Cross-correlation in band-limited ocean ambient noise fields

Observations of ambient noise in the ocean are generally band limited, because of the natural spectral shape of the noise or the restricted bandwidth of the detection system. Either way, the noise may be regarded as white noise to which a band-limiting filter has been applied. An analysis of the two-point cross-correlation function of such filtered noise is presented for two cases, isotropic and surface-generated noise. The most pronounced effects occur with high-pass and bandpass filters when the low-frequency cut-off falls well above the first few zeros in the coherence function. In this situation, the sensor separation is very many times the longest acoustic wavelength (associated with the lowest frequency) in the passband. The filtering then produces sharp pulses at correlation delays equal to the numerical value of the acoustic travel time between the sensors. Although these pulses are narrow, they have a finite width, within which a fine structure appears in the form of multiple rapid oscillations, due to the differentiating action of the filter. The number of such oscillations increases as the low-frequency roll-off of the filter becomes steeper. This fine structure is evident in several recently published experimental determinations of the cross-correlation function of band-limited ocean ambient noise.     

随机共振法估计二水听器时延差

针对二元阵航船目标方位估计问题,提出了一种基于随机共振滤波的航船辐射噪声双水听器时延差估计方法。设计了一种随机共振滤波器,提取航船螺旋桨旋转噪声在水听器接收信号中出现的本地时间,并通过滤波结果的互相关来估计目标辐射噪声分别到达两个水听器的时延差。在5 dB的输入信噪比下进行200次仿真,提出方法时延估计的归一化均方误差要低于互谱方法一个数量级。对于海试中的非合作航船,提出的方法可以在1 km距离下准确估计出目标辐射噪声到达双水听器的时延差,从而为在二元阵上实现对航船目标的准确测向提供基础。      

Evaluation of generalized cross-correlation methods for direction of arrival estimation using two microphones in real environments

The localization of sound sources, and particularly speech, has a numerous number of applications to the industry. This has motivated a continuous effort in developing robust direction-of-arrival detection algorithms, in order to overcome the limitations imposed by real scenarios, such as multiple reflections and undesirable noise sources. Time difference of arrival-based methods, and particularly, generalized cross-correlation approaches have been widely investigated in acoustic signal processing, but there is considerable lack in the technical literature about their evaluation in real environments when only two microphones are used. In this work, four generalized cross-correlation methods for localization of speech sources with two microphones have been analyzed in different real scenarios with a stationary noise source. Furthermore, these scenarios have been acoustically characterized, in order to relate the behavior of these cross-correlation methods with the acoustic properties of noisy scenarios. The scope of this study is not only to assess the accuracy and reliability of a set of well-known localization algorithms, but also to determine how the different acoustic properties of the room under analysis have a determinant influence in the final results, by incorporating in the analysis additional factors to the reverberation time and signal-to-noise ratio. Results of this study have outlined the influence of the acoustic properties analysed in the performance of these methods.     

Acoustic multipath structure in direct zone of deep water and bearing estimation of tow ship noise of towed line array

In the towed line array sonar system, the tow ship noise is the main factor that affects the sonar performance. Conventional noise cancelling methods assume that the noise is towards the endfire direction of the array. An acoustic experiment employing a towed line array is conducted in the western Pacific Ocean, and a strange bearing-splitting phenomenon of the tow ship noise is observed in the array. The tow ship noise is split into multiple noise signals whose bearings are distributed between 10° and 90° deviating from the endfire direction. The multiple interferences increase the difficulty in recognizing the target for the sonar operator and noise cancellation. Therefore, making the mechanism clear and putting forward the tow ship noise splitting bearing estimation method are imperative. In this paper, the acoustic multi-path structure of the tow ship in deep water is analyzed. Then it is pointed out that the bearing-splitting phenomenon is caused by the main lobe of direct rays and bottom-reflected rays, as well as several side lobes of direct rays. Meanwhile, the indistinguishability between the elevation angle and the bearing angle due to the axial symmetry of a strict horizontal line array causes the bearing to deviate from the endfire direction. Based on the theory above, a method of estimating bearing of the tow ship noise in deep water is proposed. The theoretical analysis results accord with the experimental results, which helps to identify the target and provide correct initial bearing guidance for noise cancelation methods.     

