Spatial attenuation of different sound field components in a water layer and shallow-water sediments

The paper presents the results of an experimental study of spatial attenuation of low-frequency vector-scalar sound fields in shallow water. The experiments employed a towed pneumatic cannon and spatially separated four-component vector-scalar receiver modules. Narrowband analysis of received signals made it possible to estimate the attenuation coefficients of the first three modes in the frequency of range of 26–182 Hz and calculate the frequency dependences of the sound absorption coefficients in the upper part of bottom sediments. We analyze the experimental and calculated (using acoustic calibration of the waveguide) laws of the drop in sound pressure and orthogonal vector projections of the oscillation velocity. It is shown that the vertical projection of the oscillation velocity vector decreases significantly faster than the sound pressure field.     

Approximate analytic representations of laws of attenuation in vector-scalar fields of multipole sources in a Pekeris waveguide

We obtain, and compare with exact solutions, the approximate analytic relations that determine, for increasing distance, irregularities of attenuation in the regular sound pressure components and orthogonal projections of the oscillation velocity vectors of low-frequency signals formed in a waveguide by various multipoles. We show that the mentioned field characteristics essentially depend on the type of multipole, the distance between the source and receivers, and the specific features of the received scalar or vector field components. It is established that the approximating dependences agree well with the exact laws of attenuation in the field and, despite the variety of dependences, they are divided into three compact groups with uniform characteristics.     

On the possibility of applying the reciprocity principle in the vector-scalar fields of mutipole sources in a waveguide

We study the applicability of the reciprocity principles in an underwater waveguide for the vector-scalar fields of multipole sources. We show analytically and numerically that multipole sources are divided into two groups according to this principle: in the first group, the sound pressure field and the horizontal projections of the vibration velocity vectors satisfy the reciprocity principle, while the vertical projections of these vectors do not. In the second group, the pressure and the horizontal projections of the vibration velocity vector do not satisfy the reciprocity principle, while their vertical projections do. We establish that the phase gradients and angles of arrival of signals in the vertical plane do not satisfy the reciprocity principle for the vector-scalar fields of volumetric sources with arbitrary directivity in the vertical plane.     

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。      

The possibility of using the equivalent plane wave model to increase the efficiency of taking bearings of low-frequency signals in shallow water

The possibility of approximating the sound field in the region of interference maxima using the equivalent plane wave model with the actual amplitude and the average ??effective?? phase velocity calculated or measured by the phase gradient at the array aperture is discussed. The method is substantiated by studying the mode, interference, and phase structures of the low-frequency sound field along with the spatial responses of an extended linear array. For bottom-moored or towed geophysical arrays whose sizes are large compared to the wavelength, both the necessity and the possibility of reducing the error in taking the bearing of a sound source in a waveguide are justified. The use of the proposed model is recommended for approximate matching of the array to the transfer function of the waveguide to reduce the bearing error.     

On the orthogonality of the phase velocity and its feasibility for plane waves

Phase and group velocities of plane waves may have different directions in a linear and homogeneous medium. This difference may lead to interesting situations such as the phase reversal propagation and the orthogonal phase velocity. Although the former has been analyzed in depth and it has found many applications, the latter has been left as a hypothesis. Actually, due to our best knowledge there is no evidence of such media where the phase velocity is orthogonal to the group velocity. In this paper we discuss this situation thoroughly. We show that a plane wave with orthogonal phase velocity cannot possess linear momentum. Moreover, we show that a plane wave without the linear momentum cannot propagate at all, so the orthogonality of the phase velocity does not take place in reality for electromagnetic plane waves.     

