弦音实验仪的设计与制作

自制了弦音实验装置,利用吉他的主体结构,设计了模仿人激励弦线的电子线路,同时测量了弦线的线密度、横波在弦线上的传播速度及弦线张力等参量.     

弦振动实验装置的改进

1引言 弦振动实验是普通物理力学中的一个必作实验.它是利用电动音叉引发弦线横波,进而形成驻波,来研究横波的叠加现象;验证横波的波长与张力、线密度的关系;并用驻波法测出电动音叉的固有频率.     

弦线中的波动方程T0（a^2y）／（ax^2）=η（a^2y）／（at^2）对大扰动也成立

一般教材中，在讨论一维弦线中的横波波动方程时，都假设弦线中的张力为恒量，在小扰动情况下，推导横波波动方程，本文在较一般情况下，推导一维弦线中的横波波动方程，并讨论弦线中张力的变化等问题，所得结果在小扰动近似下，与一般教材中的结论一致。     

弦线中的波动方程对大扰动也成立

一般教材中，在讨论一维弦线中的横波波动方程时，都假设弦线中的张力为恒量，在小扰动情况下，推导横波波动方程。本文在较一般情况下，推导一维弦线中的横波波动方程，并讨论弦线中张力的变化等问题，所得结果在小扰动近似下，与一般教材中的结论一致。     

利用横波讨论产生半波损失的条件

: 基于弦线上传播的机械横波定量讨论了产生半波损失的条件.     

对弦振动实验中振源的改进

设计制作了一种振源．将其应用于弦振动实验时,不但能像电动音叉作振源那样,可以验证弦线上驻波波长与弦线张力、密度的关系,而且还可以验证电动音叉作振源所不能验证的弦线上驻波波长与驻波频率的关系．     

弦线驻波实验的教学新视角

针对学生创造思维培养,改变通常的弦线驻波实验中验证波长与频率、外力、弦线线密度关系为核心问题的教学思路,阐述以实验理论可执行性的突破为视角进行实验教学设计思想和方法。     

关于弦线振动的横波条件

在通常的教科书中,对于弦线上的横振动,大多采用线性近似处理.典型的说法如:“看弦的一小段(x,x x)的运动,设p为弦线的单位长质量则运动方程为 对于小振动,可设θ角很小,略去θ2,有cosθ1≈cosθ2.于是由第二个方程得T1=T2=T,即在没有沿x方向的外力时弦中各点的张力是相同的.[1] 对于横振动,可以抛开小振动的假设,直接引入“横波条件”因故有 对于横向运动,则有式中d3为弦的微分弧元. 设弦线上没有振动传播时,单位长质量为p0,则代入　(3)式,得这正好是弦线上的横渡方程.这样,从横波条件出发,我们同样可以得到波动方程.不过,这时候,弦线上…     

弦振动实验的改进

弦振动实验是一个传统实验,一般由电振音叉带动有一定张力的细线作振动,它对于研究驻波、弦的基频与谐频等规律是很有意义的,但此实验存在一定的问题: 1.弦线在音叉带动下振动,固定在音叉上的端点实际上不是书点; 2.弦的振动频率不能改变,不利于研究不同频率下的振动; 3.张力改变时,线密度将变化; 4.弦线绕过滑轮加负载,滑轮阻力给实验引入误差. 我们去掉电振音叉和滑轮,改用金属弦,用磁场对通电导体的作用力去强迫弦线振动,可以较好的解决上述问题,还可以讨论振动的品质因数,实验误差也较小. 实验装置:图1为装置的示意图,将低频电流加在铜…     

弦线与音叉垂直情况下弦振动的研究

如图1所示，当音叉的位置由图la变为图fo时，则弦线驻波的波腹数减少为原来（偶数）的一半，此现象有的称为二分频现象．对此现象现已有文章‘’‘通过建立物理模型解数理方程的方法得出某些具体结果，但在教学中更希望能从基本物理规律出发对此现象做出解释．本文就此试研究如下．一、基本分析由振动和波动理论可知，要使图1装置形成稳定的驻波须有两个条件，一为弦线长度为半波长的整数倍，一为音叉给予弦线一合拍作用力对弦线总是做正功．设弦线上激发某一频率的驻波，若无能量损耗，则不需外力作用，弦线的运动状态就会一直维持下去．此…     

2019年中国声学学会青年人才托举工程项目交流会在南昌召开

为改善电磁超声换能器(electromagnetic acoustic transducer,EMAT)激发横波的特性,提出一种基于正交试验的优化方法,并利用有限元仿真软件COMSOL Multiphysics建立了横波EMAT的有限元模型,应用正交试验和极差分析法,分析了线圈各参数对电磁超声横波的近场长度和半扩散角的影响,比较了线圈各参数影响程度的大小。结果表明,激励电流的频率和线圈的尺寸对横波的传播特性有着明显影响,提高频率会使得横波的近场长度增大及半扩散角减小;其次,减小导线的宽度及间距,以缩小线圈尺寸能有效改善横波的传播特性。     

Features of wave generation by a source moving along a one-dimensional flexible guide lying on an elastic-inertial foundation

A self-consistent dynamic problem is posed for a system including a one-dimensional flexible guide (a string), elastic-inertial foundation (an array of oscillators), and moving oscillating load. The effect of the foundation parameters on the dispersion characteristics (frequency, phase velocity, and group velocity as functions of the wavenumber) of transverse waves propagating along the string has been analyzed. It has been shown that taking into account the foundation inertia leads to the presence of two critical (cutoff) frequencies. Regularities of wave generation by a source moving along the string have been analyzed.     