利用海洋环境噪声估计海流流速

为了观测海流在短时间尺度上的变化,基于被动声层析原理,提出了一种利用浅海海洋环境噪声估计海流流速的方法.通过波束形成增加噪声互相关函数的能量积累,从环境噪声互相关函数提取出两个水平阵列间的经验格林函数,利用经验格林函数的时间到达结构反演阵列间的浅海海流流速.海上实验数据处理结果表明,该方法提取了2h时间平均的经验格林函...     

Recovering the acoustic Green's function from ambient noise cross correlation in an inhomogeneous moving medium

We study long-range correlation of diffuse acoustic noise fields in an arbitrary inhomogeneous, moving fluid. The flow reversal theorem is used to show that the cross-correlation function of ambient noise provides an estimate of a combination of the Green's functions corresponding to sound propagation in opposite directions between the two receivers. Measurements of the noise cross correlation allow one to quantify flow-induced acoustic nonreciprocity and evaluate both spatially averaged flow velocity and sound speed between the two points.     

噪声交叉关联强度的时间周期调制对线性过阻尼系统的随机共振的影响

针对由加性、乘性噪声和周期信号共同作用的线性过阻尼系统, 在噪声交叉关联强度受到时间周期调制的情况下,利用随机平均法推导了系统响应的信噪比的解析表达式. 研究发现这类系统比噪声间互不相关或噪声交叉关联强度为常数的线性系统具有更丰富的动力学特性, 系统响应的信噪比随交叉关联调制频率的变化出现周期振荡型随机共振, 噪声的交叉关联参数导致随机共振现象的多样化.噪声交叉关联强度的时间周期调制的引入有利于提高对微弱周期信号检测的灵敏度和实现对周期信号的频率估计. 关键词： 随机共振 周期振荡型共振 噪声交叉关联强度 信噪比     

基于互相关的气泡速度的测量方法研究

通过对比互相关技术和多普勒技术的差别，从一个新的角度把互相关技术应用于尾流测速的研究中.实验中利用高速CCD相机拍摄模拟尾流图像.利用快速傅里叶变换实现互相关算法，为提高计算准确度采用高斯曲线拟合互相关函数的峰值并给出拟合公式，得到了亚像素级的运动速度.为标定互相关算法的准确度，文中采用图像平移的方法验证算法的可行性和可靠性.最后把算法应用于模拟尾流的实验中.该方法有望在舰船尾流速度信息的实时获取上起到重要的应用价值.     

Predicted effects of sensorineural hearing loss on across-fiber envelope coding in the auditory nerve

Cross-channel envelope correlations are hypothesized to influence speech intelligibility, particularly in adverse conditions. Acoustic analyses suggest speech envelope correlations differ for syllabic and phonemic ranges of modulation frequency. The influence of cochlear filtering was examined here by predicting cross-channel envelope correlations in different speech modulation ranges for normal and impaired auditory-nerve (AN) responses. Neural cross-correlation coefficients quantified across-fiber envelope coding in syllabic (0-5 Hz), phonemic (5-64 Hz), and periodicity (64-300 Hz) modulation ranges. Spike trains were generated from a physiologically based AN model. Correlations were also computed using the model with selective hair-cell damage. Neural predictions revealed that envelope cross-correlation decreased with increased characteristic-frequency separation for all modulation ranges (with greater syllabic-envelope correlation than phonemic or periodicity). Syllabic envelope was highly correlated across many spectral channels, whereas phonemic and periodicity envelopes were correlated mainly between adjacent channels. Outer-hair-cell impairment increased the degree of cross-channel correlation for phonemic and periodicity ranges for speech in quiet and in noise, thereby reducing the number of independent neural information channels for envelope coding. In contrast, outer-hair-cell impairment was predicted to decrease cross-channel correlation for syllabic envelopes in noise, which may partially account for the reduced ability of hearing-impaired listeners to segregate speech in complex backgrounds.     

Analysis on spatial difference of ocean ambient noise spectrum in the northern South China Sea

To study the spatial difference of ocean ambient noise in offshore China and reveal its generation mechanism, the difference between the noise level(NL) of two stations is compared based on long-term observation data collected in the same season. Considering the influence of underwater acoustic channels and the characteristics of noise sources, the transmission loss level(TL) from the port ship noise sources to the measuring station is analyzed and it shows a strong correlation between the transmission loss difference(TLD) and noise level difference(NLD) at the band of 50-500 Hz. At the same time, the analysis of ship noise source level(SL) presents a difference about 20 dB between the two ports, corresponding to the variation of the ship density before and during the fishing moratorium. The results prove that the ambient noise is closely related to ship noise sources at this frequency band, and the noise difference between the two stations results from the transmission loss and the ship noise sources.     