用于浅海有源声呐目标深度估计的匹配相位处理EI北大核心CSCD

针对浅海波导中有源声呐目标深度估计问题,提出了通过单个声源发射低频宽带信号,垂直双接收水听器接收目标回波,利用宽带目标回波比值的相位特征进行匹配相位处理的方法,不仅消除了目标散射特性对有源声呐目标深度估计的影响,而且仅需计算单程的信道传递函数,运算量小,有利于实时处理的实现需求。首先通过理论推导,定义了一个隐含目标深度、且与目标散射特性无关的声场特征——单程传播向量宽带干涉结构,在Pekeris波导条件下仿真分析了简正波阶数与单程传播向量宽带干涉结构的幅度和相位的关系,发现选取较多的简正波阶数贡献的单程传播向量宽带干涉结构进行匹配相位处理可以提高目标深度估计的性能。进一步仿真分析表明,在发射信号选取一定的时间长度和带宽的条件下,利用全部阶简正波且信噪比大于-10 dB时,方法的深度估计误差在5 m之内。最后分析了信号时间长度和处理带宽对有源声呐目标深度估计性能的影响,以及海底声速、海深和声速剖面失配时方法的鲁棒性。     

Excitation of the acoustic and leaky waves in the atmosphere by a sub-surface seismic source

We study excitation of acoustic, leaky, and surface waves by a time-harmonic force source located in a homogeneous isotropic elastic half-space contacting a homogeneous gas. The force acts in the normal direction to the interface between the media. We consider the case where the sound velocity in the gas is less than the velocity of the Rayleigh wave propagating along the surface of the solid. An expression is derived for the period-averaged radiation power of the surface Stoneley wave. The total radiation power is calculated for the acoustic wave in the gas and for the leaky pseudo-Rayleigh wave. Variations in the radiation powers of the surface and leaky waves are analyzed as functions of the source depth. If the velocities of compressional and shear waves in the elastic medium significantly exceed the sound velocity in the gas, then the radiation power of the Stoneley wave turns out to be a factor of 10⁶–10⁸ smaller than the radiation powers of other waves. The radiation power of the Stoneley wave decreases monotonically with increasing source depth, and the decrease becomes more pronounced with the increase in the difference between the acoustic impedances of the contacting media. If the shear-wave velocity in the solid is close to the sound velocity in the gas, then the radiation power of the Stoneley wave is comparable with the radiation powers of other waves and exhibits maximum at a certain source depth. For some parameters of the gas and the solid, and for certain source depths, the Stoneley wave carries away more than a half of the total radiation power. It is shown that, for certain relations between the parameters of the media, the radiation power of the Stoneley wave increases due to redistribution of the radiated power from the pseudo-Rayleigh leaky wave. The total power of these waves remains approximatly constant and, with accuracy of the order of 10⁻³, is equal to the radiation power of the Rayleigh wave at the vacuum-solid interface. It is shown that the acoustic-wave power which can be transmitted to the upper layers of the atmosphere during an earthquake does not exceed 0.01% of the total power radiated at a given frequency. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 577–592, July 2006.     

The use of effective phase velocity to decrease the direction finding error for a low-frequency signal in a waveguide

The mode, interference, and phase structures of the low-frequency sound field and the spatial responses of the extended linear array in a waveguide are studied. The wide-angle parabolic approximation and the normal mode method are used to perform calculations. A possibility of approximating the field in the zone of interference maxima by the equivalent plane wave model with the real amplitude and averaged effective phase velocity (EPV) calculated or measured from the phase gradient at the array aperture is investigated. The use of EPV in the zone of interference maxima is shown to decrease substantially the direction finding error. The conclusion is drawn that the array and the transfer function of the waveguide can be approximately matched if the speed of sound in water is substituted with the EPV in the bearing algorithm.     

Optimizing power oscillations in an ellipsometric system

Ellipsometry is a powerful and well-established optical technique used in the characterization of materials.It works by combining the components of elliptically polarized light in order to draw information about the optical system. We propose an ellipsometric experimental set-up to study polarization interference in the total internal reflection regime for Gaussian laser beams. The relative phase between orthogonal states can be measured as a power oscillation of the optical beam transmitted through a dielectric block, and the orthogonal components are then mixed by a polarizer. We show under which conditions the plane wave analysis is valid, and when the power oscillation can be optimized to reproduce a full pattern of oscillation and to simulate quarter- and half-wave plates.     

