On the possibility of using acoustic reverberation for remote sensing of ocean dynamics

The two-point correlation function of diffuse noise fields produced by distributed random sound sources carries useful information on the medium of sound propagation. Such information can be used for performing passive acoustic tomography of the ocean. In a number of cases that are important for practice, the noise field in the ocean is predominated by contributions of individual point sources. Here, a theoretical study is presented on the possibility of determining the sound speed and current velocity in the water column by the correlation processing of reverberation signals measured by two vertical receiving arrays. In other words, we study the possibility of replacing the diffuse noise produced by a great number of delta-correlated sources by waves generated by a localized source and scattered at the rough surface and bottom of the ocean for sensing the medium. The correlation function of scattered waves is calculated by using the method of small perturbations. It is shown that the correlation processing of the scattered waves offers an opportunity of measuring the acoustic nonreciprocity and reconstructing the field of sound speed in the fluid, without using any acoustiLc transceivers.     

Aberrations in materials with random inhomogeneities

Materials that consist of a random microstructure can affect ultrasonic measurements--reducing signal strength, increasing noise, and reducing measurement accuracy--through scattering and aberration of the acoustic field. To account for these adverse effects a phase screen model, alongside the stochastic wave equation, has been developed. This approach allows the field and study aberrations to be modeled from a statistical point of view. Experimental evidence of aberration and statistical properties of the measured acoustic field are shown. A measured correlation function of the acoustic field is interlinked to mean crystallite size by using a theoretical coherence function that can be mainly described by the correlation length and wave velocity variation of microstructure. The estimation of the mean crystallite size using this technique would provide some insight into material characterization.     

Acoustic sounding of the thermal and velocity structure characteristics with allowance for the turbulent attenuation

A method of simultaneous acoustic sounding of vertical profiles of the thermal and velocity structure parameters with allowance for the turbulent sound attenuation is suggested in the present work. A closed iterative algorithm that implements the suggested method is described, and results of its application for processing of measurements with the Zvuk-2 three-component Doppler sodar are given. It is demonstrated that the thermal, velocity, and acoustic refractive index structure characteristics obtained are in good agreement with the data available from the literature. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 84–90, November, 2008.     

Suitability of laser Doppler velocimetry for the calibration of pressure microphones

The details of a new approach for absolute calibration of microphones, based on the direct measurement of acoustic particle velocity using laser Doppler velocimetry (LDV), are presented and discussed. The calibration technique is carried out inside a tube in which plane waves propagate and closed by a rigid termination. The method developed proposes to estimate the acoustic pressure with two velocity measurements and a physical model. Minimum theoretical uncertainties on the estimated pressure and minimum measurable pressure are calculated from the Cramer Rao bounds on the estimated acoustic velocity amplitude and phase. These uncertainties and the minimum measurable pressure help to optimize the experimental set up. Acoustic pressure estimations performed with LDV are compared with acoustic pressures obtained with a reference microphone. Measurements lead to a minimum bias of 0.006 dB and a minimum uncertainty of 0.013 dB on the acoustic pressure estimation for frequencies 1360 Hz and 680 Hz.     

自回归状态空间模型下时变声速剖面跟踪方法

讨论了一种适用于浅海的时变声速剖面跟踪方法。该方法将时变水体声速剖面的反演问题建模为由描述声速剖面时变特性的状态方程与包含声压场局部测量信息的测量方程组成的状态-空间模型,提出利用自回归分析拟合方法将声速场扰动建模为高阶自回归演化模型,并通过集合卡尔曼滤波序贯地估计时间演化的海洋声速场。利用2001年亚洲海实验环境与声速测量数据,仿真分析了基于高阶自回归演化模型的时变声速剖面集合卡尔曼滤波估计方法。结果表明,相比于利用传统随机游走状态演化模型的估计方法,该改进方法可有效降低声速的跟踪误差,并且在较低信噪比条件下仍具有较好的跟踪性能。      

Acoustic reconstruction of the velocity field in a furnace using a characteristic flow model

An acoustic method can provide a noninvasive, efficient and full-field reconstruction of aerodynamic fields in a furnace. A simple yet reasonable model is devised for reconstruction of a velocity field in a cross section of a tangential furnace from acoustic measurements based on typical physical characteristics of the field. The solenoidal component of the velocity field is modeled by a curved surface, derived by rotating a curve of Gaussian distribution, determined by six characteristic parameters, while the nonrotational component is governed by a priori knowledge. Thus the inverse problem is translated into determination of the characteristic parameters using a set of acoustic projection data. First numerical experiments were undertaken to simulate the acoustic measurement, so as to preliminarily validate the effectiveness of the model. Based on this, physical experiments under different operating conditions were performed in a pilot-scale setup to provide a further test. Hot-wire anemometry and strip floating were applied to compare with acoustic measurements. The acoustic measurements provided satisfactory consistency with both of these approaches. Nevertheless, for a field with a relatively large magnitude of air velocities, the acoustic measurement can give more reliable reconstructions. Extension of the model to measurements of hot tangential furnaces is also discussed.     

