失谐驻波管及其极高纯净驻波场性质的研究

由直径不同的两级直圆管连接而成的两级突变截面驻波管具有失谐性,即高阶共振频率不是一阶共振频率的整数倍. 两级突变截面驻波管的失谐性质能够很好地抑制一阶共振频率激励下的大振幅非线性驻波畸变产生的高次谐波,从而获得大振幅纯净驻波场. 通过对两级突变截面驻波管失谐性质的研究,采用大功率扬声器正接等措施,利用两级突变截面驻波管的失谐性质在一阶共振频率激励下获得了184 dB的极高纯净驻波场,并对二至五阶共振频率激励下的声场进行了相应的实验研究. 在二阶、四阶共振频率激励下分别获得了180和166 dB波形比较规整的大振幅非线性驻波,并在三阶、五阶共振频率激励下观察到了谐波饱和现象和锯齿波. 关键词： 失谐驻波管 大振幅驻波 畸变 饱和     

突变截面驻波管和极高纯净驻波场的实验研究

突变截面驻波管属于失谐驻波管,即其高阶共振频率不是一阶共振频率的整数倍。通过对STAS的优化设计,利用STAS的失谐性质在一阶和二阶共振频率下激励分别获得了180 dB和177 dB的极高纯净驻波声场。尽管声压级已经很高,但在接下来的对一阶和二阶共振频率激励下的声波波形畸变和谐波饱和情况进行的实验研究中仍然没有观察到谐波饱和现象。与此同时,对三阶共振频率激励下的声场进行了实验研究,由于三阶共振频率激励下的大振幅非线性声场的二次谐波频率接近六阶共振频率,在声压级达到170 dB时观测到了三阶共振频率激励下的声波波形畸变和谐波饱和现象。      

等截面驻波管内大振幅驻波场的实验研究

通过改进等截面驻波管实验系统,在1阶峰值共振频率激励下获得了182.1 dB大振幅驻波场,并对1~5阶峰值共振频率激励下的大振幅驻波场谐波饱和情况以及波形畸变进行了实验研究。研究发现,尽管1阶峰值共振频率激励下声压级已达到182.1 dB,但波形畸变最小,谐波并未表现出饱和现象,而3阶峰值共振频率激励下的大振幅驻波场表现出了饱和趋势。对谷值共振频率激励下获得的大振幅驻波场进行对比实验研究,发现谷值共振频率激励下,1阶谷值共振频率所获得的驻波场声压级最大,但波形畸变也最大。在相同声源驱动电压下,1阶峰值共振频率激励下获得的驻波场声压级始终大于1阶谷值共振频率激励下获得的驻波场声压级。由此可见,利用扬声器在等截面驻波管中获取大振幅驻波场,驻波管由1阶峰值共振频率激励较为合适。      

大振幅驻波的实验研究──Ⅲ：三次谐波的共振

在基波声压级（Ｌｐ１）为定值（１４８ｄＢ）的条件下，三次谐波变化曲线可用一“准周期”函数描述。其周期长度为驻波管的第一共振频率Δｆ＝ｃ０／２Ｌ。三次谐波的共振现象极有规律。共振峰处自身频率（ｆ３＝３ｆ０）准确地与驻波管频率相吻合。而曲线谷值的自身频率并不与驻波管反共振频率相同。三次谐波在共振区必为峰值，在反共振区必为其谷值。而在其他频段则有尚待解释的现象。     

谐振管形状对热声发动机内非线性声场的影响

研究了谐振管一端受活塞声源激励,另一端刚性封闭条件下,管道形状对热声发动机谐振管内部非线性声场的影响。基于流体力学基本方程建立了渐变截面谐振管内一维非线性声场的模型,考虑了黏性耗散及非线性效应的影响。利用伽辽金法数值求解了该模型的速度势方程,分析了谐振管形状、活塞振动速度及激励频率对管内声场的影响。将双曲形、指数形、锥形、正弦形等四种变截面谐振管内的非线性声场与圆柱形直管的情况进行了比较。结果反映了谐振管内声场的压力波动受活塞振动速度及谐振管形状的影响;显示了当活塞振动幅度较大时,谐振管内出现的波形畸变、频率曲线偏移、共振频率滞后等非线性现象;揭示了变截面谐振管在抑制管内的高阶谐波及提高压比等方面的优越性。      

大振幅驻波的实验研究Ⅱ：驻波场谐波的饱和

大振幅驻波在激波形成前的过程中，谐波声压级随基波声压级（Ｌｐ１）增长而出现饱和现象。在Ｌｐ１＜１５３ｄＢ时，ｍ次谐波声压振幅ｐｍ与基波声压幅值ｐ１；呈ｐｍ～ｐｍ１关系。基波升高１ｄＢ，ｍ次谐波升高ｍｄＢ。之后，各次谐波均开始趋于饱和。次数越高，饱和越快，ｐｍ～ｐｍ１的关系逐渐变为ｐｍ～ｐ１的趋向。在频率域内，变化过程大致可划分为三个区域：Ｉ．线性区；Ⅱ．变化区；Ⅲ．激波区。三个区域对应了时域的激波形成。Ｌｐ１＞１６０ｄＢ后，即可认为激波已经形成。基波与电信号的关系也呈饱和现象，但与频率有关。上述结果有助于理论研究工作的进一步深入。     

