Investigation of the spatial correlations of flow noise in vector hydrophone towed linear array

Following the wall pressure spectrum of the turbulent boundary layer developed by Corcos, a method in the frequency-wavenumber space was presented to analyze the flow noise in the vector hydrophone towed linear array. The general forms of the acoustic pressure and particle velocity in the flow noise field were obtained, and the spatial correlations of the flow noise were calculated. The numerical analysis results based on wavenumber integration show that： （1） The spatial correlations of flow noise drops rapidly with increasing axial separation between the elements, so the flow noise received by different vector hydrophones usually sampled in a half-wavelength rate can be considered as independent; （2） The flow noise is highly correlated in the radial direction at low frequency, and only those of high frequency componet can be neglected.     

Research on sound wave receiving theory of inertial sphere type vector hydrophone

Sound pressure wave and particle velocity wave of the receiving theory of iner- tial sphere type vector hydrophone was studied. Based on measurement principle of particle velocity hydrophone with co-oscillating sphere, the mathematical expressions of sound wave receiving response to both rigid sphere and elastic sphere in a freely moving acoustic plane- wave field were derived, and the relationship between the frequency response curves of velocity hydrophone and its geometrical dimension and density was analyzed. In addition, according to sound wave receiving theory of spherical receiving transducer, the mathematical expression of the vector hydrophone＇s sound pressure receiving response was derived, and the laws of the pressure distribution on the surface of particle velocity hydrophone and relationships between the pressure receiving coefficient of the vector hydrophone and the parameters, such as the dimensions of the receiving surface, the radius of the particle velocity hydrophone, the layout position and radius of sound pressure hydrophone, were analyzed and calculated. The analysis method of sound wave receiving theory of vector hydrophone was established, which lays the theoretical foundation for design and development of the vector hydrophone.     

A theory on spatial correlation and vertical directivity of surface-generated ambient noise in the sea

A theoretical model of ambient sea noise including surface noisesources and stratified medium ocean is discussed.The noise sources areassumed to be statistically independent directional point sources distributedover the ocean surface,and the effects of ocean environment on ambientnoise are studied.The normal-mode theory of surface-generated noise isdeveloped,and the normaL-mode formula of the directional density func-tion suitable for small grazing angles is analytically continued for beingsuitable for great grazing angles and consistent with the ray formula.Theunified formulae for calculating the intensities,spatial correlation andvertical directivity of ambient sea noise are presented.     

Ocean dynamic noise energy flux directivity in the 400 Hz to 700 Hz frequency band

Results of field studies of underwater dynamic noise energy flux directivity at two wind speeds, 6 m/s and 12 m/s, in the 400 Hz to 700 Hz frequency band in the deep open ocean are presented. The measurements were made by a freely drifting telemetric combined system at 500 m depth. Statistical characteristics of the horizontal and vertical dynamic noise energy flux directivity are considered as functions of wind speed and direction. Correlation between the horizontal dynamic noise energy flux direction and that of the wind was determined; a mechanism of the horizontal dynamic noise energy flux generation is related to the initial noise field scattering on ocean surface waves.     

The diffracted sound field from the transition region of an axisymmetric body in water

Understanding the physical features of the diffracted sound field on the surface of an axisymmetric body is important for predicting the self-noise of a sonar mounted on an underwater platform.The diffracted sound field from the transition region of an axisymmetric body was calculated by the geometrical theory of diffraction.The diffraction ray between the source point and the receiving point on the surface of an axisymmetric body was calculated by using the dynamic programming method.Based on the diffracted sound field,a simulation scheme for the noise correlation of the conformal array was presented.It was shown that the normalized pressure of the diffracted sound field from the transition region reduced with the increases of the frequency and the curvature of the ray.The flow noises of two models were compared and a rather optimum fore-body geometric shape was given.Furthermore,it was shown that the correlation of the flow noise in the low frequencies was stronger than that in the high frequencies.And the flow noise received by the acoustic array on the curved surface had a stronger correlation than that on the head plane at the designed center frequency,which is important for sonar system design.     