Tracking of time-evolving sound speed profiles with an auto-regressive state-space model

An approach for time-evolving sound speed profiles tracking in shallow water is discussed.The inversion of time-evolving sound speed profiles is modeled as a state-space estimation problem,which includes a state equation for predicting the time-evolving sound speed profile and a measurement equation for incorporating local acoustic measurements.In the paper,auto-regression(AR) method is introduced to obtain a high-order AR evolution model of the sound speed field time variations,and the ensemble Kalman filter is utilized to track the sound speed field.To validate the approach,the accuracy in sound speed estimation is analyzed via a numerical implementation using the ASIAEX experimental environment and the sound velocity measurement data.Compared with traditional approaches based on the state evolution represented as a random walk,simulation results show the proposed AR method can effectively reduce the tracking errors of sound speed,and still keep good tracking performance at low signal-to-noise ratios.     

A multifrequency scintillation method for ocean flow measurement.

The transverse flow of inhomogeneous fluid produces fluctuation of the acoustic signal passing through it. The coherence of frequency-spaced signal fluctuation is related to the advection of the inhomogeneous medium through the sound path, thus providing a basis for the current velocity measurement. This method can be considered to be the "frequency-domain" version of the conventional scintillation approach to the current velocity registration based on the measurement of the signal correlation transmitted from the source to the two separated in space receivers (space-domain scintillation) [S. Clifford and D. Farmer, J. Acoust. Soc. Am. 74, 1826-1832 (1983)]. The sensitivity of the method depends on the features of the ocean fine structure, which is determined mainly by the internal waves and turbulence. To estimate the sensitivity of the multifrequency method of transverse current probing, the coherence function of two signals propagating through a frozen and moving internal wave field and through the turbulence is considered. The application of the multifrequency signal allows estimation of the fine-structure parameters as well as the current velocity.     

非均匀海洋环境噪声中的匹配场处理舰船辐射噪声级估计方法

在实际舰船辐射噪声测量过程中,受非均匀海洋环境噪声的影响,导致常规测量方法的性能急剧下降,基于此,提出了一种非均匀海洋环境噪声背景中的垂直阵舰船辐射噪声测量方法。根据水声信道传播理论,建立了由海面多个空时独立均匀分布噪声源构成的非均匀背景噪声场模型,推导了非均匀海洋环境噪声场中垂直阵舰船辐射噪声估计的理论公式。针对典型的浅海水声信道,进行计算机仿真实验,分析了该方法的测量性能并与常规匹配场测量方法进行对比,结果表明:(1)该方法能有效克服非均匀海洋环境噪声对测量结果的影响,测量误差较小;(2)相同测量条件下,该方法测量性能优于常规匹配场舰船辐射噪声级测量方法;(3)当信噪比满足一定要求时,测量得到的声源级与实际声源级相比,误差小于1 dB。      

基于等效源法和双面质点振速测量的声场分离方法

目前基于等效源法的声场分离方法有两种输入方式, 一种以双测量面上的声压为输入, 另一种以单测量面上的声压和质点振速为输入. 本文以双测量面上的质点振速为输入, 提出一种新的基于等效源法的声场分离方法. 首先给出了该方法的理论推导, 然后通过数值仿真和实验验证了该方法的有效性. 通过与基于双面声压测量的声场分离方法的比较, 证明了该方法在分离质点振速方面的优越性. 此外, 在仿真中还研究了干扰声源强度和测量面间距对分离精度的影响. 关键词： 声场分离 质点振速测量 等效源法 近场声全息     

Modelling the pressure field in the vicinity of a microphone membrane using PIV

A new approach for estimating the acoustic pressure in the near field of a microphone based on non-intrusive direct measurement of acoustic particle velocity is proposed.This method enables the estimation of the acoustic pressure inside a domain located in front of the microphone membrane. The acoustic pressure is calculated using the acoustic particle velocity on the frontiers of this domain and a physical model based on the Green function of the system.Results are obtained using the acoustic velocity measured with Particle Image Velocimetry (PIV) in front of a microphone excited with a plane wave inside a rectangular waveguide. They show that the diffraction of the plane wave by the microphone leads to an increase of the acoustic pressure on the microphone edge in the order of magnitude of 0.1 dB.     

海底表层沉积物声速的环境因素影响特性

应用声速通用模型开展理论分析,结合温度-压力可控实验测量分析,研究地声反演、海底原位声学测量和采样样品声学测量3种主要的海底表层沉积物声学特性测量方法中,存在的环境因素影响机制和特性。理论计算与实验测量研究具有一致性,揭示温度和静水压力对海底表层沉积物的压缩波声速的影响机制主要是通过影响孔隙海水的密度、黏度、体积弹性模量体现的。在海底4000 m以内的表层海底沉积物通常存在的海底温度和静水压力范围内,发现声速比不为常数,随着温度升高和静水压力增大而呈现出细微减小趋势,理论计算海底沉积物与底层海水声速比波动度在2%以内,实验测量声速比波动度在2.73%以内。基于声速比不为常数,提出修正声速比校正模型,为减小温度和静水压力影响因素误差,更准确地实现不同原位、实验室环境状态下的海底沉积物声速的分析和比较提供了改进方法。      

