Development of scattering surfaces for concert halls

The degree of diffusion, characterized by the “scattering coefficient” of surface materials, has been known to be one of the most important factors in determining the acoustical qualities of concert halls. Based on the suggested ISO method, which measures the random-incidence scattering coefficient of surfaces in a diffuse field, the scattering coefficients of different sizes and densities of wooden hemispheres and cubes were measured in model-scale reverberation rooms. The results show that hemispheres with a height of more than 15 cm have the highest average (500 Hz to 4 kHz) scattering coefficient. It was also found that the scattering coefficient becomes higher when the diffuser density reaches about 50% for hemispheres and 30% for cubes.     

Wall diffuser designs for acoustical renovation of small performing spaces

This paper investigates the material characteristics of diffusers for acoustical renovation of existing small performing spaces. Consideration is given to acoustical effects on sound fields through the practical cases of two performing halls: the Chamber Hall (450 seats) and the M-Theater (630 seats) in the Sejong Performing Arts Center, Seoul. The Chamber Hall was completely refurbished into a recital hall from its previous shape of a rectangular conference space. The saw-tooth shaped wall diffuser profile in the Chamber Hall was designed using glass-fiber reinforced concrete for mid-frequency sound diffusion. The M-Theater was renovated as a live and intimate space for dramatic performances with a design that included more seats on the upper floors and additional spaces above ceiling reflectors. The vertically-patterned diffuser profile with protruded cubic surfaces in the M-Theater was designed using glass-fiber reinforced gypsum for sound diffusion in the major speech frequency range. Designed diffuser profiles were evaluated for both halls by measurements of scattering and diffusion coefficients of the 1:10 scale model diffusers. The effects of diffusers in both halls were also investigated by covering the lateral walls close to the stages with reflective materials to control diffusive surfaces. As a result, spatial uniformity increased with diffusive wall profiles in both halls.     

An experimental investigation into the sound-scattering performance of wooden diffusers with different structures

An experimental investigation is performed to determine the random-incident scattering coefficients of wooden diffuser structures characterized by different materials, geometrical designs, groove depths and baffle features. In general, it is found that the reliability of the measurement results obtained using the ISO 17497-1 standard can be improved by inserting the test sample into the rotating turntable in order to reduce the edge effect. The experimental results show that a higher scattering coefficient is obtained when the diffuser is fitted with flutters, has a double-triangular upper-surface profile, and has a greater groove depth. The results and discussions presented in this study provide architectural acousticians with a useful framework for evaluating the sound-scattering performance of various wooden diffuser structures in indoor environments.     

组织模拟液Intralipid-10%的 吸收系数和各向异性因子的测量

用加吸收体法测量了组织模拟液Intralipid-10%的吸收系数和各向异性因子。运用漫射理论研究了点光源在无限大介质中的各向异性散射,实验结果得到 μ_a/D 的多个测量值的标准偏差小于0.6%,测量值与拟合值的残差方均根值为0.1%。标准偏差和残差的值表明了测量值的准确度。     

基于频域近红外光谱法的生物组织光学参数的检测与三维重构

生物组织的光学特性参数与组织的各种生理、生化以及病理过程密切相关。光学特性参数的检测与三维重构可以在一定程度上了解组织的内部结构及特性。基于频域近红外光谱法,使用美国ISS公司的频域近红外光谱检测系统,采用多光源多检测器技术,将830 nm的激光经过调制后,照射进放置了具有一定吸收系数和散射系数的异质体小球的牛奶中,检测出射光的光强和相位,然后应用光学扩散层析成像技术,通过计算含有异质体小球的牛奶的光学参数,重构出了牛奶中异质体的具体位置。实验结果表明,利用频域近红外光谱法得到的出射光的光强和相位信息,能够较准确地计算出牛奶及异质体小球的吸收系数和散射系数等组织光学参数,并可进一步定位出牛奶中异质体小球的位置。因此,可以考虑将该方法应用在分析及定位水果病变等农产品品质检测领域。     

Experimental determination of distributions of soot particle diameter and number density by emission and scattering techniques

A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504 C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701 V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation,demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.     

