对音叉接触纸张的“低音”现象研究

当振动的音叉与纸张存在较弱接触时可以产生低于音叉基频的声音,即为低音现象.在本研究中对这种低音现象进行了理论分析,采用工程学上的"悬臂梁"模型进行分析,再通过考虑其在简谐激励力下的受迫振动,提出了相应的理论模型,明确了影响低音现象的多种关键因素.对于整个系统的振动过程,在实验中以理论中所推导的因素分别从纸张的长度、厚度、宽度、褶皱程度以及音叉与纸张的接触位置这几个方面进行探究,从实验上对理论模型进行定性的验证.     

关于音叉弦振动的讨论

在大学物理教材中［１］［２］［３］常用音叉弦振动来描述驻波现象，并都认为驻波的形成原因是入射波（由音叉带动弦振动所引起的波）与反射波的干涉结果。这种解释实际上是欠妥当的。本文详细讨论了音叉弦振动形成驻波的现象及其原因。     

Python用于音叉接触纸张的泛音现象研究

以音叉接触纸张的泛音现象为研究对象,利用Python-Librosa库进行频谱分析,定性探究泛音现象的特点及其影响因素.研究发现音叉接触纸张振动会产生与音叉基频成整数倍关系的高次谐波.音叉接触距离纸张悬挂部位越远的区域,以及纸张厚度越厚,振动产生的高次谐波越多,声音越尖锐;音叉接触A4纸和白卡纸振动产生的谐波现象较为明显,接触草稿纸产生的谐波现象不明显.     

两端固定张紧弦"异常"共振现象

用弦音计做两端固定弦振动实验中出现了和音叉计不同的共振现象,如长春五光的仪器出现了 f0 ∶fk=1n∶1的共振现象,PASCO公司的产品出现了f0∶fk=12n∶1的共振现象,本文分析和解释了这些现象产生的原因,并提出了实验改进方案.     

自制音叉的对称性与非对称性破坏性实验研究

文中对自制对称正型音叉与自制非对称异型音叉受力F后音叉产生的振动的现象进行了研究,用DIS声传感数字化信息系统采集振动频率,研究频率与音叉叉股长度的关系,从而对音叉叉股对称与非对称的情况进行研究。     

基于非线性振动研究低音现象

采用音叉与纸张系统发生非线性振动的观点,对于音叉和纸张进行建模以及力学分析,预测出产生低音的频率特点.理论分析和实验结果表明:音叉振动提供驱动力,引起纸张发生非线性振动,产生低音现象.实验结果的各低分频比较好地满足了理论推导出的分数频的假设.     

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…     

音叉演示“拍”现象的改进

在讲授频率相差很小的振动合成时,常用两个音叉来演示拍现象。但由于音叉振动的音量太小,即使加了共鸣箱,在上大课时,学生也很难听清拍音。我们设计了如下方法解决这个问题。将两片压电陶瓷片用胶     

音叉受迫振动的干涉测量

通过干涉测量法得到音叉振动的幅频特性曲线,同时测量位移振幅和速度振幅.实验中使用高品质因子音叉,激励信号的电压也降到了mV量级,既可以让音叉经过较长时间消除暂态过程,又可以减缓音叉由于长时间激励所带来的升温现象.使用干涉测量法测量音叉受迫振动振幅,并给出2种数据处理方式.该方法具有驱动功率小,测量精度高,直接测量位移量,同时测量两方面信号的优点.     

干涉法测量音叉振动

采用干涉法对音叉受迫振动的位移振幅、速度振幅以及音叉振动与外加周期性驱动力的相频关系做了测量与探究.实验结果表明,位移共振频率与速度共振频率近似相等;音叉达到共振时,音叉振动与外加周期性驱动力存在π/2的相位差.利用干涉的方法可对音叉受迫振动的位移振幅、速度振幅以及音叉振动与外加周期性驱动力的相频关系同时进行研究,并且可观测到调节信号源频率至共振频率过程中相位差的改变,以及达到共振时施加外在扰动后相位差偏离π/2而后恢复的过程.     

The detection of thin embedded layers using normal incidence ultrasound

A theoretical investigation of the use of normal incidence ultrasonic reflection measurements for the detection and characterization of thin layers embedded between two much thicker media has been carried out. It has been shown that the form of the relationship between the normal incidence longitudinal reflection coefficient and frequency is defined by the reflection coefficients at zero frequency and at half the resonance frequency of the layer. The reflection coefficient at zero frequency is solely a function of the impedances of the media on either side of the layer, while that at half the resonance frequency of the layer is a function of the impedances of all three media. In general, the sensitivity of the reflection coefficient to the presence of the layer increases as the product of frequency and layer thickness increases, the maximum sensitivity being at half the resonance frequency of the layer. Unfortunately, with thin layers, it is generally not practical to test at this frequency. However, the reflection coefficient curve can, in principle, be reconstructed from data measured at lower frequencies and the sensitivity of the reflection coefficient at lower frequencies to the characteristics of the layer can be predicted from the sensitivity at half the resonance frequency. The sensitivity is also critically dependent on the relative impedances of the three media and is generally greatest when the half spaces on either side of the layer have the same impedance. With favourable impedances, it is possible to detect layers whose thickness is a small fraction of the wavelength of the ultrasonic waves employed. However, with other combinations of impedances, the detection of much thicker layers is not possible.     

