Linewidth and tuning characteristics of terahertz quantum cascade lasers

We have measured the spectral linewidths of three continuous-wave quantum cascade lasers operating at terahertz frequencies by heterodyning the free-running quantum cascade laser with two far-infrared gas lasers. Beat notes are detected with a GaAs diode mixer and a microwave spectrum analyzer, permitting very precise frequency measurements and giving instantaneous linewidths of less than -30 kHz. Characteristics are also reported for frequency tuning as the injection current is varied.     

Integrable systems,symmetries, and quantization

These notes are an expanded version of a mini-course given at the Poisson 2016 conference in Geneva. Starting from classical integrable systems in the sense of Liouville, we explore the notion of non-degenerate singularities and expose recent research in connection with semi-toric systems. The quantum and semiclassical counterparts are also presented, in the viewpoint of the inverse question: from the quantum mechanical spectrum, can one recover the classical system?     

Broadband trailing edge noise from a sharp-edged strut

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 10(5) to 5.0 × 10(5)). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.     

舰船螺旋桨轴频估计中线谱要素提取算法

舰船辐射噪声DEMON谱(调制解调谱)线谱要素提取是舰船螺旋桨轴频估计中的关键步骤。现有的线谱提取技术主要用于功率谱线谱检测,侧重于线谱频率的确定,不能满足舰船螺旋桨轴频估计需求。本文根据DEMON谱线谱的特点及人工提取线谱的过程,设计了一种新的线谱要素提取算法,改进了谱峰筛选技术,将多时刻的DEMON线谱基于最大似然估计和谱峰序列相似度加以融合决策,最终确定了线谱频率和幅值(峰值)及边界。通过实际数据验证,此线谱要素提取算法更适用于螺旋桨轴频估计,可提高轴频估计的准确率。     

Sound radiation from real airfoils in turbulence

Leading edge noise measurements and calculations have been made on a three airfoils immersed in turbulence. The airfoils included variations in chord, thickness and camber and the measurements encompass integral scale to chord ratios from 9 to 40 percent as well as 4:1 ratios of leading edge radius and airfoil thickness to integral scale. Angle of attack is found to have a strong effect on the airfoil response function but for the most part only a small effect on leading edge noise because of the averaging effect of the isotropic turbulence spectrum. Angle of attack effects can therefore be significant in non-isotropic turbulence and dependent on airfoil shape. It is found that thicker airfoils generate significantly less noise at high frequencies but that this effect is not determined solely by the leading edge radius or overall thickness. Camber effects appear likely to be small. Angle of attack effects on the response function of a strongly cambered airfoil are shown to be centered on zero angle of attack, rather than the zero lift angle of attack.     

Langevin description of markovian integro-differential master equations

For a given master equation of a discontinuous irreversible Markov process, we present the derivation of stochastically equivalent Langevin equations in which the noise is either multiplicative white generalized Poisson noise or a spectrum of multiplicative white Poisson noise. In order to achieve this goal, we introduce two new stochastic integrals of the Ito type, which provide the corresponding interpretation of the Langevin equations. The relationship with other definitions for stochastic integrals is discussed. The results are elucidated by two examples of integro-master equations describing nonlinear relaxation.     

Saxophonists tune vocal tract resonances in advanced performance techniques

The acoustical impedance spectrum was measured in the mouths of saxophonists while they played. During bugling and while playing in the very high or altissimo range, experienced players tune a strong, but relatively broad, peak in the tract impedance to select which peak in the bore impedance will determine the note. Less experienced players are unable to produce resonances with impedance peaks comparable in magnitude to those of the bore and consequently are unable to play these notes. Experienced players can also tune their tracts to select which combinations of notes are played simultaneously in multiphonics or chords, and to produce pitch bending, a technique in which notes are produced at frequencies far from those of the peak of impedance of the instrument bore. However, in normal playing in the standard range, there is no consistent tuning of the tract resonances. The playing frequency, in all cases, lies close to the peak in the impedance of the reed in parallel with the series combination of the impedances measured in the mouth and the instrument bore on either side of the reed (ZMouth+ZBore)∥ZReed.     

NOISE GENERATED BY A COANDA WALL JET CIRCULATION CONTROL DEVICE

An analysis is made of the production of sound by a hydrofoil with a Coanda wall jet circulation control (CC-) device. Three principal sources are identified in the vicinity of the trailing edge of the hydrofoil. The radiation at very low frequencies is dominated by “curvature noise” generated by the interaction of boundary layer turbulence with the rounded trailing edge of the CC-hydrofoil; this is similar in character and magnitude to the low-frequency component of the conventional trailing edge noise produced by a hydrofoil of the same chord, but with a sharp trailing edge. Higher frequency sound is produced principally at the Coanda jet slot. “Passive slot noise” is caused by the “scattering” by the slot lip of nearfield pressure fluctuations in the turbulent boundary layer of the exterior mean flow past the slot. This is of comparable intensity to high frequency, sharp-edged trailing edge noise. However, the acoustic spectrum is greatly extended to much higher frequencies if the Coanda jet is turbulent; the sound produced by the interaction of this turbulence with the lip tends to dominate the spectrum at frequencies f (Hz) greater than about U_j/h, where h is the slot width and U_jthe Coanda jet speed. Sample numerical results are presented for a typical underwater application that indicate that at this and higher frequencies the slot noise can be 20 dB or more greater than conventional trailing edge noise, although the differences become smaller as the thickness of the slot lip increases.     

