Complex calibration of a pressure gradient receiver using the reciprocity method procedure

Absolute complex free-field calibration of a pressure gradient receiver is considered. The calibration is carried out in a reflecting water tank using the reciprocity method procedure during radiation of continuous signals with linear free-field frequency modulation. To obtain the free-field frequency dependences, complex moving weighted averaging is used. It is shown that this method, developed for calibration of sound pressure receivers, allows effective recovery and measurement of the vector component of a direct acoustic wave in the presence of reflections. The method makes it possible to measure the modulus and phase angle of the complex sensitivity and reveal the insufficiencies of an experimental pressure gradient receiver.     

Instability of horizon with respect to perturbation by a scalar field

The inverse effect on the metric in the case of collapse with a spherically symmetric nonstatic scalar field is investigated. It is shown that there is not sufficient reason to neglect this effect in this case.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 115–119, March, 1978.The author is grateful to V. P. Frolov for stimulating comments and to the participants of Professor D. D. Ivanenko's seminar for discussing this paper.     

强多途环境下水听器阵列位置近场有源校正方法

针对水听器阵列近场强多途环境下的校正需求, 提出了一种高精度的多辅助源阵列位置校正方法.综合近场点源非平面波模型和泰勒近似原理, 构建了近场阵元位置二维误差模型, 进而获得信号特征向量与阵元位置误差的线性映射关系.推导并计算了校正模型的克拉美罗界(CRB).研究了多途对阵元位置校正的影响, 将多途干扰视为位置已知的相干源, 提出了多途补偿策略.理论及仿真结果表明, 近场多辅助源阵列位置校正方法具有较高的精度, 在低信噪比时接近CRB, 对于辅助源位置等误差具有一定的容忍度, 且适用于强多途环境.湖试验证了方法结论的正确性.     

The sound field in a hydroacoustic waveguide with an uneven hard bottom

An analytic representation is constructed for a nonaxially symmetric sound field to simulate a hydroacoustic waveguide the bottom of which is hard and has an axially symmetric relief. A numerical analytic method for finding the velocity potential is proposed, for which undetermined coefficients for normal modes are determined from a corresponding infinite system of algebraic equations. The sound fields are studied with for variations of the problem parameters.     

水听器线列阵近场声压测量误差实验研究

本文介绍了用于近场声压的测量水听器中阵元水听器测量声压的可靠性检验方法和实验结果,提出了元水听器测量球面波场中声传播衰减的曲线与现想球面波声场中场传播衰减的曲线对比的检验,要用球面声源声中心测量值对收发距离进行修正的方法,实验水池中对线列阵上十元水吸器在球形声源水池中进行了传播衰减曲线的测量分析,验证了基阵的弱散散射性能,表明线阵可用于距离小至1／7波长的极近场的扫描测量。     

Equilibrium with respect to pair creation in a quantizing magnetic field

The influence of quantizing magnetic fields on the production of electron-positron pairs in a hot equilibrium magnetized plasma is estimated. It is shown that with increasing strength of the external frozen magnetic field the concentration of electron-positron pairs increases.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 122–126, September, 1977.     

Measurement of sound field in cavitating media by an optical fibre-tip hydrophone

A fibre-optic technique was applied to measure the sound field in an ultrasonic cleaning vessel under practical conditions. A metal-coated fibre-tip is used as a sensor and a heterodyne interferometer detects the change in the optical path resulting from the movement of the fibre-tip in the sound field. Spectrally resolved sound field parameters such as the fundamental, the subharmonic or cavitation noise are extracted from the measurements and compared with results obtained by a piezo-electric hydrophone. It was found that the fibre sensor provides a signal related to the velocity in the sound field, but the information about cavitation-related parameters is similar to the information for pressure sensing techniques. The fibre-optic sensors have a uniquely high spatial resolution and the sound detection process is strongly influenced by single cavitation events close to the small fibre-tip. This paper shows that fibre-tip sensors are an alternative to common hydrophone techniques. They can open up new possibilities for measurement problems for which so far no solution exists, in particular when a high spatial resolution is required or when the measurement site is small.     

Sub-micron positioning of trapped ions with respect to the absolute center of a standing-wave cavity field

We demonstrate that it is possible, with sub-micron precision, to locate the absolute center of a Fabry–Pérot resonator oriented along the radiofrequency-field-free axis of a linear Paul trap through the application of two simultaneously resonating optical fields. In particular, we apply a probe field, which is near-resonant with an electronic transition of trapped ions, simultaneously with an off-resonant strong field acting as a periodic AC Stark-shifting potential. Through the resulting spatially modulated fluorescence signal, we can find the cavity center of an 11.7-mm-long symmetric Fabry–Pérot cavity with a precision of ±135 nm, which is smaller than the periodicity of the individual standing-wave fields. This can, e.g., be used to position the minimum of the axial trap potential with respect to the center of the cavity at any location along the cavity mode.     