Effect of electron-neutral collisions on the satellites of the Thomson scattering spectrum in arc plasmas

The effect of electron-neutral collisions on the high frequency spectrum of laser radiation scattered by the free electrons of a plasma is investigated for a partially ionized H₂ are plasma at atmospheric pressure. The calculations are carried out along Gorog's theory solving the linearized Boltzmann equation for electrons with a collision term. The collision integral is approximated by a Krook relaxation model with the collision frequency determined from experimental electron-atom scattering data. The collisions influence size, half width, and position of the high frequency satellites. In a H₂ arc plasma, the change of the satellites' position is negligible as well as the change in half width within experimentally attainable error limits. The change of size is of minor importance, since the general evaluation procedure uses only normalized scattering intensities. Thus, for laser scattering experiments in are plasmas the collision-free theory can be applied.     

Observation of fractional harmonics in the NMR signal from superfluid 3He-B

Unusual spin-precession states of ³He-B in which the magnetization is half the equilibrium value are investigated by continuous-wave NMR methods. Signals at frequencies equal to 1/2 and 3/2 of the magnetization precession frequency are observed in two such states. Such signals exist because the order parameter of superfluid ³He in these states precesses with frequency equal to half the magnetization precession frequency. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 200–205 (10 February 1999)     

Method of implementation of frequency encoded all optical half adder, half subtractor and full adder based on semiconductor optical amplifiers and add drop multiplexers

A novel frequency encoded all optical half adder, half subtractor and full adder are proposed. The implementation is ultrafast one and the frequency encoding makes it intensity loss dependent problem free. The use of polarization insensitive four-wave mixing makes the design polarization independent and the hardware simple. The frequency conversion by the reflective semiconductor optical amplifiers (RSOA) makes the design faster compared to other semiconductor optical amplifiers (SOA) based design.     

介质平面波导TE0模衍射场的光束参量

对于傍轴光束，提出了介质平面波导衍射场的近场模场半宽度、远场发散角半宽度和光束传输因子M^2等光束参量之间满足简单的关系式，三个光束参量均可直接由波导模式场分布精确计算，或由可实际精确测量的远场衍射频谱分布精确计算。M^2因子的计算公式可表述为以二阶矩和微分算子定义的近场模场半宽度的比值，或以二阶矩和微分算子定义的远场发散角半宽度的比值。最后推导出介质平面波导TE0模衍射场光束参量的解析函数和光束参量的范围。     

Emission of noise-like spin wave packets in the presence of three-magnon decay processes and the kinetic instability of waves in a ferromagnetic film

The features of spin wave emission from a ferromagnetic film in the direction of the propagation of a surface magnetostatic wave have been experimentally investigated at various input signal powers. Radiation in the form of two noise-like spin wave packets has been detected near the frequency corresponding to half the pump frequency. This radiation is caused by three-magnon processes of the decay of a surface magnetostatic wave and by the kinetic instability of spin waves.     

基于Kramers逃逸速率的调参随机共振机理

根据Kramers逃逸速率的特性,阐明了随机共振信号的频率被限制在Kramers逃逸速率极限值一半的范围内,这种限制是制约大频率信号产生随机共振的原因.在进一步揭示二次采样随机共振频率尺度变换机理的基础上,证明了二次采样频率尺度可以把任意信号频率映射变换到随机共振频率尺度上的结论.相对于二次采样变换方法,由于双稳系统参数的调节很难使Kramers逃逸速率的一半达到实际信号的大频率,因此系统参数只能在随机共振的小参数频率范围内调节来实现随机共振. 关键词： 双稳随机共振 二次采样频率变换 系统参数调节 Kramers逃逸速率     

Spot size measurement of a flash-radiography source using the pinhole imaging method

The spot size of the X-ray source is a key parameter of a flash-radiography facility, and is usually quoted as an evaluation of the resolving power. The pinhole imaging technique is applied to measure the spot size of the Dragon-I linear induction accelerator, by which a two-dimensional spatial distribution of the source spot is obtained.Experimental measurements are performed to measure the spot image when the transportation and focusing of the electron beam are tuned by adjusting the currents of solenoids in the downstream section. The spot size of full-width at half maximum and that defined from the spatial frequency at half peak value of the modulation transfer function are calculated and discussed.     

压电条SH声辐射场研究

具有一定宽度和厚度的无穷长压电条置于均匀各向同性半空间上, 压电条在电激励下产生机械振动从而在整个空间产生声场. 将压电条取为6mm晶系, 其对称主轴沿长度方向. 针对这 种压电换能器结构, 深入研究了SH波的激发和辐射特性. 首先将压电条中的声场展开为傅里 叶级数, 而将压电条下的无界半空间中的声场展开为傅里叶积分, 然后根据边界条件得到了 整个空间中声场的求解方案, 通过数值模拟计算了声场的分布特性, 并和传统方法进行了定 量的对比和分析. 最后采用最陡下降法(鞍点法)研究并得到了压电条声辐射场的远场近似表 达式, 分析了声场的指向性规律. 结果表明只有当频率f和压电条宽度a的乘积 fa<1 kHz·m 时, 传统方法得到的结果是可靠的，当频率升高时，压电条内应力和位移呈振荡式分布，频率愈高振荡愈激烈，且与传统方法的差异愈大.本文对声学微系统与声传感器件的研究具有 重要意义. 关键词： 压电条 6mm晶系 表面位移 指向性因子     

半波长管传声损失分析

1/4波长管和Herschel-Quincke(HQ)管具有良好消声潜力,在其固有频率附近具有很高的消声量级,为了将这种消声潜力在更小的安装空间内和更宽的频带上发挥出来,设计了一种新的半波长管,通过对多管传声损失理论模型的推导,运用数值计算的方法,分析了传声损失的影响因素并对多分支管模型进行宽频带尺寸设计,最终实现在3...