海军工程大学振动与噪声研究所武汉430033）

叶轮产生离散噪声的原因是叶片间周向夹角均匀布置，因此降低叶轮离散噪声的有效手段就是破坏叶片周向夹角的均匀布置，通过合理布置各叶片夹角降低叶轮离散噪声。本文提出了采用叶片周向角不等距布置降低叶轮总离散噪声A声级的一种优化设计模型，模型充分考虑了A计权函数与波动力对脉动流场响应参数SC乘积有关的综合衰减曲线的高低频段衰减较大的特性和人可听域频率范围在20∽20000Hz之间的特点，通过算例验证采用该模型可以降低总离散噪声A声级。     

低A声级贯流风机叶片周向角不等距优化设计

叶轮产生离散噪声的原因是叶片间周向夹角均匀布置,因此降低叶轮离散噪声的有效手段就是破坏叶片周向夹角的均匀布置,通过合理布置各叶片夹角降低叶轮离散噪声。本文提出了采用叶片周向角不等距布置降低叶轮总离散噪声A声级的一种优化设计模型,模型充分考虑了A计权函数与波动力对脉动流场响应参数Sc乘积有关的综合衰减曲线的高低频段衰减较大的特性和人可听域频率范围在20-2000Hz之间的特点,通过算例验证采用该模型可以降低总离散噪声A声级。     

平交路口交通噪声的测量分析与模型计算结果

本文给出了对有交通控制信号的十字型平交路口,交通噪声(以下简称路口噪声)的测量及统计分析结果。指出用统计声级L_50和L_90来衡量时,路口噪声比所谓自由车流噪声要高出3—7dB(A);而用L_10和等效声级L_eq的衡量时,两者几乎没有区别,虽然两者给人的感觉有所不同。通过与点声源路口噪声模型的计算结果对比,得出路口噪声统计声级L_50增高是因为两条道路相交,横路上车辆的运动噪声对总噪声能量的贡献。文中还讨论了路口噪声的统计分布和瞬时噪声级与交通控制信号之间的关系等问题。     

交流文章摘要

蔡国本(上海市环境监测中心) 1981—1985年上海市交通噪声监测对市中心区三十六条主要繁忙道路进行了定期定点监测。其中有62.6％监测声级超过本市环境噪声标准分析五年来监测资料发现本市交通噪声在低车流、低车速多声源的情况下声级与机动车流量在某些方面仍有一定的相互关系。噪声对人体听觉系统影响的调查报告,九省市噪声调查研究协作组     

低反射光学功能涂层材料的组成与反射特性研究

从光与涂层的作用机理出发,建立了涂层对特定波长光的反射率模型,系统分析了涂层中颜料、基料用量对涂层反射率的影响,并且对理论模型进行了实验验证,理论模拟曲线与实验规律相符。该模型可以为低反射率光学功能涂层的设计和制备提供理论上的指导。     

近场测量中采用多元接收计算舰船辐射噪声总声级

主要阐述在海上布放多个接收水听器来测量舰船近场辐射噪声,以实现准确计算被测舰船水下辐射噪声宽带总声级。     

扩压器几何参数对离心风机噪声的影响

本文试验研究了扩压器几何参数对一高速离心风机的噪声的影响。扩压器的几何参数包括叶片数、叶轮与扩压器的径向间隙和倾斜前缘倾角以及它们的耦合作用对风机噪声的影响。试验结果表明:(1)风机A声级噪声随扩压器叶片数增加而下降,但气动性能也随之下降;(2)扩压器前缘半径从R_3/R_2=1.03增加到1.07,在设计点风机A声级噪声降了约3 dB(A),继续增大至1.09则基本不变;(3)适当倾斜扩压器前缘可有效降低风机噪声,在设计点30°倾角扩压器相应的风机A声级噪声下降了约3.6 dB(A);(4)倾斜扩压器前缘与增大径向间隙的降噪效果不能叠加。     

降低通风机噪声的试验研究

本文介绍了在通风机内部结构上采取降低噪声的两种措施及试验研究结果.一种是对风机叶片的进、出气边作锯齿形处理,另一种是对风机叶片的出气边作仿生学处理.试验结果表明,锯齿形处理可降低A声级噪声2—5％,尤以齿距z=0.1b齿高h=0.1b的锯齿形处理为佳,试验结果还表明,“风机叶片的仿生学处理可以降低A声级噪声,降噪幅度在风量一定的条件下主要取决于仿生学处理的叶片相对长度。ΔL/L.上述方法是值得推广的降低通风机噪声的有效措施.     

基于传声器阵列过顶测量结果的飞机起落架噪声研究

应用由111个传声器组成的平面传声器阵列对当前流行的民用客机进场着陆过程中的机体噪声源进行了实验测量,本对七架窄体客机和七架宽体客机的起落架噪声进行了分析,得到了起落架噪声的频谱特性、指向特性和声级变化。研究发现,起落架噪声的频谱是由宽频随机噪声与一些较为明显的单噪声源组成,起落架噪声的指向性类似于一个水平放置的偶极子。不同飞机起落架噪声的声级相差较大,这说明可以通过重新结构设计降低起落架噪声。     

数值模拟不等距叶片对贯流风机的影响

贯流风机的通过频率(BPF)是其重要的噪声频率.降低BPF噪声可以降低基频处的声压级,其中一种方法是采用不等距叶片.本文采用3种叶片距分布形式,采用realizable k-ε两方程和大涡模拟(LES)湍流模型模拟了风机的内流场,计算线性欧拉方程(LEE)中声源项得到声源位置及强度,采用基于Lighthill声类比的FW-H积分方程获得了叶轮和蜗舌处偶极子型的离散噪声频谱.比较了不同叶片距对风机性能,噪声特别是BPF噪声的影响.计算结果表明在对性能影响较小的情况下,不等距叶轮可降低BPF噪声和总A声级噪声.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.