Aerodynamic sound generation of flapping wing

The unsteady flow and acoustic characteristics of the flapping wing are numerically investigated for a two-dimensional model of Bombus terrestris bumblebee at hovering and forward flight conditions. The Reynolds number Re, based on the maximum translational velocity of the wing and the chord length, is 8800 and the Mach number M is 0.0485. The computational results show that the flapping wing sound is generated by two different sound generation mechanisms. A primary dipole tone is generated at wing beat frequency by the transverse motion of the wing, while other higher frequency dipole tones are produced via vortex edge scattering during a tangential motion. It is also found that the primary tone is directional because of the torsional angle in wing motion. These features are only distinct for hovering, while in forward flight condition, the wing-vortex interaction becomes more prominent due to the free stream effect. Thereby, the sound pressure level spectrum is more broadband at higher frequencies and the frequency compositions become similar in all directions.     

An ensemble source spectra model for merchant ship-radiated noise

This paper presents an evaluation of the classical model for determining an ensemble of the broadband source spectra of the sound generated by individual ships and proposes an alternate model to overcome the deficiencies in the classical model. The classical model, proposed by Ross [Mechanics of Underwater Noise (Pergamon, New York, 1976)] postulates that the source spectrum for an individual ship is proportional to a baseline spectrum with the constant of proportionality determined by a power-law relationship on the ship speed and length. The model evaluation, conducted on an ensemble of 54 source spectra over a 30-1200-Hz to 1200-Hz frequency band, shows that this assumption yields large rms errors in the broadband source level for the individual ships and significantly overestimates the variability in the source level across the ensemble of source spectra. These deficiencies are a consequence of the negligible correlation between the source level and the ship speed and the source level and the ship length. The alternate model proposed here represents the individual ship spectra by a modified rational spectrum where the poles and zeros are restricted to the real axis and the exponents of the terms are not restricted to integer values. An evaluation of this model on the source spectra ensemble indicates that the rms errors are significantly less than those obtained with any model where the frequency dependence is represented by a single baseline spectrum. Furthermore, at high frequencies (400 to 1200 Hz), a single-term rational spectrum model is sufficient to describe the frequency dependence and, at the low frequencies (30 to 400 Hz), there is only a modest reduction in the rms error for a higher order model. Finally, a joint probability density on the two parameters of the single term model based on the measured histograms of these parameters is proposed. This probability density provides a mechanism for generating an ensemble of ship spectra.     

消声室测量法研究弦槌对钢琴声音的影响

为研究弦槌性能,有很多研究者采用了各种机械测量方法,而本论文则希望通过声学测量方法来研究。我们实测了两套不同弦槌对钢琴声音频谱的影响,具体做法是在消声室中使用3种力度弹奏88个琴键并进行录音。频谱分析的结果表明两套弦槌在中力度弹奏时得到的频谱基本一致,但在大力度和小力度弹奏时差别较大,其差别表现为：在低频端,珠江钢琴弦槌激发的高次谐波分量要大于并多于德国Abel公司生产的弦槌;随着频率的升高,该现象逐渐减弱,并出现相反的现象。根据Donal E. Hall的弦槌理论可知,造成这些差异的原因是弦槌硬度和初始能量的影响效果在不同力度下有所不同,如欲考察弦槌宽度和质量的影响适合在大力度下弹奏,而考察弦槌硬度的影响适合在小力度下弹奏。实验结果表明,这种声学测量实验方法主要使用常规的声学仪器,具有简单、易行的优点,可以作为辅助弦槌的设计和改进工作的一种有效方法。     

复频聚焦超声换能器水中焦区商场特性的研究

本文对新研制的一种复频聚焦超声换能器的辐射声场进行了实验研究,测定了辐射压在轴向上的分布曲线,进而确定了焦区位置,这与理论所得结果符合较好,测定了辐射声压在焦平面上的分布曲线,并对焦区声场进行了定位,最后对换能器在水中焦区辐射声的频谱进行了研究,不仅观测到了两个源波,而且还观测到了和频波,差频波以及倍频波,证实了声散射声效应的存在。     