基于数值方法的激光海面漫反射通信能量分布特性

 针对激光点对点通信方式的不足,提出了利用海面作为激光漫反射媒介进行组网通信的设想,采用前后向迭代的数值方法结合Green函数谱积分加速算法(FBM/SAA)对激光海面漫反射通信的能量分布特性进行了研究。通过对激光光束入射海面后产生的散射场的分析、计算及实验验证,得出了较为准确的2维激光海面双站散射系数,并对激光光束入射海面后的散射场进行了分析。研究结果表明：入射激光束经过粗糙海面散射后,能量大部分集中在前向散射区域上,而后向散射强度很弱,且在散射场的边缘处能量迅速衰减,说明了激光海面漫反射组网通信方法的可行性。通过与前后向迭代法和Kirchhoff近似方法的计算结果的比较,说明了FBM/SAA是一种高效、准确的计算方法。     

Diffusion coefficient of krypton atoms in helium gas at low and moderate temperatures

In the present work, using the Chapman–Enskog method for dilute gases, the diffusion coefficients of ground krypton atoms in a very weakly ionized helium buffer gas are revisited. The calculations are carried out quantum mechanically in the range of low and moderate temperatures. The ¹ Σ⁺ potential-energy curve via which Kr approaches He is constructed from the most recent ab initio energy points. The reliable data points used in the construction are smoothly connected to adequate long- and short-range forms. The calculations of the classical second virial coefficients and the Boyle temperature of the helium–krypton mixture are also discussed. These coefficients and their variations in terms of temperature are analysed by adopting the constructed HeKr potential and the Lennard–Jones form that fits it. The diffusion and elastic cross sections are also explored and the resonance features they exhibit are closely examined. The variation law of the diffusion coefficients with temperature is determined for typical values of density and pressure. The coefficients show excellent agreement with the available experimental data; the discrepancies do not exceed 5%.     

脑组织漫反射光谱与约化散射系数的斜率估算法

约化散射系数是生物组织光学研究中的一个重要的组织光学参数。利用微光纤探头与光纤光谱仪组成漫反射光谱采集系统对脑组织约化散射系数的实时在位测量进行研究。通过悬乳液与模拟胶模型实验推导了700～850nm波段约化散射系数与漫反射光谱斜率绝对值的关系式,用模型对公式的计算精度进行验证,利用该系统实时在位测量了20只大鼠脑组织在700nm、750nm和800nm处的约化散射系数。经统计分析,大鼠脑白质约化散射系数在上述波长处分别为30±5cm-1、28±5cm-1和25±6cm-1,脑灰质约化散射系数则分别为14±3cm-1、13±3cm-1和12±3cm-1。脑组织约化散射系数实时在位测量对脑外科微创手术中的组织识别具有重要的参考意义。     

Onset of 3D collective surface diffusion in the presence of lateral interactions: Na/Cu(001)

3He spin-echo measurements are used to follow the picosecond motion of sodium atoms on a copper (001) substrate. 2D correlated motion arising from repulsive adsorbate interactions is observed for coverages as low as 0.04 ML. At coverages greater than 0.05 ML there is a pronounced onset of motion perpendicular to the surface. The perpendicular motion is thermally activated and seems related to the basic translational hopping diffusion process. The correlated motion is modeled successfully using a molecular dynamics simulation and a dipolelike lateral interaction. A simple model which relates the apparent height of the atom with its local coverage is shown to reproduce the experimental observations.     

Mean field of a low-frequency sound wave propagating in a fluctuating ocean

Statistical characteristics of low-frequency sound waves propagating over long distances in a fluctuating ocean are important for many practical problems. In this paper, using the theory of multiple scattering, the mean field of a low-frequency sound wave was analytically calculated. In these calculations, the ratio of the sound wavelength and the scale of random inhomogeneities can be arbitrary. Furthermore, the correlation function of inhomogeneities is expressed in terms of a modal spectrum (e.g., internal waves modes). The obtained mean sound field is expressed as a sum of normal modes that attenuate exponentially. It is shown that the extinction coefficients of the modes are linearly related to the spectrum of random inhomogeneities in the ocean. Measurements of the extinction coefficients can therefore be used for retrieving this spectrum. The mean sound field is calculated for both 3D and 2D geometries of sound propagation. The results obtained can be used to study the range of applicability of the 2D propagation model.     