昆特管实验原理分析

根据声学原理对昆特管演示实验进行分析,给出了对实验现象细节的一种物理解释,即在昆特管中除主驻波模式之外还激励起了最低一级高次驻波模式振动,昆特管内的声波场实际上是主驻波模式和最低一级高次驻波模式叠加形成的声波场.     

大振幅驻波的实验研究──Ⅰ：二次谐波的特性

本文对一维大振幅驻波场进行了较为全面的实验研究，介绍了实验研究系统，着重讨论二次谐波的特性、实验研究结果表明：二次谐波的量值在基波产压级为定值的条件下是驱动频率的函数．其变化曲线可用一简谐函数近似描述之。大振幅驻波场在频域内可划分为四个区域。在称之为“稳定区”内，其二次谐波最小值可用马大猷教授所提出的“大振幅驻波理论”求得。理论与实验符合良好。用更进一步发展的理［８］，能定性地对二次谐波的变化趋势作出圆满解释。     

激光驻波场形成Cr原子沉积纳米光栅的初步实验研究

利用激光驻波场会聚原子沉积纳米结构的技术可以用来研制纳米结构长度传递标准.当激光驻波场的频率大于原子的共振频率时，原子由于受到偶极力的作用将被会聚到驻波的波节处.采用了一种新的技术方案减小了实验对大的激光功率的要求.利用激光驻波场会聚准直性较好的Cr原子，并使其沉积在硅基片上形成纳米光栅结构.经原子力显微镜测试表明纳米光栅的周期为215 nm. 关键词： 激光偶极力 纳米计量 原子沉积 蓝失谐     

驻波激光场中囚禁离子的线性熵和量子态转移研究

运用量子纠缠和线性熵理论,研究了驻波激光场中囚禁离子的线性熵和量子态转移.讨论了相干角、离子的相对位相、离子与驻波激光场之间的耦合强度以及失谐量、Lamb-Dicke参数对离子线性熵的影响.结果表明,在一定的条件下可以实现囚禁离子的内态到振动态的相干转移,线性熵随时间的演化呈现非周期性的振荡行为.离子线性熵的最大值随着相干角、离子与激光场之间的耦合强度以及失谐量的增大而减小,随着Lamb-Dicke参数的增大而增大.并且可以通过调节驻波激光场来调节离子与驻波激光场之间的耦合强度和失谐量,从而达到对离子线性熵的控制与操纵,理论上提供了一种调控纠缠的方式.     

Two-photon standing-wave fluorescence correlation spectroscopy

A fluorescence correlation spectroscopy experiment that combines two-photon excitation and a standing-wave interference pattern is presented. The experimental correlation function can be analyzed using a simple expression involving (1) an exponential decay with time constant tau(f), which reflects diffusion across the interference fringes, and (2) a longer-lived decay with time constant tau(omega), which reflects diffusion in and out of the focal spot. The diffusion of Rhodamine 110 in water and ethylene glycol is measured using this method. The ability to simultaneously measure diffusion on two different time and lengthscales makes this experiment especially useful in environments where anomalous diffusion is suspected.     

Experimental investigation on standing-wave tube with abrupt section and extremely nonlinear pure standing-wave field

Standing-wave tube with abrupt section （STAS） was a dissonant standing-wave tube whose higher resonance frequencies were not integral multiplies of the first one. Making use of the dissonant property of STAS and through the optimization of the system, extremely nonlinear pure standing-wave field of 180 dB at the first resonance frequency and that of 177 dB at the second resonance frequency have been obtained. At the two resonance frequencies, distortion of waveform and saturation of harmonics were studied experimentally, but saturation did not appear even though under such high sound pressure levels. However, while nonlinear sound field was experimentally studied at the third resonance frequency, it was found that the frequency for the second harmonic of the third resonance frequency was close to the sixth resonance frequency of the STAS and the distortion of waveform and the saturation of harmonies appeared as the sound pressure level approached 170 dB.     

