Reciprocal maximum-length sequence pairs for acoustical dual source measurements

In this paper we propose and demonstrate a method to obtain simultaneous dual source-receiver impulse responses in acoustical systems using binary maximum-length sequences (MLS). A binary MLS and its reversed-order sequence form a reciprocal MLS pair. Their correlation property includes a two-valued "pulse-like" autocorrelation function and a relatively smaller-valued cross-correlation function. This unique property, along with other number-theory properties, makes the reciprocal MLS pair suitable for simultaneous dual source cross-correlation measurements. In the measurement of a dual source system, each of the reciprocal MLS pairs simultaneously excite one of two separate sources, one or several receiver signals cross-correlate in turn with each of the MLS pairs, resulting in impulse responses associated with two separate sources. The proposed method is particularly valuable for system identification tasks with multiple sound/vibration sources and receivers that have to be accomplished in a limited time period. A fast algorithm called a fast MLS transform is exploited for the cross-correlation. In this paper we propose a fast MLS transform pair for the reciprocal MLS pairs. Its efficiency lies in the requirement of one single permutation matrix for a pair of two fast MLS transforms. Its feasibility and usefulness in the acoustical measurements are demonstrated using experimental results.     

Arrival-time structure of the time-averaged ambient noise cross-correlation function in an oceanic waveguide

Coherent deterministic arrival times can be extracted from the derivative of the time-averaged ambient noise cross-correlation function between two receivers. These coherent arrival times are related to those of the time-domain Green's function between these two receivers and have been observed experimentally in various environments and frequency range of interest (e.g., in ultrasonics, seismology, or underwater acoustics). This nonintuitive result can be demonstrated based on a simple time-domain image formulation of the noise cross-correlation function, for a uniform distribution of noise sources in a Pekeris waveguide. This image formulation determines the influence of the noise-source distribution (in range and depth) as well as the dependence on the receiver bandwidth for the arrival-time structure of the derivative of the cross-correlation function. These results are compared with previously derived formulations of the ambient noise cross-correlation function. Practical implications of these results for sea experiments are also discussed.     

A high-speed all-digital technique for cycle-resolved 2-D flow measurement and flow visualisation within SI engine cylinders

Particle image velocimetry (PIV) is now a well-established planar flow measurement technique for the study of in-cylinder flow fields in internal combustion engines. Here the authors describe a turnkey, high-speed digital imaging system that provides combined real-time flow visualisation and rapidly processed PIV data in an industrial optical research engine facility. The system is based on commercially available, high-speed imaging and laser technology and conventional digital cross-correlation processing to provide cycle-resolved PIV data and flow visualisation within timescales appropriate for engine development. A simple variation on the synchronisation scheme also allows the acquisition of tens to thousands of flow visualisation sequences and PIV maps at the same crank angle, thus giving the potential for the study of cycle-to-cycle flow variability and its effect on combustion stability in a suitably instrumented optical engine. The technique may also find applications in other unsteady or oscillatory flows of importance in aerodynamics, acoustics, mixing, and heat transfer.     

量子声学及其应用

量子声学是声学的一个前沿和刚刚活跃起来的分支,是继电子学、光子学之后发展迅速的一门新学科,本文就量子声学的诞生,定义和所涉及的内容进行了讨论,较为系统地介绍了与量子声学有关的理论、材料、器件和应用进展。     

Multiple scattering in a reflecting cavity: application to fish counting in a tank.

Classical fisheries acoustics techniques are useless in the presence of multiple scattering or reflecting boundaries. A general technique is developed that provides the number and the scattering strength of scatterers in motion placed inside a highly reflecting cavity. This approach is based on multiple scattering theory. The idea is to measure the average effect of the scatterers on the acoustic echoes of the cavity interfaces. This leads to the measure of the scattering mean free path, a typical length that characterizes the scattering strength of the cloud of scatterers. Numerical results are shown to agree with a simple theoretical analysis. Experiments are performed with fish in a tank at two different scales: ultrasonic frequency (400 kHz) in a 1.4-l beaker with 1-cm-long fish as well as fisheries acoustics frequency (12.8 kHz) in a 30-m3 tank with 35-cm-long fish. These results have interesting applications to fish target strength measurement and fish counting in aquaculture.     

Comparison of EMT and MST approaches in studying linear acoustics in bubbly liquid

Two kinds of approaches have been used in literature to study the linear acoustics in a liquid-bubble mixture: The effective medium theory approach and the multiple scattering theory approach. Theoretical comparisons of mathematical formulations and intrinsic relations between these two approaches are given in this paper and show their consistency in dealing with the linear acoustics in bubbly liquids. Possible applications of these two methods are also generally discussed.     