带宽对平顶高斯宽带激光传输特性的影响

研究了横向分布为平顶高斯模式的宽带激光在自由空间的传输特性,分析了频带宽度对其强度分布的影响。首先推导了平顶高斯模式宽带激光的每一频率分量在自由空间的传输公式,然后将频率域的传输公式通过傅里叶变换得到时间域的传输公式。基于推导的公式通过数值算例给出了不同频带宽度激光光束的强度分布结果,详细分析了空间模式的变化和时空耦合现象。研究结果表明:随着传输距离增大,平顶高斯空间模式趋于高斯分布。同时随着带宽增大,平顶高斯模式宽带激光表现出更强的时空耦合现象,并导致其空间横向分布发生明显变化。变化的主要特征为波前的强度分布随带宽增大而变窄,波后的强度分布则随带宽增大而远离轴中心向外扩展。     

Difference between a photon's momentum and an atom's recoil

When an atom absorbs a photon from a laser beam that is not an infinite plane wave, the atom's recoil is less than variant Planck's k in the propagation direction. We show that the recoils in the transverse directions produce a lensing of the atomic wave functions, which leads to a frequency shift that is not discrete but varies linearly with the field amplitude and strongly depends on the atomic state detection. The same lensing effect is also important for microwave atomic clocks. The frequency shifts are of the order of the naive recoil shift for the transverse wave vector of the photons.     

Analysis of Dispersion in Ferrite-superconductor Layered Structures

This paper studies electromagnetic wave propagation in hybrid structures consisting of ferrite and superconducting films. Dispersion relations for transverse electric mode of propagation are derived. Numerical results are presented for two specific structures viz. a ferrite/superconductor structure and a ferrite/superconductor/ferrite structure and dependence of propagation on various structural parameters and frequency are determined. The non-reciprocal nature of propagation is also established.     

Wave propagation in fluid-conveying viscoelastic single-walled carbon nanotubes with surface and nonlocal effects

In this paper, the transverse wave propagation in fluid-conveying viscoelastic single-walled carbon nanotubes is investigated based on nonlocal elasticity theory with consideration of surface effect. The governing equation is formulated utilizing nonlocal Euler-Bernoulli beam theory and Kelvin-Voigt model. Explicit wave dispersion relation is developed and wave phase velocities and frequencies are obtained. The effect of the fluid flow velocity, structural damping, surface effect, small scale effects and tube diameter on the wave propagation properties are discussed with different wave numbers. The wave frequency increases with the increase of fluid flow velocity, but decreases with the increases of tube diameter and wave number. The effect of surface elasticity and residual surface tension is more significant for small wave number and tube diameter. For larger values of wave number and nonlocal parameters, the real part of frequency ratio raises.     

自适应光学对大气扰动波前的补偿效果研究 I.水平大气传输时的结果分析

讨论了光波在水平大气湍流中传输时的情况。计算结果表明自适应光学系统的补偿效果与光波传播路径上的横向风有很大的关系。大气湍流的强度越大，自适应光学系统的截止频率越高，横向风的影响也越大。计算结果还表明在相同的ｒ０或者相同的大气湍流强度下，球面波所受大气扰动的自适应光学补偿效果受横向风的影响要比平面波的厉害。本文还将理论分析的结果和数值模拟的结果进行了比较，表明数值模拟结果受横向风的影响比理论分析结果受横向风的影响大。最后，文中分析了以上现象并对此作出了合理的解释     

Relativistic string in a pulse

I study a relativistic open string coupling through its endpoints to a plane wave with arbitrary temporal profile. The string’s transverse oscillations respond linearly to the external field. This makes it possible to solve the classical equations, and to calculate the quantum-mechanical S-matrix in closed form. I analyze the dynamics of the string as the characteristic frequency and duration of the pulse are continuously varied. I derive, in particular, the multipole expansion in the adiabatic limit of very long wavelengths, and discuss also more violent phenomena such as shock waves, cusps, and null brane intersections. Apart from their relevance to the study of time-dependence in superstring theory, these results could have other applications, such as the teleportation of gravitational wave bursts by cosmic strings.     

Transverse plane wave analysis of short elliptical chamber mufflers: An analytical approach

Short elliptical chamber mufflers are used often in the modern day automotive exhaust systems. The acoustic analysis of such short chamber mufflers is facilitated by considering a transverse plane wave propagation model along the major axis up to the low frequency limit. The one dimensional differential equation governing the transverse plane wave propagation in such short chambers is solved using the segmentation approaches which are inherently numerical schemes, wherein the transfer matrix relating the upstream state variables to the downstream variables is obtained. Analytical solution of the transverse plane wave model used to analyze such short chambers has not been reported in the literature so far. This present work is thus an attempt to fill up this lacuna, whereby Frobenius solution of the differential equation governing the transverse plane wave propagation is obtained. By taking a sufficient number of terms of the infinite series, an approximate analytical solution so obtained shows good convergence up to about 1300 Hz and also covers most of the range of muffler dimensions used in practice. The transmission loss (TL) performance of the muffler configurations computed by this analytical approach agrees excellently with that computed by the Matrizant approach used earlier by the authors, thereby offering a faster and more elegant alternate method to analyze short elliptical muffler configurations.     

Propagation of electromagnetic waves in a waveguide with a modulated anisotropic insert

The propagation of transverse electric (TE) and transverse magnetic (TM) waves in a waveguide of arbitrary cross section with an anisotropic periodically space-and time-modulated insert is considered. Under the assumption that the modulation indices are small, analytical expressions for the TE and TM fields in the domain of weak interaction between the signal wave and modulating wave are derived. The frequency near which strong interaction between these waves occurs is found, and specific features of the fields in this domain are revealed.