薄膜超构表面声扩散体扩散性能研究*

针对传统的声扩散体结构复杂、加工不便的缺点,该文提出了一种薄膜型声学超材料扩散体,通过有限元软件建立了扩散体的有限元模型。分析了薄膜张力、质量块大小及背腔深度对声波相位调控的影响,并与传统施罗德扩散体的扩散性能进行比较,发现调控后的薄膜超构表面声扩散体对声能的吸收降低,并使入射声能更均匀地向全方向扩散。在相同的尺寸情况下,与传统施罗德扩散体相比,薄膜超构表面声扩散体在所需频段内扩散性能更优,结构更优,体积更小,质量更轻,加工更方便,更适用于工程应用。     

三维空间域多普勒功率谱及其多天线系统性能

在三维(3 dimensional,3D)空间域信道建模中,针对波达信号仰角(elevation angle,EA)在不同覆盖区散射体环境中的分布,提出了指数型EA仰角概率密度函数并对其信道特征实现建模.在假设波达信号方位谱为均匀分布时,采用近似算法在对称和非对称两种情况下导出其多普勒功率谱密度(power spectral density,PSD)闭合表达式.从分析结果可发现PSD与仰角函数式及波达信号边界角度参数βmin和βmax关系密切,并与多普勒频移密切相关.对该三维空间域中的多输入多输出(multiple input multiple output,MIMO)多天线信号衰落相关性(spatial fading correlation,SFC)进行了推导和仿真.结果表明MIMO多天线阵元间SFC与βmin和βmax关系密切,而EA仰角概率密度函数参数n对SFC影响较小.本文引入的指数型EA仰角概率密度函数可应用于多种无线通信环境下的信道参数估计,且与传统模型对比表明本模型的信道参数估计结果符合理论和经验值,拓展了对3D空间域统计信道的建模与MIMO多天线分析计算.     

阶梯状声扩散体形体优化方法

应用时域有限差分法计算阶梯状声扩散体扩散系数,结合免疫遗传算法分别对周期排布的六阶和十二阶阶梯状声扩散体的形体进行优化。首先,验证了时域有限差分法计算声场的有效性,之后采用两种方式进行优化设计:第一种是直接对多周期重复排布的扩散体形体进行优化;第二种首先对单周期扩散体形体进行优化,再将该扩散体重复周期排布。结果显示:第一种优化方式能获得扩散性能更优的周期排布的扩散体。单周期扩散性能最优的扩散体重复一定周期后,其性能不会保持最优,相反,扩散性能并非最优的单周期扩散体重复一定周期后的扩散性能却优于前者;扩散体总长度一定条件下,增加扩散体的阶次、降低重复周期数,可有效提升扩散体的扩散性能。因此,直接对多周期重复排布的高阶次扩散体形体进行优化,能够获得扩散性能最优的周期重复排布的扩散体。      

A simplified method to determine the coverage dependence of surface diffusion coefficients

A new method to determine the coverage dependence of surface diffusion coefficients from coverage-gradient experiments is presented. The initial coverage-gradient employed is an evacuated circular region surrounded by a constant coverage. In order to demonstrate this method, Fick's second law equation was numerically solved using coverage-dependent surface diffusion coefficients. An analysis of the results demonstrates that the coverage dependence of the surface diffusion coefficient can be determined using a simple extrapolation procedure. The coverage dependence can be obtained for a wide range of functional forms for the surface diffusion coefficient. The extrapolation procedure requires only that the number of adsorbates that have diffused back into the initially evacuated region be measured as a function of time. This method should be useful in the interpretation of coverage-gradient surface diffusion experiments, especially those performed using laser-induced thermal desorption techniques.     

Volumetric diffusers: pseudorandom cylinder arrays on a periodic lattice

Most conventional diffusers take the form of a surface based treatment, and as a result can only operate in hemispherical space. Placing a diffuser in the volume of a room might provide greater efficiency by allowing scattering into the whole space. A periodic cylinder array (or sonic crystal) produces periodicity lobes and uneven scattering. Introducing defects into an array, by removing or varying the size of some of the cylinders, can enhance their diffusing abilities. This paper applies number theoretic concepts to create cylinder arrays that have more even scattering. Predictions using a boundary element method are compared to measurements to verify the model, and suitable metrics are adopted to evaluate performance. Arrangements with good aperiodic autocorrelation properties tend to produce the best results. At low frequency power is controlled by object size and at high frequency diffusion is dominated by lattice spacing and structural similarity. Consequently the operational bandwidth is rather small. By using sparse arrays and varying cylinder sizes, a wider bandwidth can be achieved.     