Flow spectra from spectral power density calculations for pulsed Doppler

Range gated pulsed Doppler can be used to make localized velocity measurements within a blood vessel. Both the transducer and the sample volume are of finite size, and this prohibits the measurement of velocity at a point. A spectral flow profile can be created by stepping a sufficiently small sample volume across the lumen of a vessel. However no such set of spectra will correspond directly to the true velocity profile. In this study we developed a systematic theoretical treatment which allows Doppler spectral power density (SPD) functions to be calculated under a very wide range of conditions. Simulated flow spectra were created from sets of these spectra. The model is based on the beam intensity weighted volume method and incorporates, through the idea of a spread function, Guidi's individual flow line spectrum. Our method can be applied for different spread functions; with beam profiles which are uniform, Gaussian or arbitrarily narrow (needle beam); with range gated sample volumes which can be maximal (CW-type) or minimal (PW-type); and for beams which intersect the flow tube axis, or are off centre. Under all conditions we find the spread function parameter k, equal to the ratio of the central Doppler shift to half the bandwidth, plays a key role. After formulating the model analytically, we sought simplifications to allow results to be obtained from simple, practical formulae. Spread and unspread SPD functions are in most cases given as single integrals which contain measurable physical parameters and can be easily evaluated numerically. Model results are presented for flow spectra of parabolic flow, illustrating the interplay between different factors in determining the appearance of spectral flow profiles.     

A direct method for acoustic impedance measurement based on the measurement of electrical impedance of acoustic transmitter

A simple and straight forward method for acoustic impedance measurement is presented and evaluated. In this method a speaker is used as the signal source. The relationship between the electrical impedance of the speaker and its acoustical load is developed and studied. It is shown that the electrical current and voltage of the speaker relate to the acoustical pressure and volume velocity. The mechanical and acoustical impedances are therefore easily derived by measuring the electrical current and voltage of the circuit. The proposed method yield itself to the automatic computer measurement and can be used for the field and in situ measurements.Comparison of the measurement data with those from other methods proves the applicability and accuracy of the proposed method.     

Characterization of acoustic streaming in water and aluminum melt during ultrasonic irradiation

It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density.     

回旋电子束横纵速度比测试技术

摘要:通过对回旋电子束形成的电位和电场分布,推导出电子束横纵速度比测量的计算式.提出将电子束作传输线内导体等效,采用陶瓷电容分压器来测量速度比的方法.介绍了该测量装置的设计和加工工艺,及陶瓷电容分压器的标定方法和结果分析,并介绍了初步的原理性实验结果.该测试方法装置简单,测试电子束横纵速度比时不会破坏电子束的特性,能够...     

Imaging of echo data of submarine sedimentary beds layer-by-layer with acoustic wave equation refraction correction method

I.IntroductionThcobjcctofsubmarincstratasurvcyisforgco1ogicalintcrprctation,inwhichsubmarinccchodataarcuscd.Rightgcologicalintcrprctationisbascdonthedatawithhighsignal-to-noiseratio(S/N)andhighrcsoIution.ThcusualmcthodofimprovingS/Nistocollcctwide-anglcechodatahyasystcmcomposcdofanacousticsourceandahydrophoncsfrcamcrtOwedbchindaprofilerandstackthcdataincommon-dcpth-point(CDP)gathcr,whichcansupprcsssomctypcsofintcrfcrcnce.a..s[1'2].Bcsidcs,combincdmcthodsoffic1dco11cctionandfi1teringmcthods…     

用声波方程对海底地层回波数据进行逐层折射校正成像

用地层剖面仪加一条拖曳水听器阵组成的海底地层测量系统采集海底地层广角回波数据，对广角回波数据进行共深度点叠加得到的叠加剖面可用于地质解释。当海底地层声速有横向变化时，由于波的折射效应的影响，叠加剖面不能真实反映海底下地层构造的情况，应用于地质解释很可能会导致错误的结果。在已知海底地层声速的情况下，用声波方程Ｋｉｒｃｈｈｏｆｆ积分方法对叠加数据进行正反向两次波场延拓，可消除上伏地层声速横向变化引起下伏地层回波波场的畸变，获得更精确反映海底下地层构造的声波图像。     

Correlation techniques in dynamic light scattering

The motion of small scatterers in a laser beam results in Doppler shifts of the scattered light. Various homodyne or heterodyne experiments may be used to measure such shifts. The stochastic nature of spatial particle arrangements and of the photon-detection process leads to statistic data processing schemes like temporal correlation or the computation of structure functions. Photon correlation is one of these schemes and has found numerous applications in velocimetry and Brownian motion studies. Topics of current interest are dead-time corrections, the use of photon structure functions, and multiple tau measurements, which access large ranges of time constants in a single run.More recent data processing techniques are recurrence rate correlation for the immediate determination of velocity correlation functions in seeded fluid flows and the measurement of amplitude-weighted phase structure functions, which is able to resolve very small particle displacements otherwise completely obscured by random Brownian motion. Rate correlation found applications in hydrodynamic studies of the route to turbulence, while the major use of phase structure function processing is a very significant increase in the sensitivity of electrophoretic mobility measurements by light scattering.