Experimental verification of the acoustic performance of diffusive roadside noise barriers

The acoustic performance of pairs of diffusive roadside barriers is tested experimentally on a 1:10 scale model, and compared to that of more traditional specularly reflecting barriers. Significant attenuation benefits are detected not only in the shadow zone behind the barriers, but also in the unprotected zone immediately above the barriers, thus proving that diffusive traffic faces of the barriers may effectively help in counteracting multiple reflection effects. In addition, a radiosity-based theoretical model developed for the evaluation of the sound field behind pairs of diffusive noise barriers is described, and its ability to predict the extra SPL attenuation deriving from the replacement of geometrically reflecting barriers with diffusely reflecting barriers is verified.     

垒温对硅衬底GaN基蓝光LED发光效率的影响

用MOCVD技术在硅衬底上生长了GaN基蓝光LED外延材料,研究了有源层多量子阱中垒的生长温度对发光效率的影响,获得了不同电流密度下外量子效率(EQE)随垒温的变化关系。结果表明,在860~915℃范围内,发光效率随着垒温的上升而上升。当垒温超过915℃后,发光效率大幅下降。这一EL特性与X光双晶衍射和二次离子质谱所获得的阱垒界面陡峭程度有明显的对应关系,界面越陡峭则发光效率越高。垒温过高使界面变差的原因归结为阱垒界面的原子扩散。垒温偏低使界面变差的原因归结为垒对前一个量子阱界面的修复作用和为后一个量子阱提供台阶流界面的能力偏弱。外延生长时的最佳垒温范围为895~915℃。     

斜轨法测量重力加速度影响因素的探讨

斜轨法是测量重力加速度的一种常用方法,但光电门挡光片的形状和宽度以及导轨的倾斜角度对测量结果有着直接的影响。本文从实验角度探讨上述几个因素对测量结果的影响并与当地重力加速度的标称值(广州9.788 m/s2)进行对比。测量结果表明:在导轨的垫片高度为2 cm时,使用条形挡光片的测量值均大于标称值;而U型挡光片宽度大于13 cm时测量值大于标称值,而小于13 cm时测量值则均小于标称值。在相同的导轨倾斜角下用U型挡光片时,两挡光片间距越大,测量值越大;而使用条形挡光片时,挡光片宽度越大,测量值就越小。     

轻轨桥架声屏障降噪效果的测量与分析

在轻轨桥架两侧设置声屏障是控制轨道交通噪声的主要措施之一，对上海轻轨明珠线桥架两侧设置不同高度声屏障实际降噪效果的测量与分析，对声屏障的实用设计有重要的参考价值。     

Resonant tunneling of spin-wave packets via quantized states in potential wells

We have studied the tunneling of spin-wave pulses through a system of two closely situated potential barriers. The barriers represent two areas of inhomogeneity of the static magnetic field, where the existence of spin waves is forbidden. We show that for certain values of the spin-wave frequency corresponding to the quantized spin-wave states existing in the well formed between the barriers, the tunneling has a resonant character. As a result, transmission of spin-wave packets through the double-barrier structure is much more efficient than the sequent tunneling through two single barriers.     

Quantum nature of proton transferring across one-dimensional potential fields

Proton transfer plays a key role in the applications of advanced energy materials as well as in the functionalities of biological systems.In this work,based on the transfer matrix method,we study the quantum effects of proton transfer in a series of one-dimensional(1 D) model potentials and numerically calculate the quantum probability of transferring across single and double barriers(wells).In the case of single barriers,when the incident energies of protons are above the barrier height,the quantum oscillations in the transmission coefficients depend on the geometric shape of the barriers.It is found that atomic resonant tunneling(ART) not only presents in the rectangular single well and rectangular double barriers as expected,but also exists in the other types of potential wells and double barriers.For hetero-structured double barriers,there is no resonant tunneling in the classical forbidden zone,i.e.,in the case when the incident energy(E_i) is lower than the barrier height(E_b).Furthermore,we have provided generalized analysis on the characteristics of transmission coefficients of hetero-structured rectangular double barriers.     

