Predicting reverberation times in a simulated classroom

By varying the sound-absorption treatments in a simulated classroom, experimental results were compared with analytical and computer predictions of reverberation time. Analytical predictions were made with different absorption exponents, which are the result of different weighting procedures involving room surface areas and the sound-absorption coefficients. Sound scattering was found to influence measured reverberation times. With the amount of sound scattering provided, more accurate analytical predictions were obtained with absorption exponents that give reverberation times longer than those obtained with the Sabine absorption exponent, which consistently underpredicted reverberation times. However, none of the absorption exponents could be singled out as more adequate because of similar average accuracy. Computer predictions of reverberation time were accomplished with two commercially available ray-based programs, RAYNOISE 3.0 and ODEON 2.6, with specular and calibrated diffuse reflection procedures. Neither type of procedure, in either program, was more accurate than the best analytical predictions. With RAYNOISE, neither the specular nor the calibrated diffuse reflection procedure could be singled out as more adequate. For ODEON, the calibrated diffuse reflection procedure gave consistently more accurate predictions than its specular reflection procedure, with the best accuracy of the computer predictions.     

Optimization by simulated annealing: Quantitative studies

Simulated annealing is a stochastic optimization procedure which is widely applicable and has been found effective in several problems arising in computeraided circuit design. This paper derives the method in the context of traditional optimization heuristics and presents experimental studies of its computational efficiency when applied to graph partitioning and traveling salesman problems.     

Irreducible finite-size effects in the surface free energy of NaCl crystals from crystal-nucleation data

In this Letter we report a simulation study in which we compare the solid-liquid interfacial free energy of NaCl at coexistence, gamma_{LS}, with the value that follows from the height of the homogeneous nucleation barrier. The two estimates differ by more than 100%. Smaller discrepancies are found for gamma_{LS} of hard-sphere and of Lennard-Jones particles. We consider a variety of possible causes for this discrepancy and conclude that it is due to a finite-size effect that cannot be corrected for by any simple thermodynamic procedure. By taking into account the finite-size effects of gamma_{LS} obtained in real nucleation experiments, we obtain quantitative agreement between gamma_{LS} estimated in the simulations and derived from the experiments. Our finding suggests that most published solid-liquid surface free energies derived from nucleation experiments will have to be revised.     

An analytic and parameter-free wavefunction for studying the stability of three-body systems

An analytic wavefunction is proposed for the ground state of general atomic three-body systems in which two light particles are negatively charged and the third (heavy) is positively charged. By construction the wavefunction (i) has the same analytical form for all systems; (ii) is parameter-free; (iii) is nodeless; (iv) satisfies all two-particle cusp conditions; and (v) yields reasonable ground state energies for several three-body systems, including the prediction of a bound state for H^??, D^??, T^?? and Mu^??. Simple polynomial fits are provided for certain important subcases, allowing for a rapid estimate of the ground state energy and of the stability of three-body systems.     

从非线性与统计物理多重角度探讨数值地震预测

在科技文献中,地震常被比喻为非线性动力学过程或统计物理中的相变过程.文章探讨了如何从非线性力学中的分岔理论以及统计物理内的朗道相变理论出发,从势磊穿越,临界涨落与临界慢化等多个角度来分析和了解地震发生的全过程.文章作者试图在这些非线性力学与统计物理的基础上,综合地震过程中在时间与空间上应出现的前兆,解释如何可能做出具有普适性的数值地震预测.     

从非线性与统计物理多重角度探讨数值地震预测

在科技文献中,地震常被比喻为非线性动力学过程或统计物理中的相变过程。文章探讨了如何从非线性力学中的分岔理论以及统计物理内的朗道相变理论出发,从势磊穿越,临界涨落与临界慢化等多个角度来分析和了解地震发生的全过程。文章作者试图在这些非线性力学与统计物理的基础上,综合地震过程中在时间与空间上应出现的前兆,解释如何可能做出具有普适性的数值地震预测。     

半导体量子点形成临界尺寸的定量预测

半导体量子点是一类具有显著量子效应的零维量子结构,自组的模型系统,表现为Stranski-Krastanov型生长.其特征为,当超过3-4个Ge单原子层（浸润层）时,则由二维层状生长转变为三维岛状生长.Ge/Si量子点是初期形成的与衬底共格无位错的三维岛,岛表面由{105}晶面组成.文章作者利用第一性原理计算和介观理论模拟相结合的连续式多尺度（sequential multi-scale）方法,第一次对纯Ge和GeSi合金量子点在Si（001）表面的成核临界尺寸进行了定量的理论预测,同时研究了岛边缘的应力不连续对量子点稳定性的影响,实现了对Ge/Si量子点的形成和稳定性定量的理论研究。     

半导体量子点形成临界尺寸的定量预测

半导体量子点是一类具有显著量子效应的零维量子结构,自组的模型系统,表现为Stranski-Krastanov型生长.其特征为,当超过3—4个Ge单原子层（浸润层）时,则由二维层状生长转变为三维岛状生长.Ge/Si量子点是初期形成的与衬底共格无位错的三维岛,岛表面由{105}晶面组成.文章作者利用第一性原理计算和介观理论模拟相结合的连续式多尺度（sequential multi-scale）方法,第一次对纯Ge和GeSi合金量子点在Si(001)表面的成核临界尺寸进行了定量的理论预测,同时研究了岛边缘的应力不连续对量子点稳定性的影响,实现了对Ge/Si量子点的形成和稳定性定量的理论研究.     