南海北部海洋环境噪声谱级空间差异性分析

为研究中国近海海洋环境噪声的空间差异性,揭示其形成原因,以同一海域同一季节两个测量站位的长期观测数据为研究对象,对比两个站位噪声谱级的差异.结合海洋信道和噪声源特性的影响,分析港口强噪声源到噪声采集站位的传播损失,发现在50~500 Hz频段传播损失差值和噪声谱级差值呈现较强相关性;对港口航船噪声源级的分析发现,该频段内源级相差20 dB左右,与休渔期前后航船密度差异相对应。试验分析和研究结果表明,南海北部海域50~500 Hz频段内海洋环境噪声与航船噪声源密切相关,两站位噪声空间差异由海区传播条件差异与航船噪声源级差异共同引起。      

分布式矢量传感器对宽带源被动定位的方法研究

本文给出了两种基于矢量传感器的被动定位方法。一般来说，对目标进行被动定位可以通过方位信息或时延信息来实现。单个矢量传感器就可以实现目标方位估计，而对多个矢量传感器接收的信号进行互相关运算即可得到目标对各矢量传感器的时延信息。本文先给出了利用方位信息进行被动定位的方法，然后给出了利用分布式矢量传感器进行方位一时延信息联合被动定位的新方法，并通过仿真分析对这两种方法的定位性能进行了比较。     

Stochastic Resonance in Linear Region of a Single-Mode Laser： Effects of Amplitude Modulation of Signal

A single-mode laser noise model driven by quadratic colored pump noise and amplitude modulation signal is proposed. The real and imaginary parts of the pump noise are assumed to be cross-correlation. The effect of cross-correlation of noise and amplitude modulation of signal on laser statistical properties is studied by using the linearized approximation. The analytic expression of signal-to-noise ratio (SNR) is calculated. It is found that the phenomena of stochastic resonance (SR) respectively exist in the curves of the SNR versus the noise cross-correlation coefficient λ and the SNR versus the pump parameter a, as well as the SNR versus the signal frequency ω in our model. It is shown that there are three different typies of SR in the model: the conventional form of SR, the SR in the broad sense, and the bona fide SR.     

透射边界条件在点声源海底地震波数值计算中的应用

航行舰船在海底岩土层中引起的弹性波被称为舰船地震波,主要由舰船低频辐射噪声引起,可用于识别舰船目标。本文采用大型有限元软件ANSYS中的流固耦合计算模块,结合多次透射公式(MTF)人工边界条件,对较为简单的水平分层海洋环境下低频点声源海底地震波进行了数值计算,并为了分析数值计算方法的精度,将数值计算结果与波数积分方法结果进行了比较。频率较低时两种方法的声传播损失曲线符合较好,频率较高时结果存在一定差别;两种计算方法所得声传播频率特性曲线的形式有所不同,但曲线总体变化趋势基本一致;而海底表面竖直位移、水平位移、竖直加速度和水平加速度的频率特性曲线基本符合。由此认为,基于多次透射公式人工边界,采用大型有限元软件ANSYS中的流固耦合模块,对较复杂环境下低频点声源海底地震波问题进行数值计算是基本可行的。     

基于时延神经网络模型的舰船辐射噪声目标识别*

水声目标被动识别是水声信号处理领域的研究热点之一。海洋环境中存在的不规则噪声干扰,使得基于传统方法的水声目标被动识别技术在实际的应用场景中效果不佳。本文采用一种基于时延网络(Time Delay Neural Network,TDNN)模型的舰船辐射噪声目标识别方法,该方法利用目标的短时平稳特性和长时关联特性对目标的声纹特征进行建模,使用梅尔谱图提取目标信号的初级特征,再通过融合注意力机制和时延神经网络的深度学习模型实现高级特性提取,最后再利用余弦相似度实现不同目标的类别划分。该方法在ShipsEar数据集和自行采集的数据进行测试验证,目标识别准确率分别达到79.2%和73.9%,可证明本文方法的有效性。     