Performance of profiled single noise barriers covered with quadratic residue diffusers

The paper describes an investigation about the acoustic performance of noise barriers with quadratic residue diffuser (QRD) tops, and with T-, Arrow-, Cylindrical and Y-shape profiles. A 2D boundary element method (BEM) is used to calculate the barrier insertion loss. The results of rigid and with absorptive coverage are also calculated for comparisons. Using QRD on the top surface of almost all barrier models presented here is found to improve the efficiency of barriers compare with using absorptive coverage at the examined receiver positions. T-shape and Arrow-shape barriers are also found to provide better performance than other shapes of barriers. The best shape of barriers for utilising QRD among the tested models is the T-shape profile barrier. It is found that reducing the design frequency of QRD shifts the performance improvement towards lower frequency, and therefore the most efficient model for traffic noise is a barrier covered with a QRD tuned to around 400 Hz.     

Noise control by sonic crystal barriers made of recycled materials

A systematic study of noise barriers based on sonic crystals made of cylinders that use recycled materials like absorbing component is reported here. The barriers consist of only three rows of perforated metal shells filled with rubber crumb. Measurements of reflectance and transmittance by these barriers are reported. Their attenuation properties result from a combination of sound absorption by the rubber crumb and reflection by the periodic distribution of scatterers. It is concluded that the porous cylinders can be used as building blocks whose physical parameters can be optimized in order to design efficient barriers adapted to different noisy environments.     

Multi-affine analysis of typical currency exchange rates

The resonance splitting in finite semiconductor superlattices which consist of a number of electric barriers is investigated. It is found that (n-1)-fold splitting for n-barrier tunneling obtained in periodic superlattices of identical barriers no longer holds for superlattices which are periodically juxtaposed with two different building barriers. In general, one resonant domain in the former splits into two resonant subdomains in the latter, and splitting occurs each time when two new barriers are added. The results indicate that the resonance splitting is determined not only by the structure but also by the parameters of building blocks. Received: 12 October 1997/ Revised and Accepted: 5 January 1998     

The barrier systematics of fusion and capture of heavy systems within the surface friction model

Available experimental fusion and capture barriers for heavy systems are compared to calculated barriers within the surface friction model. The average trend of the experimental surplus energies over the Bass barriers is reproduced more satisfactorily than with dynamical extra-extra push calculations.     

Linear and non-linear energy barriers in systems of interacting single-domain ferromagnetic particles

A comparative analysis between linear and non-linear energy barriers used for modeling statistical thermally-excited ferromagnetic systems is presented. The linear energy barrier is obtained by new symmetry considerations about the anisotropy energy and the link with the non-linear energy barrier is also presented. For a relevant analysis we compare the effects of linear and non-linear energy barriers implemented in two different models: Preisach-Néel and Ising-Metropolis. The differences between energy barriers which are reflected in different coercive field dependence of the temperature are also presented.     

Distinction between two molecular mechanisms for deformation of glassy amorphous polymers

During deformation of glassy amorphous polymers the rotation of rigid chain segments around skeletal bonds is restricted simultaneously by configurational (intramolecular) and chain-chain (intermolecular) energy barriers. These barriers are modeled in a self-consistent manner for six polymers by use of an approximate analytical treatment. The comparative contribution of these barriers to the small-strain modulus of the bulk solid is used as a basis for distinguishing between two mechanisms of stiffening. With polycarbonate, polyphenylene oxide, and an aromatic polyimide, in which the rigid chain segment is relatively long, the modulus derives its value primarily from the resistance to displacements across chains due to intermolecular barriers. With vinyl polymers such as polystyrene, poly(vinyl chloride) and poly(methyl methacrylate), however, in which the rigid chain segment is short, resistance to displacements along the main chain due to intramolecular barriers contributes equally significantly to the modulus. Our calculations also show that the length of the rigid chain segment, acting as a mechanical moment arm, affects the resistance to intramolecular displacements much more than does the height of the rotational energy barrier.     