Study of the reciprocity relations for a nonlinear multipole in inhomogeneous magnetic field

The reciprocity relations for a matrix of nonlinear resistances of a multipole placed in an inhomogeneous magnetic field are obtained based on the material equation of a nonlinear inhomogeneous nonstationary conducting medium in the Landau collision integral approximation. The question about the measured potentials of the multipole terminals in the quasi-stationary mode is discussed. A method for testing the obtained reciprocity relations has been proposed and the experimental data have been presented. It has been shown that the reciprocity relations are valid for a nonlinear multipole within the electric measurement error.     

HLS注入段束流位置探头定标电场的拟合和误差计算

 描述了束流位置探测器(BPM)探头定标、定标电场的拟合和误差计算。该定标系统采用了具有调节精度为10μm垂直放置真空管道的天线模拟定标装置和分辨率高达1μm外差式、窄带频域信号处理电子学线路。并推导出针对合肥光源（HLS）定标系统条件下的电场拟合函数及相关的BPM-Mapping图和误差计算结果,较精确地计算出该高精度定标系统误差。     

Laboratory calibration of star sensor with installation error using a nonlinear distortion model

The high-precise star sensor calibration method requires high-accurate turntable, collimator, star point plate or other high-precision devices that are very expensive. We present a simple and available method to calibrate the principal point, focal length, radial distortion, tangential distortion and installation error of star sensor in laboratory, and without having high accurate or expensive devices. The calibration model takes the ordinary camera calibration methods and installation error into account. The installation error is modeled by combination of three typical effects: the installation of pan-tilt-zoom (PTZ) initial status, PTZ and charge-coupled device, which result in six parameters. The proposed procedure consists of a closed-form solution, followed by a nonlinear refining based on maximum likelihood criterion. Our calibration method is validated through simulation and real data that shows the superiority with respect to the traditional methods and has the same level as the state-of-the-art methods. The accuracy of our calibration method is 0.015° in the root of mean square distances between testing points and projected ones.     

Instability of a charged plane interface between two liquids with respect to the tangential discontinuity of a time-dependent velocity field

The differential equation that describes the evolution of perturbations of a charged plane boundary between immiscible liquids when the upper liquid moves relative to the lower one with a time-dependent velocity parallel to the boundary is the Hill equation. In this system, the interface can exhibit instabilities of three types at various values of physical parameters: the Kelvin-Helmholtz, Tonks-Frenkel, and parametric instability. When physical parameters have certain values, the interface that is unstable with respect to surface charge and the tangential discontinuity of the velocity field across the interface can be parametrically stabilized.     

Reduction of thermo-elastic damping with a secondary elastic field

Thermo-elastic damping is the dominant mode of energy loss due to the coupling of thermal and elastic fields in a body vibrating at or near resonant frequency. While the literature contains both exact and numerical schemes to quantify it, no technique is available yet to reduce thermo-elastic damping. We address this issue by introducing a secondary elastic field to derive an exact expression that predicts linear reduction in thermo-elastic damping with respect to frequency. Contrary to the current understanding, introduction of a static axial stress in addition to the flexural stresses is shown to increase quality factor and resonant frequency simultaneously.     

室内声场简正波法的数值计算方法和混响场中声强测量的误差

简正波法是求解室内声场的一种较准确的数值计算方法,准确度由所取级数的项数而定,本文详细讨论了数值计算方法、级数的收敛性,并以图线来表明在声源点级数的发散及在声源附近计算的困难,本文还讨论声强测量的理论误差,用一般声强计在混响场中测量声强,误差是很大的,以致所得结果完全不可信任,如将传声器易位、前后两次测得的声强取平均,测量误差就可大大减小,若能采用精密仪器,使两个通道的相位差小于0.05°,则测量结果的统计误差可控制在合理范围内。     

The maximum possible hydrophone size in the studies of the ultrasonic field structure

A simple formula relating the admissible error in the reconstructed field structure to the hydrophone radius, the wavelength, and the hydrophone-piston radiator distance is obtained. Examples of applying the derived relationships to analyzing the structure of the acoustic field produced by a piston radiator are considered.