境噪声空间相关性模型一般假设海面噪声源是非相关的,这种假设可以解释一些物理现象,但是与真实情况并不符合。考虑到风成噪声与海浪运动的相关性,引入海浪谱,得到噪声源的相关函数,利用Kuperman-Ingenito (K/I)噪声模型,计算海洋环境噪声的垂直相关性。通过仿真结果与实验数据对比可以看出,在高风速下,假设噪声源不相关时计算得到的噪声场垂直相关性与实验结果相差较大,而利用由海浪谱导得的噪声源的相关函数计算得到的噪声场的垂直相关性与实验结果符合较好。      

海面噪声的空间相关与垂直方向性理论

本文讨论由海面噪声源与分层介质海洋所组成的海洋环境噪声模型。假设噪声源是统计独立分布在海面上的指向性点源,研究噪声源特性以及传播条件对海洋环境噪声场的影响。发展了海面噪声的简正波理论,并在一定条件下将简正波理论得到的适合于小掠角范围的噪声场方向密度函数解析延拓至大掠角范围,使之与射线理论表式衔接起来,获得了计算海洋环境噪声强度、空间相关与方向密度函数的简明表式。     

冬季冲绳海域地形及黑潮对声传播的影响

冲绳海域地形复杂且冬季存在较强的黑潮海洋锋,利用数值实验研究斜坡地形和海洋锋同时存在时由浅海至深海的声传播特性。海洋模式数值预报环境数据表明,分布于冲绳海槽斜坡上方的海洋锋导致该海域上层水体声速存在水平变化,纬度越高,水平变化越大。利用抛物方程声场模型计算声传播损失,通过简正模态分析存在表面声道的环境中声能量分布,利用声线轨迹图解释海底斜坡和海洋锋对声传播的影响。结果表明:声源频率低于表面声道截止频率时,声传播主要受海底地形影响;声源频率高于表面声道截止频率时,位于表面声道内的声源激发的声能量主要在表面声道传播,部分声能量从表面声道泄漏沿斜坡向深海传播,位于表面声道深度以下的声源激发的声能量主要沿斜坡向深海传播,斜坡地形导致表面声道下方至共轭深度这一深度范围呈现为声影区,海洋锋的存在可导致表面声道传播损失变化明显,影响程度与声源深度有关。     

等声速环境中目标前向声散射简正波耦合的垂直阵空域响应特征

水下目标的前向声散射会引起声波传播过程中的简正波耦合效应,使得接收声场结构发生变化,研究目标前向散射引起的垂直阵列上空域响应变化特征,可实现对直达波强干扰背景下的前向散射检测。通过将垂直阵波束形成技术分别用于信道中目标散射场理论模型计算数据和湖上实验验证数据,分析了等声速环境中目标前向声散射简正波耦合的垂直阵空域响应特征。结果表明,目标靠近接收端时前向散射引起的声波垂直达到结构与无目标时相比差异显著,高阶简正波向低阶简正波转化导致信号到达时延宽度展宽,采用指向水平方向的窄波束可显著提取目标前向散射引起的接收声波变化特征。      

考虑噪声源深度分布的海洋环境噪声模型及地声参数反演

考虑到海洋环境噪声源深度分布不集中,建立了噪声源随深度分布的海洋环境噪声模型,分析了源深度对噪声场垂向特征的影响并从简正波角度予以解释,发现海底声阻抗和声源深度都显著影响由海洋环境噪声获得的等效海底反射损失大掠射角部分,进而将该模型用于地声参数反演.两段实测噪声数据200—525 Hz频段的反演结果表明:基于海洋环境噪声的地声参数反演最优值与声传播的反演结果相近;源平均深度最优值随频率增加有变小的趋势,说明随频率增加环境噪声主要贡献源逐渐由航船转为风浪;当海况大于3级时,400 Hz以上频段噪声源深度平均值很小,与Monahan气泡理论的描述一致.     

一种海洋环境噪声分步反演地声参数方法

分析不同海底参数对环境噪声垂向空间特性的敏感度,根据海底密度对环境噪声垂直指向性的小掠射角部分不敏感,而对等效海底损失的大掠射角部分相对敏感的特点,提出了一种海洋环境噪声分步反演地声参数方法:先用环境噪声垂直指向性小掠射角部分反演海底声速、衰减;之后利用大掠射角部分来反演海底密度。仿真算例和海上实验数据处理结果验证了该方法的有效性。结果表明:分步反演加强了匹配物理量对海底参数敏感部分的关注,在反演结果精度(海底密度)以及反演效率上都有所改善,具有一定可行性。      

风成噪声背景下垂直阵阵列信噪比随声源深度的变化规律

风成噪声是海洋中最广泛存在的环境噪声、是被动声纳处理性能的重要影响因素.它具有一个典型的物理性质是很大程度上只对应中高阶模态.阵列信噪比计入了阵列采样辐射声强、背景噪声功率和阵处理增益,是决定声纳阵列处理性能的关键物理量.本文研究了受风成噪声模态结构影响,典型夏季浅海环境中垂直阵阵列信噪比随声源深度的变化关系.在简正波模深函数采样完整的假设条件下,理论证明了阵列信噪比随声源深度的变化可近似为低阶模态幅度强度(模深函数模值的平方)随深度变化的线性叠加,且模态阶数越低,贡献越大;并且,在强风成噪声背景、显著负梯度环境下该变化规律可由1阶模态幅度强度随深度的变化近似独立表征.以上结果表明,在同一声源距离条件下,声源置于水体下半部分时的阵列信噪比比置于海面附近更大,并且在位于1阶模态峰值点所在深度附近时达到最大.典型负声速梯度浅海环境中的仿真实验结果对理论分析进行了验证,并表明在一定条件下阵列信噪比随声源深度的变化与声源距离近似无关.     