Design of sonar transducer based on the laser-induced sound

A design of a kind of sonar transducers was presented. An experimental system was developed for producing the sound signal based on the laser-induced breakdown and bubble oscillation. The power spectrum and the ambiguity function were analyzed. The range resolution and the Doppler resolution constant of the sound signal were determined. It is basically proved in theory that both the range resolution and directivity of the sonar transducer are very high. The transducer can be a sound source of high-resolution sonars.     

Detection of number of sources and DOA estimation based on the combined information processing of pressure and particle velocity using acoustic vector sensor array

In order to solve the problem of DOA(direction of arrival)estimation of underwater remote targets,a novel subspace-decomposition method based on the cross covariance matrix of the pressure and the particle velocity of acoustic vector sensor arrays(AVSA)was proposed. Whereafter,using spatio-temporal virtual tapped-delay-line,a new eigenvector-based criteria of detection of number of sources and of subspace partition is also presented.The theoretical analysis shows that the new source detection and direction finding method is different from existing AVSA based DOA estimation methods using particle velocity information of acoustic vector sensor(AVS)as an independent array element.It is entirely based on the combined information processing of pressure and particle velocity,has better estimation performance than existing methods in isotropic noise field.Computer simulations with data from lake trials demonstrate,the proposed method is effective and obviously outperforms existing methods in resolution and accuracy in the case of low signal-to-noise ratio(SNR).     

THE ACOUSTICAL QUALITY OF HALLS——A COMPARISON OF SIMULATION TO EXPERIMENT

The acoustical quality of an ordinary hall,namely the reverberation time and the spatial nonuni-formity of sound pressure level in audience area,was simulated on a microcomputer.Comparisonbetween calculation,measurement and simulation of reverberation time shows that the reverberationtime from simulation is closer to measurement than from calculation,if the absorption of the audience isproperly chosen,in the frequency range from l25 to 4000 Hz;the average deviation is less than 10%.For the spatial nonuniformity of sound pressure level,there is a good correspondency botween thesimulation and the measurement.In addition,some details in simulation were discussed.     

Application and measurement of underwater acoustic reciprocity transfer functions in reverberant sound field

The reciprocity measurement theory in anomalous reverberant sound fields was investigated.An improved method Was proposed due to the interrelated errors.The source volume velocity Was corrected by spatial average of measurement results and evaluation of the reverberant sound field influence on acoustic energy density.The result was validated in underwater experiment,corrected reciprocity measurement results were almost the same as direct measurement results.It indicates that reverberant sound field does not affect the validitv of the principle,but influences the obtainment of source volume velocity,then influences the measurement of transfer functions with the principle.The proposed method is simple and effective in anomalous reverberant sound fields.The study mav be valuable for the applications which are based on the principle.     

RESEARCH AND TECHNICAL NOTES LETHAL THRESHOLD CURVE OF GUINEA-PIGS IN HIGH-INTENSITY SOUND FIELD

Some results from the study of the physiological effects of high level broad-band noise fields(165-173 dB)on guinea-pigs are presented in this paper.The lethal time for guinea-pigs undervarious sound pressure levels was determined.The logarithm of exposure time and the sound pres-sure level had good linear correlation.The experimental results showed that lethal time was decreasedby half if the sound pressure level was increased by 3 dB.These results agreed with the law ofequal energy and showed the death of guinea-pigs depended on the noise-dose,i.e.the noise in-tensity times the exposure duration.The average value of the lethal noise-dose is a constant,about33 J/mm~2.     

Spatial correlation and coherence in reverberant acoustic fields: Extension to microphones with arbitrary first-order directivity

Spatial correlation and coherence functions in reverberant sound fields are relevant to the acoustics of enclosed spaces and related areas. Theoretical expressions for the spatial correlation and coherence functions between signals representing the pressure and/or the components of the particle velocity vector in a reverberant sound field are established in the literature and most of these have also been corroborated with measurements [F. Jacobsen, J. Acoust. Soc. Am. 108, 204-210 (2000)]. In the present paper, these expressions are generalized to microphones of first-order directivity, whereby the directivity can be expressed in terms of pressure and pressure gradient. It is shown that the resulting spatial correlation and coherence functions can be expressed in terms of the established spatial correlation and coherence functions. The derived theoretical expression for the spatial coherence function is validated with a modeled diffuse sound field. Further, it is compared with the experimental coherence obtained from the reverberant tails of room impulse responses measured with two common surround sound microphone setups in a concert and a lecture hall.     