Resonance interaction of an edge-dislocation wall with a traveling sound wave

The dynamics of an infinite edge-dislocation wall and fragments of this wall in their interaction with a monochromatic sound wave with a nonzero wave vector has been investigated taking into account the mass of dislocations. It has been shown that the drift velocities of the wall fragments significantly increase when the sound wave frequency approaches the natural frequencies of small-amplitude vibrations, including zero frequency. For the infinite wall, a similar increase in the drift velocity is observed for low frequencies and for frequencies close to half the maximum frequency of small-amplitude vibrations of the edge-dislocation wall. The resonance increase in the drift velocity at low frequencies of the sound wave can be important for practical applications.     

On the electromagnetic energy density and energy transfer rate in a medium with dispersion due to conduction

The simplest dispersion relation determined by dissipation due to conduction is considered; the electromagnetic energy density in a plane monochromatic wave and its (phase and group) velocity are determined, as well as the energy and momentum transfer rates. It is shown that the energy density at low frequencies in this case has the form of the electrostatic density, in which the permittivity is replaced by its real part, and the energy transfer rate in a plane electromagnetic wave is equal to the phase velocity. The group velocity may exceed the speed of light.     

On the sound sources of screech tones radiated from choked circular jets

The generation mechanism of the screech tone in the helical oscillation mode is mainly investigated using a series of instantaneous schlieren photographs. From the photographs, five evanescent sound sources are observed as prominent points along the jet axis. The sound source for the dominant helical oscillation mode is found to be the second prominent point which moves along a circular orbit in a plane perpendicular to the jet axis and just downstream of the rear edge of the third shock cell. It is shown that the speed of a moving sound source is supersonic and that the Mach cone generated by the moving sound source forms the helical-shaped wave front of the screech tone for the helical oscillation mode of the jet. This idea of the moving sound source is well supported by a measured directionality of the screech tone. Sound sources of the other oscillation modes appearing in the other pressure ratio ranges are also described.     

Attenuation Regularities of Vector-Scalar Fields of Multipole Sources in Interference Maxima Regions

Limiting dependences determining the attenuation regularities (related to an increase in distance) of sound pressure and orthogonal components of vibration velocity vector in the regions of interference maxima of signals formed in a Pekeris waveguide by multipoles of different orders, have been obtained and numerically analyzed.     

Near fields scattered by an underwater finite cylindrical baffle

In this work,acoustic vector characteristics of near fields scattered by an underwater finite cylindrical baffle are investigated theoretically and experimentally.The analytic expressions for the scattered pressure and particle velocity are derived using the elastic thin shell theory.Calculations are presented for the scattered near fields of the pressure,the particle velocity and the intensity.It is found that the pressure and the particle velocity fields near the surface of the cylindrical baffle are characterized by complex interference structure,particle velocity directions and the source bearings are not consistent.The phase difference between the pressure and the particle velocity is not zero and the intensity vector does not reflect the sound bearings.It can be noted that the distortions of the fields will make the original vector signal processing method based on the free space assumption be no longer applicable in the presence of the cylindrical baffle.These results can serve as a basis of the application for the acoustic vector sensor on board.     

Mean force on a small sphere in a sound field in a viscous fluid

A mean force exerted on a small rigid sphere by a sound wave in a viscous fluid is calculated. The force is expressed as a sum of drag force coming from the external steady flow existing in the absence of the sphere and contributions that are cross products of velocity and velocity derivatives of the incident field. Because of the drag force and an acoustic streaming generated near the sphere, the mean force does not coincide with the acoustic radiation pressure, i.e., the mean momentum flux carried by the sound field through any surface enclosing the sphere. If the sphere radius R is considerably smaller than the viscous wave penetration depth delta, the drag force can give the leading-order contribution (in powers of delta/R) to the mean force and the latter can then be directed against the radiation pressure. In another limit, delta< or =R, the drag force and acoustic streaming play a minor role, and the mean force reduces to the radiation pressure, which can be expressed through source strengths of the scattered sound field. The effect of viscosity can then be significant only if the incident wave is locally plane traveling.     