Extraction of acoustic normal mode depth functions using vertical line array data

A method for extracting the normal modes of acoustic propagation in the shallow ocean from sound recorded on a vertical line array (VLA) of hydrophones as a source travels nearby is presented. The mode extraction is accomplished by performing a singular value decomposition (SVD) of individual frequency components of the signal's temporally averaged, spatial cross-spectral density matrix. The SVD produces a matrix containing a mutually orthogonal set of basis functions, which are proportional to the depth-dependent normal modes, and a diagonal matrix containing the singular values, which are proportional to the modal source excitations and mode eigenvalues. The conditions under which the method is expected to work are found to be (1) sufficient depth sampling of the propagating modes by the VLA receivers; (2) sufficient source-VLA range sampling, and (3) sufficient range interval traversed by the source. The mode extraction method is applied to data from the Area Characterization Test II, conducted in September 1993 in the Hudson Canyon Area off the New Jersey coast. Modes are successfully extracted from cw tones recorded while (1) the source traveled along a range-independent track with constant bathymetry and (2) the source traveled up-slope with gradual changes in bathymetry. In addition, modes are successfully extracted at multiple frequencies from ambient noise.     

In-duct identification of fluid-borne source with high spatial resolution

Source identification of acoustic characteristics of in-duct fluid machinery is required for coping with the fluid-borne noise. By knowing the acoustic pressure and particle velocity field at the source plane in detail, the sound generation mechanism of a fluid machine can be understood. The identified spatial distribution of the strength of major radiators would be useful for the low noise design. Conventional methods for measuring the source in a wide duct have not been very helpful in investigating the source properties in detail because their spatial resolution is improper for the design purpose. In this work, an inverse method to estimate the source parameters with a high spatial resolution is studied. The theoretical formulation including the evanescent modes and near-field measurement data is given for a wide duct. After validating the proposed method to a duct excited by an acoustic driver, an experiment on a duct system driven by an air blower is conducted in the presence of flow. A convergence test for the evanescent modes is performed to find the necessary number of modes to regenerate the measured pressure field precisely. By using the converged modal amplitudes, very-close near-field pressure to the source is regenerated and compared with the measured pressure, and the maximum error was −16.3 dB. The source parameters are restored from the converged modal amplitudes. Then, the distribution of source parameters on the driver and the blower is clearly revealed with a high spatial resolution for kR<1.84 in which range only plane waves can propagate to far field in a duct. Measurement using a flush mounted sensor array is discussed, and the removal of pure radial modes in the modeling is suggested.     

Spatial coherence and cross correlation of three-dimensional ambient noise fields in the ocean

Ambient acoustic noise fields in the ocean are generally three dimensional in that they exhibit vertical and horizontal directivity. A model of spatially homogeneous noise is introduced in which the directionality is treated as separable, that is, the overall directionality of the field is the product of the individual directivities in the horizontal and vertical. A uni-modal von Mises circular distribution from directional statistics is taken to represent the noise in the horizontal, whilst the vertical component is consistent with a surface distribution of vertical dipoles. An analysis of the coherence and cross correlation of the noise at two horizontally aligned sensors is developed. The coherence function involves a single integral over finite limits, whilst the cross-correlation function, derived on the assumption that the noise has been pre-whitened, is given by an integral with limits that depend on the correlation delay time. Although the cross-correlation function does not exhibit delta functions that could be identified with the Green's function for propagation between the two sensors in the field, it does drop abruptly to zero at numerical time delays equal to the travel time between the sensors. Hence the noise could be used to recover the sound speed in the medium.     

Eigenmode analysis of phased-coupled VCSEL arrays using spatial coherence measurements

We apply the modal coherence theory to evaluate the spatial mode structure of a 2×2 phase-coupled array of vertical cavity surface emitting lasers (VCSELs). The eigenmode structure is extracted for different pump currents by measuring the degree of spatial coherence of all VCSEL pairs in the array. The results reveal the impact of optical disorder and spatial hole burning on the modal discrimination. The approach is useful more generally for the evaluation of spatial mode content of other laser array.     

Experimental investigation of an inversion technique for the determination of broadband duct mode amplitudes by the use of near-field sensor arrays

This paper is an experimental investigation of an inverse technique for deducing the amplitudes of the modes radiated from a turbofan engine, including schemes for stablizing the solution. The detection of broadband modes generated by a laboratory-scaled fan inlet is performed using a near-field array of microphones arranged in a geodesic geometry. This array geometry is shown to allow a robust and accurate modal inversion. The sound power radiated from the fan inlet and the coherence function between different modal amplitudes are also presented. The knowledge of such modal content is useful in helping to characterize the source mechanisms of fan broadband noise generation, for determining the most appropriate mode distribution model for duct liner predictions, and for making sound power measurements of the radiated sound field.     

深海不完整声道下反转点会聚区研究

近期南海远程声传播实验数据的处理分析表明在深海不完整声道中声道轴以下存在一种会聚区,该会聚区相比于海面附近的上反转点会聚区在远距离处具有更高的会聚增益.本文利用射线简正波理论确定了水中反转型焦散线和海面反射型焦散线位置,对比发现实验中观测到的深海大深度会聚区位置与水中反转型焦散线位置一致,证明该会聚区是由大量简正波同相叠加形成的下反转点会聚区,其在深海声道轴以下的一定深度范围内都具有会聚效应,研究了该会聚区的形成条件以及声源深度变化对会聚区焦散结构的影响,对比了远距离处上下反转点会聚区的传播损失以及会聚区宽度,分析表明第七个下反转点会聚区的会聚增益仍不小于10 dB,研究了声速垂直结构变化对下反转点会聚区的影响,理论分析结果与实验数据吻合较好.