Comparison of sound power radiation from isolated airfoils and cascades in a turbulent flow

An analytical model of the sound power radiated from a flat plate airfoil of infinite span in a 2D turbulent flow is presented. The effects of stagger angle on the radiated sound power are included so that the sound power radiated upstream and downstream relative to the fan axis can be predicted. Closed-form asymptotic expressions, valid at low and high frequencies, are provided for the upstream, downstream, and total sound power. A study of the effects of chord length on the total sound power at all reduced frequencies is presented. Excellent agreement for frequencies above a critical frequency is shown between the fast analytical isolated airfoil model presented in this paper and an existing, computationally demanding, cascade model, in which the unsteady loading of the cascade is computed numerically. Reasonable agreement is also observed at low frequencies for low solidity cascade configurations.     

Collective oscillations of the magnetic moments of a chain of spherical magnetic nanoparticles with uniaxial magnetic anisotropy

Magnetic particles moving freely in a fluid can organize dense phases (3D clusters or linear chains). We analyze the spectrum of magnetic oscillations of a chain of spherical magnetic particles taking into account the magnetic anisotropy of an individual particle for an arbitrary relation between the anisotropy energy and the energy of the dipole interaction of particles. For any relation between these energies, the spectrum contains three branches of collective oscillations: a high-frequency branch and a weakly split doublet of low-frequency branches. The frequency of the high-frequency branch is determined by a stronger interaction, while the frequencies of the low-frequency branches are determined by the weakest interaction. Accordingly, the dispersion is maximal for oscillations formed by the dipole-dipole interaction of particles, which have high frequencies in the case of a strong dipole interaction or low frequencies in the case of a strong anisotropy.     

一种脉冲星信号模拟新方法

在脉冲星导航地面实验中,为解决地面无法接收脉冲星辐射的X射线信号的问题,必须在实验系统中模拟脉冲星辐射信号. 本文提出两种全新脉冲星信号模拟方法. 第一种方法基于统计物理模型,该方法克服了常用泊松模型模拟方法中仿真适应性窄、仿真速度慢和时间分辨率低等不足,实现了对任意随机过程的脉冲信号进行快速高时间分辨率的仿真;第二种方法基于优化统计模型,该方法针对非齐次泊松过程信号的特性进行优化,仿真速度比常用泊松模型模拟方法提高至少30 倍,对于低流量脉冲星,甚至提高4个数量级,可实现纳秒级时间分辨率仿真. 关键词： 脉冲星导航 非齐次泊松过程 X射线光子信号     

泊松统计的无偏估计

当泊松分布的平均值依赖于被估参数时, 从单次实验结果给出的关于参数的最大似然估计一般是统计学意义上有偏的. 借助矩的表达形式, 提出了一种关于被估参数的无偏估计方法.     

Analysis of Optical Disk Readout Signal Deterioration Caused by Dust on the Substrate

A noise power spectrum estimation method is presented for optical disk readout signal influenced by dust on the substrate. The noise due to dust has been modeled using geometrical optics and assuming the Poisson distribution for the dust particles. The noise power spectrum consists of two components: one comes from the effect of individual dust particles passing across the light beam, and the other comes from the effect of overlapping area of a particle and the inverted image of another particle. The feasibility of a thin-substrate optical disk was studied using this model. Dust will not have a serious effect on the readout signal when a 0.6 mm-thick substrate is used in place of the conventional 1.2 mm-thick substrate.     

VIBRATION AND STABILITY OF ROTATING POLAR ORTHOTROPIC ANNULAR DISKS SUBJECTED TO A STATIONARY CONCENTRATED TRANSVERSE LOAD

In the present paper, natural frequencies and stability of a spinning polar orthotropic disk subjected to a stationary concentrated transverse load are investigated. The analysis of the free vibration of a spinning disk is performed first to find natural frequencies and corresponding vibration modes. The resulting eigenfunctions obtained from the free vibration are used as deflection functions of the forced vibration of a disk where the load is modelled as a mass-spring-dashpot system fixed in space. By using the Galerkin approximation method, eigenvalues of the whole system are determined. Results show that disks with higher values of modulus ratios or the Poisson ratios have higher natural frequencies, and the stability of the whole system can be improved by raising the value of the modulus ratio or lowering that of the Poisson ratio.     

Relation between single neuron and population spiking statistics and effects on network activity

To simplify theoretical analyses of neural networks, individual neurons are often modeled as Poisson processes. An implicit assumption is that even if the spiking activity of each neuron is non-Poissonian, the composite activity obtained by summing many spike trains limits to a Poisson process. Here, we show analytically and through simulations that this assumption is invalid. Moreover, we show with Fokker-Planck equations that the behavior of feedforward networks is reproduced accurately only if the tendency of neurons to fire periodically is incorporated by using colored noise whose autocorrelation has a negative component.