Increasing the frequency resolution in the processing of acoustic signals by sliding complex weighted averaging

We investigated the possibilities of estimating the transient impedance of an emitter and receiver in a free field, and we present our results, which are based on sliding complex weighted averaging of the frequency dependence of emitter and receiver’s transient impedance in a nonanechoic bath. We show that the procedure makes it possible to reduce reflection effects and to form free-field conditions in baths with various reflections that differ sharply in shape and many times in size. We suggest a procedure of repeat use of sliding complex weighted averaging together with spectrometry of reflection functions, which makes it possible to increase the frequency resolution of the method.     

The multi-dimensional limiters for solving hyperbolic conservation laws on unstructured grids

Novel limiters based on the weighted average procedure are developed for finite volume methods solving multi-dimensional hyperbolic conservation laws on unstructured grids. The development of these limiters is inspired by the biased averaging procedure of Choi and Liu [10]. The remarkable features of the present limiters are the new biased functions and the weighted average procedure, which enable the present limiter to capture strong shock waves and achieve excellent convergence for steady state computations. The mechanism of the developed limiters for eliminating spurious oscillations in the vicinity of discontinuities is revealed by studying the asymptotic behavior of the limiters. Numerical experiments for a variety of test cases are presented to demonstrate the superior performance of the proposed limiters.     

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.     

Free-field calibration of a pressure gradient receiver in a reflecting water tank using a linear frequency-modulated signal

This work continues a study of the method for constructing the frequency dependence for a projector-receiver pair in a free field by complex moving weighted averaging of the frequency dependence for a pair measured in the field of a reflecting water tank. The method is applied to the free-field calibration of a pressure gradient receiver using a reference hydrophone when radiating a complex linear frequency-modulated (LFM) signal. To improve the estimates of this method, we edited the initial frequency dependences using functions in the form of the product of the complex LFM projector current multiplied by the powerlaw function of the LFM signal frequency. We consider ways to use a priori information both to improve the results obtained by complex moving weighted averaging and to estimate the distortions introduced by this method are considered.     

Green function simulation of Hamiltonian lattice models with stochastic reconfiguration

We apply a recently proposed Green function Monte Carlo procedure to the study of Hamiltonian lattice gauge theories. This class of algorithms computes quantum vacuum expectation values by averaging over a set of suitable weighted random walkers. By means of a procedure called stochastic reconfiguration the long standing problem of keeping fixed the walker population without a priori knowledge of the ground state is completely solved. In the model, which we choose as our theoretical laboratory, we evaluate the mean plaquette and the vacuum energy per plaquette. We find good agreement with previous works using model-dependent guiding functions for the random walkers. Received: 23 November 1999 / Published online: 25 February 2000     

Some Weighted Averaging Methods for Gradient Recovery

We propose some new weighted averaging methods for gradient recovery, and present analytical and numerical investigation on the performance of these weighted averaging methods. It is shown analytically that the harmonic averaging yields a superconvergent gradient for any mesh in one-dimension and the rectangular mesh in two-dimension. Numerical results indicate that these new weighted averaging methods are better recovered gradient approaches than the simple averaging and geometry averaging methods under triangular mesh.     

非下采样变换的红外与可见光图像融合

基于非下采样Contourlet变换（NSCT）,提出了一种红外和可见光图像融合算法。针对低频子带系数和各带通方向子带系数分别提出了基于图像物理特征的系数加权选择方式与基于区域能量匹配的系数选择方式,即低频基于区域梯度信息、高频基于区域特征因子的加权与选择结合的图像融合算法。实验结果表明：非下采样Contourlet变换具有较快的运算速度,且经非下采样变换后能量更加集中,可提供更多的图像信息。相对于基于像素的图像融合算法,本文的图像融合算法具有更高的融合性能,是一种更适合图像融合的多尺度几何分析（MGA）工具。     