A model for the spatial coherence of arbitrarily directive noise in the depth-stratified ocean

Though referred to as noise, the ambient ocean soundscape carries valuable information about the physical ocean environment. To study this information, Kuperman and Ingenito introduced a model for spatial coherence in a depth stratified ocean arising from the vertically directive diffuse acoustic noise produced by bubbles distributed throughout the surface. Here the model is adapted to incorporate horizontal directivity as well, making it possible to include additional noise contributions from directive features such as storms, biologics, shipping, and wave breaking. As an analytic approach, the model can serve as a computationally light complement to existing methods.     

Array gain for a conformal acoustic vector sensor array:An experimental study

An acoustic vector sensor can measure the components of particle velocity and the acoustic pressure at the same point simultaneously, which provides a larger array gain against the ambient noise and a higher angular resolution than the omnidirectional pressure sensor. This paper presents an experimental study of array gain for a conformal acoustic vector sensor array in a practical environment. First, the manifold vector is calculated using the real measured data so that the effects of array mismatches can be minimized. Second, an optimal beamformer with a specific spatial response on the basis of the stable directivity of the ambient noise is designed, which can effectively suppress the ambient noise.Experimental results show that this beamformer for the conformal acoustic vector sensor array provides good signal-tonoise ratio enhancement and is more advantageous than the delay-and-sum and minimum variance distortionless response beamformers.     

利用矢量海洋环境噪声提取声场格林函数

考虑到矢量水听器在垂直方向上具有8字形指向性,能够有效抑制远方非平稳噪声源的干扰,提出了一种矢量环境噪声相关函数(NCF)提取声场时域格林函数(TDGF)的方法。基于简正波理论建立了声压和垂直振速垂直相关性模型。在此基础上,给出了声压和垂直振速相关函数提取声场纵向格林函数的过程.数值仿真对比和实验数据分析表明,相对于声压提取方法,垂直振速提取方法能够有效消除直达波前出现的亮纹与亮区干扰。此外,对于同等时间长度噪声序列,声压提取方法只提取到直达波路径,而垂直振速提取方法还提取到了我们更为关心的海底反射路径。利用直达波与海底反射波到达时延差估计的海深与实测海深吻合较好。      

Imaging of directional distributed noise sources

This study relates to the acoustic imaging of noise sources that are distributed and strongly directional, such as in turbulent jets. The goal is to generate high-resolution noise source maps with self-consistency, i.e., their integration over the extent of the noise source region gives the far-field pressure auto-spectrum for a particular emission direction. Self-consistency is possible by including a directivity factor in the formulation of the source cross-spectral density. The resulting source distribution is based on the complex coherence, rather than the cross-spectrum, of the measured acoustic field. For jet noise, whose spectral nature changes with emission angle, it is necessary to conduct the measurements with a narrow-aperture array. Three coherence-based imaging methods were applied to a Mach 0.9 turbulent jet: delay-and-sum beamforming; deconvolution of the beamformer output; and direct spectral estimation that relies on minimizing the difference between the measured and modeled coherences of the acoustic field. The delay-and-sum beamforming generates noise source maps with strong spatial distortions and sidelobes. Deconvolution leads to a five-fold improvement in spatial resolution and significantly reduces the intensity of the sidelobes. The direct spectral estimation produces maps very similar to those obtained by deconvolution. The coherence-based noise source maps, obtained by deconvolution or direct spectral estimation, are similar at small and large observation angles relative to the jet axis.     