Experimental study of large-amplitude standing-wave field obtained in a standing-wave tube with uniform cross section

A large-amplitude standing-wave field of 182.1 dB is obtained under the excitation at the resonant frequency of the lst-order peak of the sound pressure transfer function in an improved standing-wave tube experimental system,and saturation of harmonics and waveform distortion are investigated experimentally for the large-amplitude standing-wave fields obtained under the excitations at the resonant frequencies of the 1 st-to the 5 th-order peaks.The results show that although the sound pressure level has reached 182.1 dB under the excitation at the resonant frequency of the 1 st-order peak,the waveform distortion is the minimum and the harmonic saturation is not observed.However,the large-amplitude standing-wave field excited at the resonant frequency of the 3 rd-order peak exhibits the trend of the harmonic saturation.Comparison of the large-amplitude standing-wave fields obtained under the excitations at valley resonant frequencies shows that the standing-wave field excited at the resonant frequency of the 1 st-order valley has the largest SPL,but also has the largest waveform distortion.Under the same source-driving voltage,the standing-wave field excited at the resonant frequency of the 1 st-order peak always has greater SPL than the standing-wave field excited at the resonant frequency of the 1 st-order valley.Hence,to obtain a large-amplitude standing-wave field,it's better to excite at the resonant frequency of the 1 st-order peak of the SPTF by using loudspeaker in a standing-wave tube with uniform cross section.     

Experimental study on the standing-wave tube with tapered section and its extremely nonlinear standing-wave field

A standing-wave tube with tapered section(STTS) was evolved from a standing-wave tube with abrupt section(STAS) whose abrupt section was replaced with tapered section. The research was intended to compare the acoustic properties and the extremely nonlinear pure standing waves of STTS with those of STAS.The acoustic properties of the STTS were studied with transfer matrix.It was proved,like the STAS,that the STTS was dissonant standing-wave tube.With its dissonant property,the 181 dB extremely nonlinear pure standing wave was obtained in the STTS excited at its first resonance frequency.Then the comparative experimental studies on the saturation properties of the extremely nonlinear standing waves were carried out in the STTS and the STAS with the same length.It was found that the STTS could suppress the harmonics and meanwhile reduce energy loss of the standing wave more effectively.Compared with the STAS,under the same voltage of loudspeaker,the STTS obtained a higher extremely nonlinear pure standing wave.Moreover,it was found for the STTS that the third harmonic of the third resonance frequency was close to the seventh resonance frequency of sound source impedance,to which the valley value of the sound pressure level transfer function corresponded.Because of this,the third harmonic increased rapidly with the increase of fundamental wave and tended to saturate.     

Adiabatic following in standing-wave diffraction of atoms

We report experiments on the diffraction of atoms from a standing light wave in the channeling regime, characterized by long interaction time and large potential height. The observed far-field diffraction patterns depend specifically on the way in which the potential is switched on and off. For fast switching, the evolution is non-adiabatic and many diffraction orders are populated. For slow switching, however, the evolution is adiabatic and the number of populated diffraction orders decreases dramatically. The experiments are performed in two different setups employing rubidium and argon atoms, respectively. In one of the setups, we study the dependence of the diffraction pattern on the interaction time, in the other setup that on the incidence angle. Received: 30 November 1998 / Revised version: 5 July 1999 / Published online: 8 September 1999     

弦线驻波实验装置改进

本文对典型弦线驻波实验中数据不够准确、驻波不稳定及实验可重复性差等现象进行了分析,明确其根本原因在于实验装置设计方案存在缺陷.提出了实验装置改进方案,一是引入拉力传感器取代狭缝刀口和悬挂重物,改变反射波的形成机理,并以弦线张力直接测量取代砝码重力间接计算;二是引入机械结构微调弦线长度以改变弦线张力和弦线上驻波波长.实验结果证明,改进后的实验装置能明显改善上述问题,该装置还可实施研究驻波规律的多个实验方案。     

Pulsed standing-wave mirror for neutral atoms and molecules

Reflection of neutral atoms and molecules by a pulsed standing wave with a duration on the order of nanoseconds is studied. It is shown that, with a suitable choice of the laser parameter values, each period of the standing-wave pattern functions as an independent mirror, thus providing a novel way to manipulate large samples of neutral gas-phase particles even with a single laser pulse. At moderate field intensities, the pulsed standing-wave mirror would be directly applicable, e.g., for the manipulation of buffer-gas cooled molecules.     

驻波热声系统的自激振荡机理

热声发动机的起振过程是一个产生并维持自激振荡的过程, 研究热声自激振荡机理有助于进一步了解热声效应的实质. 根据热声网络理论, 建立了驻波热声发动机的整机网络. 将热声网络比拟成电网络, 利用厄米特式计算了输入热声网络的视在功流, 功流平衡对应自激, 在角频率虚部为零的情况下计算了热声发动机的阈值温度和运行频率. 结果表明, 计算值与实验值符合得较好, 充气压力与阈值温度和运行频率的耦合关系大致相同. 所得结论有助于进一步探究热声效应机理以及热声发动机系统的优化设计.     

Optimal profile for a Gaussian standing-wave atom-optical lens

We have used a Gaussian standing-wave atom-optical lens to focus a thermal atomic beam. We examine the effect of variations in the intensity profile along the direction of the atomic beam on the performance of our atom-optical lens. For a constant focal-length atom-optical lens, we find that the resolution and contrast of the standing-wave lens are independent of the intensity profile.