Lowering of calculated fission barriers of superheavy nuclei

The effect of angular momentum explicit conservation is evaluated in the framework of models which utilize single-particle wavefunctions to calculate fission barriers. Using simple assumptions, and extrapolating from known data, the calculated syperheavy barriers are lowered by 2 – 2.5 MeV. Fission barriers for the actinides are unaffected.     

Critical distance for fusion and fusion barriers of very-heavy-ion collisions

For the fusion barriers of very-heavy-ion collisions we derive a simple analytical formulae which is based on the concept of the critical distance for fusion. The analytical expression reproduces the experimental fusion barriers within a large range of target-projectile combinations.     

Scattering of massless Dirac particles by oscillating barriers in one dimension

We study the scattering of massless Dirac particles by oscillating barriers in one dimension. Using the Floquet theory, we find the exact scattering amplitudes for time-harmonic barriers of arbitrary shape. In all cases the scattering amplitudes are found to be independent of the energy of the incoming particle and the transmission coefficient is unity. This is a manifestation of the Klein tunneling in time-harmonic potentials. Remarkably, the transmission amplitudes for arbitrary sharply-peaked potentials also become independent of the driving frequency. Conditions for which barriers of finite width can be replaced by sharply-peaked potentials are discussed.     

Molecular torsion: Comparison of harmonic oscillator and free rotor bases

In this work we describe a method of solving the eigenvalue problem of torsional motion in polyatomic molecules by expanding the eigenfunctions in a harmonic oscillator basis. By comparing the eigenvalues obtained with a harmonic oscillator basis and those calculated with a free rotor basis, it is found that a basis with only 5–7 oscillator functions will reproduce the low-lying energy levels for large torsional barriers with the same accuracy as a basis with about 10 free rotor functions. The method has been applied to a calculation of the barriers to internal rotation for methyl ammonium chloride. The barriers were obtained from a least squares fit of the torsional frequencies of methyl ammonium chloride and seven of its deuterated derivatives, treating the molecule as a double rotor. The method is recommended for the calculation of large rotational barriers from spectroscopic data on systems in any state of aggregation, and may therefore be useful in the testing of theoretical models.     

氧在金属及其氧化物中的表面及扩散势垒的计算

使用一种较简单的计算方法，研究了氧在简单金属及其氧化物中的表面穿透势垒及扩散势垒，发现表面势垒及扩散势垒值依赖于材料结构及弛豫能力．部分数据与现有的实验值作了比较，两者基本相符． 关键词：     

A STUDY OF VARIOUS BARRIERS IN ENCLOSED SOUND FIELD BY USING THE COMPUTER PROGRAM—SOFIS

In the study of the behaviors of barriers in an enclosed field, one should take into account such phenomena as sound energy reflection, absorption, scattering and diffraction. Therefore, the study is much more difficult than that in free field. In this paper, sound barriers are classified into four kinds according to their size, number and shape. Each kind of barriers is modelled by a corresponding method based on a computer program—SOFIS. The program combines the ray-tracing technique and statistical method. The impulse response and some acoustical parameters such as sound pressure level at different positions can be calculated by the program, no matter there are a certain kind of barriers in the field or the field is empty. The ray-tracing program and the algorithms for various barriers are validated by the comparison between measurement and prediction of the reverberation room and the anechoic room of the Northwestern Polytechnic University.     

Spatiotemporal distribution of nucleation events during crystal growth

We consider irreversible second-layer nucleation that occurs when two adatoms on a terrace meet. We solve the problem analytically in one dimension for zero and infinite step-edge barriers, and numerically for any value of the barriers in one and two dimensions. For large barriers, the spatial distribution of nucleation events strongly differs from rho(2), where rho is the stationary adatom density in the presence of a constant flux. Theories of the nucleation rate omega based on the assumption that it is proportional to rho(2) are shown to overestimate omega by a factor proportional to the number of times an adatom diffusing on the terrace visits an already visited lattice site.