基于局部位移校正的磁共振图像相干平均

多次扫描相干平均是提高磁共振图像信噪比的常用方法,但如果在多次扫描过程中病人发生自主或不自主的运动,使得图像中的组织发生位移,简单相干平均图像会导致图像模糊.本文受非局域均值算法的启发,提出了一种基于局部位移校正的相干平均方法.该算法通过比较多次采集的图像中组织结构的局部相似性,找出图像间的局部位移,利用该信息修正位移后进行加权平均,从而达到提高图像信噪比的目的.我们用模型及真实的肝脏弥散数据进行了实验.实验结果表明,对于不同次采样间存在运动的磁共振图像,该算法可有效地提高信噪比并保持结构边缘;其结果优于简单的相干平均,去噪效果也优于经典的非局域均值算法.     

A New Model of Interval-Valued Intuitionistic Fuzzy Weighted Operators and Their Application in Dynamic Fusion Target Threat Assessment

Existing missile defense target threat assessment methods ignore the target timing and battlefield changes, leading to low assessment accuracy. In order to overcome this problem, a dynamic multi-time fusion target threat assessment method is proposed. In this method, a new interval valued intuitionistic fuzzy weighted averaging operator is proposed to effectively aggregate multi-source uncertain information; an interval-valued intuitionistic fuzzy entropy based on a cosine function (IVIFECF) is designed to determine the target attribute weight; an improved interval-valued intuitionistic fuzzy number distance measurement model is constructed to improve the discrimination of assessment results. Specifically, first of all, we define new interval-valued intuitionistic fuzzy operation rules based on algebraic operations. We use these rules to provide a new model of interval-valued intuitionistic fuzzy weighted arithmetic averaging (IVIFWAA) and geometric averaging (IVIFWGA) operators, and prove a number of algebraic properties of these operators. Then, considering the subjective and objective weights of the incoming target, a comprehensive weight model of target attributes based on IVIFECF is proposed, and the Poisson distribution method is used to solve the time series weights to process multi-time situation information. On this basis, the IVIFWAA and IVIFWGA operators are used to aggregate the decision information from multiple times and multiple decision makers. Finally, based on the improved TOPSIS method, the interval-valued intuitionistic fuzzy numbers are ordered, and the weighted multi-time fusion target threat assessment result is obtained. Simulation results of comparison show that the proposed method can effectively improve the reliability and accuracy of target threat assessment in missile defense.     

Resolution enhancement by subtraction of confocal signals taken at different pinhole sizes

Subtractive imaging in confocal fluorescence light microscopy is based on the subtraction of a suitably weighted widefield image from a confocal image. An approximation to a widefield image can be obtained by detection with an opened confocal pinhole. The subtraction of images enhances the resolution in-plane as well as along the optic axis. Due to the linearity of the approach, the effect of subtractive imaging in Fourier-space corresponds to a reduction of low spatial frequency contributions leading to a relative enhancement of the high frequencies. Along the direction of the optic axis this also results in an improved sectioning. Image processing can achieve a similar effect. However, a 3D volume dataset must be acquired and processed, yielding a result essentially identical to subtractive imaging but superior in signal-to-noise ratio. The latter can be increased further with the technique of weighted averaging in Fourier-space. A comparison of 2D and 3D experimental data analysed with subtractive imaging, the equivalent Fourier-space processing of the confocal data only, and Fourier-space weighted averaging is presented.     

Frequency response testing in a noisy environment or with a limited power supply

The current trend in dynamic testing is towards short duration, transient testing in the field followed by detailed computer analysis of the data at some later time. For many applications, tests must be carried out in a noisy environment, such as in frequency response studies of operating machinery and vibration tests on buildings or ships where the excitation power source is small and produces a low level response compared with ambient vibration levels. The use of time domain averaging is demonstrated and it is shown to be a suitable signal enhancement procedure for testing in poor signal to noise ratio conditions, when using a transient excitation technique. The limitations and accuracy of the technique are discussed and examples of practical application are given.     