Measurements of restricted diffusion using an oscillating gradient spin-echo sequence

An oscillating gradient spin-echo (OGSE) pulse sequence was used to measure the apparent diffusion coefficient (D(app)) of water in the short diffusion time regime in the presence of restrictions. The diffusion coefficients of water in a simple water sample and a water and oil mixture were measured to be the same for different periods of the gradient oscillation, as expected when there are no restriction effects. The D(app) of water in the spaces between closely packed beads was also measured as a function of the gradient oscillation periods in the range 11 to 80 ms. The D(app) of water in restricted systems varies with the period of the gradient oscillation and the dispersion depends on the scale of the restriction. For a sample of packed beads of diameter 9.1 +/- 0.7 microm, the pore surface-to-volume ratio was estimated experimentally by this method to be 1.3 +/- 0.1 microm(-1), corresponding to a mean pore diameter of 6.4 +/- 0.7 microm. A Monte Carlo computer simulation of the NMR OGSE signal from the spins diffusing in a system of compartments was also implemented and the D(app) demonstrated similar behavior with gradient oscillation periods.     

Self- and mutual-diffusion coefficients measurements by 31P NMR 1D profiling and PFG-SE in dextran gels

31P NMR 1D profiling was successfully introduced to measure macroscale mutual-diffusion coefficients (D(m)) of phosphate ions in dextran gels. Series of 1D profiles describing the phosphate concentration along cylindrical dextran gels were acquired at different times. These profiles that included over 600 points could be fitted using equations derived from Fick's law, with D(m) as the single fitting parameter. Release and penetration profiles were recorded providing two alternative approaches for allowing the determination of D(m). The D(m) values were compared with microscale self-diffusion coefficients (D(s)) measured by pulsed field gradient spin echo (PFG-SE) technique. D(m) values, measured between 25 and 45 degrees C, were systematically lower than D(s). The experimental diffusion time and the associated diffusion length of D(s) (60 ms, 10 microm) are short compared to those of D(m) (up to 18 h, 50 mm). These scale differences are considered to be the origin of different D(s) and D(m) and provide information relative to the network in these gels.     

Exact Stationary States of a Two-Dimensional Transport Model

Previous calculations of transport coefficients of particle systems in two or more dimensions have used computer simulations or various approximations, like those implicit in the Boltzmann equation or in the standard probability theory of diffusion. Here an exactly solved model of crystal growth in three dimensions is transformed into a 2D model of steady particle transport. Then the exact, many-body stationary distribution and exact transport coefficients can be found as well as correlation functions. The model is highly asymmetric. In the transverse direction, particles are strongly attracted, so that long transverse rows of particles tend to form at low temperature. Kinks in these rows are rare, so the transverse positions of kinks effectively move continuously on a large distance scale.     

Dependence of temporal diffusion spectra on microstructural properties of biological tissues

The apparent diffusion coefficient (ADC) measured using magnetic resonance imaging methods provides information on microstructural properties of biological tissues, and thus has found applications as a useful biomarker for assessing changes such as those that occur in ischemic stroke and cancer. Conventional pulsed gradient spin echo methods are in widespread use and provide information on, for example, variations in cell density. The oscillating gradient spin echo (OGSE) method has the additional ability to probe diffusion behaviors more readily at short diffusion times, and the temporal diffusion spectrum obtained by the OGSE method provides a unique tool for characterizing tissues over different length scales, including structural features of intracellular spaces. It has previously been reported that several tissue properties can affect ADC measurements significantly, and the precise biophysical mechanisms that account for ADC changes in different situations are still unclear. Those factors may vary in importance depending on the time and length scale over which measurements are made. In the present work, a comprehensive numerical simulation is used to investigate the dependence of the temporal diffusion spectra measured by OGSE methods on different microstructural properties of biological tissues, including cell size, cell membrane permeability, intracellular volume fraction, intranucleus and intracytoplasm diffusion coefficients, nuclear size and T₂ relaxation times. Some unique characteristics of the OGSE method at relatively high frequencies are revealed. The results presented in the paper offer a framework for better understanding possible causes of diffusion changes and may be useful to assist the interpretation of diffusion data from OGSE measurements.