碳纤维复合材料内部缺陷深度的定量红外检测

利用脉冲红外热成像技术对碳纤维复合材料试件内部的模拟脱黏缺陷的深度进行测量, 研究在被测物热属性参数未知情况下,碳纤维增强塑料中缺陷深度的测量方法. 分析了平板材料在脉冲热源激励下的一维热传导模型;给出了内部缺陷深度的红外测量原理; 选用对数温度二阶微分峰值时刻作为特征时间测量缺陷深度; 考虑单点标定测量深度可能产生较大的随机误差,提出利用最小二乘法多项式拟合建立阶梯件中阶梯深度与其对应的对数温度时间二阶微分曲线峰值时间两者之间的标定关系式的方法, 选择在相对误差平方和最小情形下的拟合关系式作为脱黏缺陷深度测量的标定关系式. 实验结果表明,利用该方法测量脱黏缺陷深度的精度优于单点法标定测量结果, 实现了在被检测材料热属性参数未知的情况下仍能较准确地测量脱黏缺陷深度.     

Quantitative assessment and prediction of contact area development during spherical tip indentation of glassy polymers

This paper describes the development of the contact area during indentation of polycarbonate. The contact area was measured in situ using an instrumented indentation microscope and compared with numerical simulations using an elasto-plastic constitutive model. The parameters in the model were obtained using macroscopic tests. Indentations were performed on samples with different thermal histories and at different speeds. For all cases, the numerical model correctly predicted the development of the contact area during indentation. For increasing strain rates, the contact area decreased at equal indentation depths. Annealing the samples resulted in a smaller contact area at equal indentation depth. Using only numerical simulation, it was also shown that pile-up around the indenter resulted from localization effects and was, thus, promoted by strain-softening properties of the indented material. Strain hardening, on the other hand, will tend to promote sink-in. Finally, we performed simulations of load relaxation during indentation. The results indicate that about 40% of the total observed relaxation may be assigned to plastic effects.     

Towards a parameter-free characterization of charge transfer via hopping: the case of tris(8-hydroxyquinolato) aluminum

By using calculations based on density functional theory, we investigate the physical factors determining the elemental charge transfer in Alq3, taken as a prototype of molecular systems in condensed phase. The effect of the environment on the hopping of the charge carrier is evaluated self-consistently in a model in which an Alq3 dimer is embedded in an ensemble of permanent and polarizable dipoles, including orientational disorder and the presence of impurities. The results indicate that the origin of the activation barrier is mainly extrinsic and that the local orientation of the dipole moments plays a major role. The influence of nonadiabaticity is also studied and found to be more important for a hole than for an electron.     

Short first-passage times

For diffusion in a monotonie potential field the probability distribution of first-passage times is computed in the limit of short times. The relation to the familiar long-time regime is pointed out.     

Sensitivity of ray travel times

Ray in a waveguide can be considered as a trajectory of the corresponding Hamiltonian system, which appears to be chaotic in a nonuniform environment. From the experimental and practical viewpoints, the ray travel time is an important characteristic that, in some way, involves an information about the waveguide condition. It is shown that the ray travel time as a function of the initial momentum and propagation range in the unperturbed waveguide displays a scaling law. Some properties of the ray travel time predicted by this law still persist in periodically nonuniform waveguides with chaotic ray trajectories. As examples we consider few models with special attention to the underwater acoustic waveguide. It is demonstrated for a deep ocean propagation model that even under conditions of ray chaos the ray travel time is determined, to a considerable extent, by the coordinates of the ray endpoints and the number of turning points, i.e., by a topology of the ray path. We show how the closeness of travel times for rays with equal numbers of turning points reveals itself in ray travel time dependencies on the starting momentum and on the depth of the observation point. It has been shown that the same effect is associated with the appearance of the gap between travel times of chaotic and regular rays. The manifestation of the stickiness (the presence of such parts in a chaotic trajectory where the latter exhibits an almost regular behavior) in ray travel times is discussed. (c) 2002 American Institute of Physics.     

Calculation of spin-lattice relaxation times

A general expression is found for the probability for a direct (single-phonon) relaxation transition for arbitrary symmetry of the surroundings of a paramagnetic ion having an effective spin of S = 1/2. The expression is compared with experiment for the case of tetragonal symmetry. Since certain approximations frequently used in the calculations are not used here, a markedly improved agreement is found with the experimental relaxation times.Translated from Izvestiya VUZ. Fizika, No. 2, pp. 36–39, February, 1970.     

Ricci recurrent space times

The algebraic restrictions on the Ricci tensor in a Ricci-recurrent space-time are determined. The restrictions imposed on the Petrov type of the Weyl tensor are also given.     

Calculations of anharmonic phonon relaxation times

Anharmonic phonon relaxation times in Ge are calculated using (i) an isotropic continuum model, and (ii) a dispersive model. A complete spectrum of calculated results is presented. Frequency-averaged values for normal- and umklapp-three-phonon relaxation times are also calculated. A comparison is made between our findings and some earlier works, and disagreements are discussed. The results are applied in calculating some results in the theory of lattice thermal conductivity. From February 1976: Department of Theoretical Physics, The University, Newcastle-Upon-Tyne, NE1 7RU U.K.