A comparison of adaptive procedures for rapid and reliable threshold assessment and training in naive listeners

In psychoacoustic studies there is often a need to assess performance indices quickly and reliably. The aim of this study was to establish a quick and reliable procedure for evaluating thresholds in backward masking and frequency discrimination tasks. Based on simulations, four procedures likely to produce the best results were selected, and data collected from 20 naive adult listeners on each. Each procedure used one of two adaptive methods (staircase or maximum-likelihood estimation, each targeting the 79% correct point on the psychometric function) and two response paradigms (3-interval, 2-alternative forced-choice AXB or 3-interval; 3-alternative forced-choice oddball). All procedures yielded statistically equivalent threshold estimates in both backward masking and frequency discrimination, with a trend to lower thresholds for oddball procedures in frequency discrimination. Oddball procedures were both more efficient and more reliable (test-retest) in backward masking, but all four procedures were equally efficient and reliable in frequency discrimination. Fitted psychometric functions yielded similar thresholds to averaging over reversals in staircase procedures. Learning was observed across threshold-assessment blocks and experimental sessions. In four additional groups, each of ten listeners, trained on the different procedures, no differences in performance improvement or rate of learning were observed, suggesting that learning is independent of procedure.     

Filtering of Trends with Forbush Effects of Time Series of Observations Performed by Cosmic-Ray Monitoring Systems

A method for filtering trends with Forbush effects of the time series of observations performed by cosmic-ray monitoring systems is proposed. A filtering algorithm based on sliding local models and weighted averaging is developed. A mathematical technique for optimizing the filter parameters is implemented. The results of observations from a neutron monitor are used as an example to test the developed filtering algorithm and estimate its efficiency.     

Perturbational approach for the curvature correction to the surface tension via the Hartree-Fock approximation for the plane surface

It is pointed out that the series expansion, used semi-classically to derive a closed formula for the curvature energy, breaks down in the case of the Hartree-Fock approach due to energy density and density fluctuations. The specific curvature energy of a Fermi gas is explicitly shown to be orders of magnitude smaller than that obtained by the standard perturbational procedure. Averaging energy density and density before the evaluation of the standard expression yields reasonable values for the curvature energy in the case of the Fermi gas. Such an averaging procedure is proposed as a way out of the dilemma found in Hartree-Fock calculations of the curvature energy with the standard formula. It is shown that a careful averaging is necessary for the calculation of curvature energies, whereas for surface properties of plane systems rough averaging is also sufficient.     

旋转不变的非局域均值算法在磁共振图像去噪中的应用

磁共振成像（MRI）实验时常采用多次扫描累加平均提高图像信噪比（SNR）,但当扫描过程中运动引起图像变形时,简单地累加平均就无法奏效.为此,本研究组曾提出一种匹配加权平均方法（MWA）提高图像的信噪比.在此基础上,该文提出一种旋转不变的非局域均值算法（RINLM）,即选取圆形邻域区域并将其划分为一系列以中心像素为圆心的等面积圆环,再计算模式的相似性.RINLM算法可以更好地利用图像中旋转的冗余信息、找到更多的相似结构,提高算法的去噪性能.我们把该方法应用于低信噪比图像序列的平均和去噪中,可以更好地处理旋转的局部运动.与非局域均值算法（NLM）相比,RINLM算法可以进一步提高图像的信噪比;与MWA方法相比,其与RINLM算法的结合可以进一步提高磁共振图像序列信噪比,更好的保持图像边缘信息.     

Vibration of a rotating shaft with randomly varying internal damping

A simple Jeffcott rotor is considered with both external and internal damping. Coefficient of internal damping is subject to temporal random variations which may occasionally bring the rotor into the domain of dynamic instability. The corresponding sporadic outbreaks in the rotor's vibrational response (whirl) are studied by applying the Krylov-Bogoliubov averaging method to the complex equation of motion and using parabolic approximation for the random coefficient of the internal damping. This results in an explicit analytical solution for the radius of whirl which may be used for predicting reliability of the rotor. Furthermore, a convenient procedure is described for interpreting measured on-line test data for the rotor. Namely, the mean value of the coefficient of internal damping as well as its standard deviation and mean frequency of temporal variations may be estimated directly from the trace of whirl radius which exhibits spontaneous random outbreaks in response.     