通信声学的发展及研究现状

人类作为信息交流的主要信息产生源和信息接收槽,语言声学、心理声学和生理声学在信息技术中发挥着越来越重要的作用。作为声学与信息技术相互渗透的产物-通信声学,近十几年来随着数字信号处理技术的发展,其研究和应用领域得到了很大的扩展。本文在简述通信声学产生和发展的必然性和必要性之后,主要介绍了通信声学目前的研究领域及其应用。     

A new code construction with zero cross correlation based on BIBD

This paper presents a newly constructed zero cross correlation code (ZCC) which is based on BIBD (balanced incomplete block design) code. The ZCC (C, w) code is a family of binary sequences of length C and constant Hamming-weight w. Such codes find applications in spectral amplitude-coding optical code division multiple access (SAC-OCDMA). The constructing ZCC codes have a size of C ? N ÿ w + 1, where N is the number of users and C is any prime number. The proposed construction method is not complicated compared to the existing ones.     

基于超结构光纤光栅的正交四相光码分多址编/解码器

正交四相序列编码相比于二相序列具有更大的码字容量和更好的互相关特性,因此更适用于光码分多址(OCDMA)无源光接入网.提出并实现了一种基于超结构光纤光栅的正交四相光码分多址编/解码器.该编/解码器采用A族四相序列作为地址码,在制作的过程中仅需一个均匀相位掩模板即可实现编码功能,并且在性能上与传统工艺制作的编码器相当.为了与不同波长信道相匹配,提出了变信道编码技术,仿真结果表明采用该技术的编码器具有更高频谱效率,因此得到更好的编/解码性能.对一个码长63,长度4.1 cm的正交四相编/解码器进行了信息速率为2.5 Gb/s.码片速率为156 Gchip/s的编/解码实验,取得了较好的编/解码效果.     

基于三元伪随机序列集的多声源房间脉冲响应测量方法*

在多输入多输出系统中,诸多情况下需并行测量任意输入输出之间的脉冲响应。虽然利用以Kasami序列集为代表的二元伪随机序列集良好的相关特性可以实现多声源并行测量,但是囿于其编解码信号相同且为二元信号的特点,使得其相关特性仍存在较大的可优化空间。该文提供了一种基于三元伪随机序列集的多声源房间脉冲响应测量方法,具有更好的相关特性,提高了测量准确度。仿真和实验验证了该测量方法的可行性和准确性。在封闭空间几何模型构建等领域中,可使用该方法提高测量效率并降低由于各通道不同步导致的误差。     

An electric-field cross-correlation fourier transform millimeter wave spectrometer

The principle and fundamental structure of a millimeterwave electric-field cross-correlation Fourier transform spectrometer are presented. In the spctrometer, two pseudorandom binary sequences (PRBS) with controllable phase variation are used to modulate individually two wide bandwidth microwave signals with variable phase delay are obtained. And the Fourier transform of the crosscorrelation function of these microwave signals is the cross-power spectrum. Some experimental results about the spectrometer are given in 8mm-wave range.Supported by Foundation of Specialities Opened to Doctorate Study.     

飞秒光频梳的任意长绝对测距

高精度测距在工业、航空航天、科学研究等方面都具有重要应用, 而不断发展的激光测距技术始终处于前沿研究领域. 本文研究飞秒光频梳绝对测距技术, 拓展光梳在长度测量领域的应用. 在利用脉冲激光进行任意绝对长度测量中常用到飞行时间法, 然而其测量分辨力受限于电子器件的带宽, 仅为毫米量级. 为克服这一缺点, 本文研究了光梳多脉冲序列之间的时间相干性, 结合多脉冲序列干涉法和飞行时间法提出了任意长绝对测距的方法, 搭建了基于改进型Michelson干涉原理的任意绝对测长系统, 通过同时测量多脉冲序列的一阶和二阶互相关信号, 可以分别计算出飞行时间的时间差, 即可得到被测距离. 利用光梳作为光源进行了0.6m的绝对测距实验, 将测量结果与高精度激光位移传感器的测量值进行比较, 实验结果表明本系统具有良好的测量线性度, 并且测距精度可达±0.5μm. 关键词： 飞秒光频梳 任意长绝对测距 飞行时间法 多脉冲序列干涉法     

Mean-field HP model, designability and alpha-helices in protein structures

Analysis of the geometric properties of a mean-field HP model on a square lattice for protein structure shows that structures with a large number of switchbacks between surface and core sites are chosen favorably by peptides as unique ground states. Global comparison of model (binary) peptide sequences with concatenated (binary) protein sequences listed in the Protein Data Bank and the Dali Domain Dictionary indicates that the highest correlation occurs between model peptides choosing the favored structures and those portions of protein sequences containing alpha helices.     