Stream ambient noise, spectrum and propagation of sounds in the goby Padogobius martensii: sound pressure and particle velocity

The most sensitive hearing and peak frequencies of courtship calls of the stream goby, Padogobius martensii, fall within a quiet window at around 100 Hz in the ambient noise spectrum. Acoustic pressure was previously measured although Padogobius likely responds to particle motion. In this study a combination pressure (p) and particle velocity (u) detector was utilized to describe ambient noise of the habitat, the characteristics of the goby's sounds and their attenuation with distance. The ambient noise (AN) spectrum is generally similar for p and u (including the quiet window at noisy locations), although the energy distribution of u spectrum is shifted up by 50-100 Hz. The energy distribution of the goby's sounds is similar for p and u spectra of the Tonal sound, whereas the pulse-train sound exhibits larger p-u differences. Transmission loss was high for sound p and u: energy decays 6-10 dB10 cm, and sound pu ratio does not change with distance from the source in the nearfield. The measurement of particle velocity of stream AN and P. martensii sounds indicates that this species is well adapted to communicate acoustically in a complex noisy shallow-water environment.     

一种矢量水听器基阵模拟器的设计

矢量水听器的每个阵元同时测量声场中声矢量和质点振速的3个分量,相对于声压水听器阵来说,矢量阵获取声场中更多的信息。利用矢量阵所获得速度场的信息可以去除目标方位估计中的 180°模糊。模拟器可以模拟实际海洋环境中目标的辐射特性和噪声特性,应用模拟器可以有效地缩短声纳的研制周期。本文提出一种矢量水听器基阵模拟器的设计方案,该方案解决了矢量阵中宽带信号的90°移相问题、时延精确控制问题和宽带噪声的谱状控制问题。     

快速随机粒子网格法的气动噪声预测方法

耦合随机湍流速度生成模型与线化欧拉方程技术,形成了一套具备模拟噪声在非均匀流场中传播能力的气动噪声混合预测方法。该混合方法的随机湍流速度生成模型采用了快速随机粒子网格法,为声传播模拟提供了可靠的源项。而噪声的传播计算选用线化欧拉方程,其空间离散采用9点5阶的色散保持关系格式,时间推进选用了高精度大时间步长的6级4阶龙格库塔格式,远场边界应用了无分裂形式的理想匹配层边界条件。首先,选用高斯脉冲传播算例对线化欧拉方程的时空离散格式、远场无反射边界条件进行了验证分析。然后,计算分析各向同性湍流的空间相关性验证湍流速度生成模型的可靠性。最后,基于已搭建的气动噪声混合预测方法进行了30P30N三段翼缝翼噪声的计算分析。计算分析可知:监测点处功率谱密度曲线、噪声指向性等计算结果与参考文献结果取得了较好的一致性。数值计算结果表明所建立的气动噪声混合预测方法能有效预测二维复杂构型的气动噪声问题。      

Passive acoustic tomography of the ocean using arrays of unknown shape

It was recently established that ocean acoustic tomography based on an inversion of ray travel times can be implemented without use of any dedicated sound sources by cross-correlating the ambient noise recorded on two line arrays, the shapes of which are known. In contrast to active tomography, the amount of useful information from noise interferometry is proportional to the product of the numbers of receivers in the two arrays. In our study based on the 2D and 3D numerical experiments, we examine a hypothesis concerning the feasibility of simultaneous performance of a passive ray tomography and passive positioning of arrays through cross-correlation of ambient or shipping noise. The numerical experiments are conducted under conditions close to those of a field experiment on passive ocean tomography. It is demonstrated that, when using arrays of 20–40 hydrophones, the sound velocity profile and the array shape can be found from noise correlation to an accuracy adequate for oceanological and acoustic applications.     

Highly directional acoustic receivers

The theoretical directivity of a single combined acoustic receiver, a device that can measure many quantities of an acoustic field at a collocated point, is presented here. The formulation is developed using a Taylor series expansion of acoustic pressure about the origin of a Cartesian coordinate system. For example, the quantities measured by a second-order combined receiver, denoted a dyadic sensor, are acoustic pressure, the three orthogonal components of acoustic particle velocity, and the nine spatial gradients of the velocity vector. The power series expansion, which can be of any order, is cast into an expression that defines the directivity of a single receiving element. It is shown that a single highly directional dyadic sensor can have a directivity index of up to 9.5 dB. However, there is a price to pay with highly directive sensors; these sensors can be significantly more sensitive to nonacoustic noise sources.