声纳导流罩吸声障板声学特性的研究

本文研究内容属于舰艇声兼容性技术研究的一部分。应用声学软件SYSNOISE计算了,声纳定向发射时某试验用声纳导流罩及尾部吸声障板的声场特性,同时进行相同工况的水下模型试验。计算结果与试验结果吻合良好,说明应用声学软件对声纳声场进行计算是一种可行的方法,有可能推广至其他的水下模型的声场计算。结果数据反映了吸声障板对声纳定向发射时声场的影响,为舰艇总体声纳导流罩内吸声障板的结构设计、声兼容设计提供依据。     

Exchanged ridge demodulation of time-scale manifold for enhanced fault diagnosis of rotating machinery

The vibration or acoustic signal from rotating machinery with localized fault usually behaves as the form of amplitude modulation (AM) and/or frequency modulation (FM). The demodulation techniques are conventional ways to reveal the fault characteristics from the analyzed signals. One of these techniques is the time-scale manifold (TSM) ridge demodulation method with the merits of good time–frequency localization and in-band noise suppression properties. However, due to the essential attribute of wavelet ridge, the survived in-band noise on the achieved TSM will still disturb the envelope extraction of fault-induced impulses. This paper presents an improved TSM ridge demodulation method, called exchanged ridge demodulation of TSM, by combining the benefits of the first two TSMs: the noise suppression of the first TSM and the noise separation of the second TSM. Specifically, the ridge on the second TSM can capture the fault-induced impulses precisely while avoiding the in-band noise smartly. By putting this ridge on the first TSM, the corresponding instantaneous amplitude (IA) waveform can represent the real envelope of pure faulty impulses. Moreover, an adaptive selection method for Morlet wavelet parameters is also proposed based on the smoothness index (SI) in the time-scale domain for an optimal time-scale representation of analyzed signal. The effectiveness of the proposed method is verified by means of a simulation study and applications to diagnosis of bearing defects and gear fault.     

船舶噪声激励夜光虫发光特性实验研究

对船舶噪声刺激下的生物发光特性展开研究,发光生物为夜光虫。利用声学换能器模拟船舶辐射噪声,在含有发光生物夜光虫的实验水槽中进行生物发光实验。声学换能器发出不同频率,对夜光虫受不同频率噪声激励下的发光特性进行记录。通过对不同激励频率下夜光虫发光图像的分析处理,得到不同频率激发下生物发光点图像,经统计分析得到了夜光虫在船舶噪声激励下的发光特性,得出夜光虫发光点数随频率和声源级的增加而增加的结论。     

The statistical fluctuation of a single-mode laser system driven by coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts

Using the linear approximation method, this paper studies the statistical property of a single-mode laser driven by both coloured pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts, and calculates the steady-state mean normalized intensity fluctuation and intensity correlation time. It analyses the influences of the modulation signal, the net gain coefficient, the noise and its correlation form on the statistical fluctuation of the laser system respectively. It is found that the coloured pump noise modulated by the signal has a great suppressing action on the statistical fluctuation of the laser system; the pump noise self-correlation time and the specific frequency of modulation signal have the result that the statistical fluctuation tends to zero. Furthermore, the `colour' correlation of pump noise has much influences on the statistical fluctuation of the laser system. Increasing the intensity of pump noise will augment the statistical fluctuation of the laser system, but the intensity of quantum noise and the coefficient of cross-correlation between its real and imaginary parts have less influence on the statistical fluctuation of the laser system. Therefore, from the conclusions of this paper the statistical property can be known and a theoretical basis for steady operation and output of the laser system can be provided.     

Influence of Signal and Noise on Statistical Fluctuation of Single-Mode Laser System

On the basis of calculating the steady-state mean normalized intensity fluctuation of a signal-mode laser system driven by both colored pump noise with signal modulation and the quantum noise with cross-correlation between its real and imaginary parts, we analyze the influence of modulation signal, noise, and its correlation form on the statistical fluctuation of the laser system. We have found that when the amplitude of modulation signal weakens and its frequency quickens, the statistical fluctuation will reduce rapidly. The by reducing the intensity of pump noise and quantum noise. statistical fluctuation of the laser system can be restrained Moreover, with prolonging of colored cross-correlation time, the statistical fluctuation of laser system experiences a repeated changing process, that is, from decreasing to augmenting, then to decreasing, and finally to augmenting again. With the decreasing of the value of cross-correlation coe~cient, the statistical fluctuation will decrease too. When the cross-correlation form between the real part and imaginary part of quantum noise is zero correlation, the statistical fluctuation of laser system has a minimum. Compared with the influence of intensity of pump noise, the influence of intensity of quantum noise on the statistical fluctuation is smaller.