单边噪声源环境下的格林函数提取

对单边噪声源环境下空间中存在散射体时的格林函数到达时间结构提取展开了实验性研究。通过对位于沙滩上的传声器上记录的海浪噪声进行互相关处理,成功提取出两传声器间的格林函数到达时间结构。到达时间结构中观察到伪散射路径,并且伪散射路径在幅值上远高于散射路径。针对上述现象给出了理论及仿真解释:当传声器对受噪声源单边照射时,广义光学定理失效,伪散射路径将会出现;同时,由于伪散射路径由所有方向噪声源贡献,散射路径仅有稳相点附近声源贡献,因此伪散射路径在幅值上可能高于散射路径。结果表明,在单边噪声源环境下,基于互相关技术可以提取伪散射路径。伪散射路径的短时稳定提取可以为散射体无源成像提供更多信息,在海洋声学、地震学等领域具有潜在应用价值。     

旋转圆柱目标的动态散斑统计特性(英)

高斯光束照射以固定角速度旋转的粗糙圆柱体表面,反射空间形成随着被测表面运动变化的动态散斑。研究了照射面积大于圆柱曲面条件下在远场衍射区形成的动态散斑统计特性,得到了不同入射条件及不同圆柱时动态散斑强度起伏空间-时间归一化相关函数。结果表明：随着圆柱半径、旋转速度以及照射光斑有效面积的增大,动态散斑强度起伏相关性快速减小;当圆柱旋转速度恒定,动态散斑光强起伏相关函数的峰值随着探测点之间距离的增大而减小,但峰值的位置随之而增大;在近似点照射情况下随着圆柱半径的增大空间相关长度基本不变,而相关时间性明显增大。     

旋转圆柱目标的动态散斑统计特性(英)

高斯光束照射以固定角速度旋转的粗糙圆柱体表面,反射空间形成随着被测表面运动变化的动态散斑。研究了照射面积大于圆柱曲面条件下在远场衍射区形成的动态散斑统计特性,得到了不同入射条件及不同圆柱时动态散斑强度起伏空间-时间归一化相关函数。结果表明:随着圆柱半径、旋转速度以及照射光斑有效面积的增大,动态散斑强度起伏相关性快速减小;当圆柱旋转速度恒定,动态散斑光强起伏相关函数的峰值随着探测点之间距离的增大而减小,但峰值的位置随之而增大;在近似点照射情况下随着圆柱半径的增大空间相关长度基本不变,而相关时间性明显增大。     

混沌伪随机序列的谱熵复杂性分析

为了准确分析混沌伪随机序列的结构复杂性,采用谱熵算法对Logistic映射、Gaussian映射和TD-ERCS系统产生的混沌伪随机序列复杂度进行了分析.谱熵算法具有参数少、对序列长度N(惟一参数)和伪随机进制数K鲁棒性好的特点.采用窗口滑动法分析了混沌伪随机序列的复杂度演变特性,计算了离散混沌系统不同初值和不同系统参数条件下的复杂度.研究表明,谱熵算法能有效地分析混沌伪随机序列的结构复杂度;在这三个混沌系统中,TD-ERCS系统为广域高复杂度混沌系统,复杂度性能最好;不同窗口和不同初值条件下的混沌系统复杂度在较小范围内波动.为混沌序列在信息安全中的应用提供了理论和实验依据.     

A stochastic model for acoustic attenuation

Propagation of waves in gases or liquids has been observed for a long time in homogeneous or non-homogeneous media. In acoustics, attenuation is a significant problem which is studied mainly through the 'equations of change' of fluid mechanics. These equations are based only on the macroscopic characteristics of the medium. Microscopic variations, related to other phenomena like Brownian motion or critical opalescence, are not taken into account. This paper provides a random one-ray model. This model explains the proportionality between the standard attenuation and the product between length and frequency squared in a logarithmic scale. The wave is shown to be necessarily associated with noise, even if this noise cannot be observed by devices. Furthermore, the 'coefficient of variation' defined in turbulent environments can be explained as a random version of the usual coefficient of attenuation.     

宽带参量阵技术在近海探测中的研究

文章针对非线性声学宽带参量阵技术应用于近海海底剖面绘制及掩埋物探测进行了分析研究。对于宽带参量阵声呐产生的差频信号利用Berktay包络解调理论和KZK(Khokhlov-Zabolotskaya-Kuznetsov)方程,对其进行了理论分析,水池实验研究和时域数值模拟,并结合声呐方程计算,说明该技术用于近海探测的可行性。本文同时结合脉冲压缩技术对产生的宽带差频信号做匹配处理,用于提高设备的垂直分辨率。最后在海试实验中,利用文中设计的参量阵声呐,分别使用宽带差频信号和单频差频信号对浅海海底沉积层和海底掩埋管线进行探测,取得了清晰的剖面图和掩埋物探测结果,验证了宽带参量阵技术在海底探测中的有效性和实用性。     

Modeling multiple time scales during glass formation with phase-field crystals

The dynamics of glass formation in monatomic and binary liquids are studied numerically using a microscopic field theory for the evolution of the time-averaged atomic number density. A stochastic framework combining phase-field crystal free energies and dynamic density functional theory is shown to successfully describe several aspects of glass formation over multiple time scales. Agreement with mode coupling theory is demonstrated for underdamped liquids at moderate supercoolings, and a rapidly growing dynamic correlation length is found to be associated with fragile behavior.