Ultrasound fields in attenuating media

For medical ultrasonic imaging and for nondestructive testing, the attenuation of pressure waves and the resulting shift in wave velocity are important features in commonly used transmission media such as biological tissue. An algorithm for the numerical evaluation of pressure field distributions generated by ultrasonic transducers is presented. The attenuation and dispersion of the sound transmission medium are taken into consideration. The sound fields are computed numerically for continuous wave as well as pulse excitation. The transducer has plane or gently curved geometry and is embedded in a plane rigid baffle. The numerically determined pressure fields are presented as 3D plots, as gray-scale images for a fixed time stamp (like a snapshot), or as isobars regarding the maximum values over time for each local point in the area under investigation. The algorithm described here can be utilized as a tool for design of ultrasound transducers, especially array antennas.     

深海中利用单水听器的影区声源无源测距测深方法

在典型深海情况下当声源与接收水听器位于海水表层时,在影区内由声源海底接收器、声源海面海底接收器、声源海底海面接收器和声源海面海底海面接收器4条声线形成声场干涉结构,声强随着频率具有两种干涉周期,随着收发距离的增加而增大,分别随着声源深度、接收水听器深度的增加而减小。因此由单水听器记录的声场干涉结构即可实现宽带声源目标的无源测距测深,仿真分析验证了其有效性。在南海深海声学实验中观测到海面宽带噪声源在声场影区所形成的声场干涉结构,数据分析结果验证了深海声场干涉结构用于声源无源定位的有效性。与传统无源定位方法相比,该方法不需要宽带引导声源、精确的海底声学参数和大规模的拷贝场计算。      

同振球型矢量水听器声波接收理论研究

对同振球型矢量水听器声压和质点振速的声波接收理论进行了研究。以同振球型振速水听器测量原理为基础,推导了自由运动刚性球体和弹性球体的声波接收响应数学表达式,分析了振速水听器几何尺寸、平均密度与其频响特性曲线之间的关系;另外,根据球面接收器的声波接收理论,推导了矢量水听器声压接收响应数学表达式,通过理论分析和数值计算,研究了振速水听器表面上的声压分布规律以及声压水听器的声波接收压力系数与其接收面的大小、质点振速水听器的半径、布放的位置和半径等参数之间的关系;从理论上建立了矢量水听器声波接收理论模型和分析方法,为矢量水听器的设计和研制提供了理论依据。      

Laser-Doppler Vibrometer measurements of acoustic-to-seismic coupling in unconsolidated soils

Measurements of acoustic-to-seismic coupling ratio, i.e. the ratio of the pressure exerted by an acoustic wave at a point on the surface to the acoustic particle velocity generated at the surface at that point, may be used to determine both elastic and structural properties of poroelastic materials. The sound pressure is measured using a microphone and, usually, the velocities are measured using geophones. Problems with geophone sensors have been shown to include both mass loading of the soil and coupling resonances within the frequency range of interest. The latter can lead to inaccurate amplitude and phase measurements. In an attempt to overcome these problems, the use of a Laser-Doppler vibrometer (LDV) has been investigated. Previous work with compacted plane soil surfaces has been extended to loosely consolidated soils. Good agreement has been found between geophone and LDV measurements of vertical particle velocity for a continuous wave sound source. Problems with poor LDV signal-to-noise ratio in unconsolidated materials have been overcome using local ground treatment. Subsequent modelling shows reasonable agreement between the data and the predicted values of material properties.