Non-linear parameter estimation with Volterra series using the method of recursive iteration through harmonic probing

Volterra series provides a platform for non-linear response representation and definition of higher order frequency response functions (FRFs). It has been extensively used in non-parametric system identification through measurement of first and higher order FRFs. A parametric system identification approach has been adopted in the present study. The series response structure is explored for parameter estimation of polynomial form non-linearity. First and higher order frequency response functions are extracted from the measured response harmonic amplitudes through recursive iteration. Relationships between higher order FRFs and first order FRF are then employed to estimate the non-linear parameters. Excitation levels are selected for minimum series approximation error and the number of terms in the series is controlled according to convergence requirement. The problem of low signal strength of higher harmonics is investigated and a measurability criterion is proposed for selection of excitation level and range of excitation frequency. The procedure is illustrated through numerical simulation for a Duffing oscillator. Robustness of the estimation procedure in the presence of measurement noise is also investigated.     

Stability Analysis of an Inverted Pendulum Subjected to Combined High Frequency Harmonics and Stochastic Excitations

Stability of vertical upright position of an inverted pendulum with its suspension point subjected to high frequency harmonics and stochastic excitations is investigated. Two classes of excitations, i.e., combined high frequency harmonic excitation and Gaussian white noise excitation, and high frequency bounded noise excitation, respectively, are considered. Firstly, the terms of high frequency harmonic excitations in the equation of motion of the system can be set equivalent to nonlinear stiffness terms by using the method of direct separation of motions. Then the stochastic averaging method of energy envelope is used to derive the averaged Ito stochastic differential equation for system energy. Finally, the stability with probability 1 of the system is studied by using the largest Lyapunov exponent obtained from the averaged Ito stochastic differential equation. The effects of system parameters on the stability of the system are discussed, and some examples are given to illustrate the efficiency of the proposed procedure.     

IN-SERVICE IDENTIFICATION OF NON-LINEAR DAMPING FROM MEASURED RANDOM VIBRATION

A non-linearly damped single-degree-of-freedom (s.d.o.f.) system under broadband random excitation is considered. A procedure for in-service identification of the damping characteristic from measured stationary response is described. The procedure is based on the stochastic averaging method. The explicit analytical solution is obtained for the integral equation, which relates the desired damping characteristics to the apparent force in the shortened equation for the slowly varying response amplitude, and thus to the measured probability density of the amplitude. The approach is of a non-parametric nature, which makes it convenient for testing hypotheses of damping mechanisms from measured random vibration data. Extensive results of numerical tests for the procedure are presented.     

Full wave-field reflection coefficient inversion

This paper develops a Bayesian inversion for recovering multilayer geoacoustic (velocity, density, attenuation) profiles from a full wave-field (spherical-wave) seabed reflection response. The reflection data originate from acoustic time series windowed for a single bottom interaction, which are processed to yield reflection coefficient data as a function of frequency and angle. Replica data for inversion are computed using a wave number-integration model to calculate the full complex acoustic pressure field, which is processed to produce a commensurate seabed response function. To address the high computational cost of calculating short range acoustic fields, the inversion algorithms are parallelized and frequency averaging is replaced by range averaging in the forward model. The posterior probability density is interpreted in terms of optimal parameter estimates, marginal distributions, and credibility intervals. Inversion results for the full wave-field seabed response are compared to those obtained using plane-wave reflection coefficients. A realistic synthetic study indicates that the plane-wave assumption can fail, producing erroneous results with misleading uncertainty bounds, whereas excellent results are obtained with the full-wave reflection inversion.