航空发动机噪声在大气中衰减算法分析与软件设计

在准确测量航空发动机噪声时必须考虑其在大气传播过程中的衰减特性,大气声吸收系数是衡量衰减特性的重要指标,本文通过纯音法对大气声吸收系数进行计算,并编制专用软件分析了相同条件下相对湿度与大气声吸收系数的关系。结果表明,相同大气温度下大气声吸收系数与频率直接相关,相同频率下大气温度高于20℃,大气声吸收系数则随着相对湿度的变化呈现出平稳趋势。即使相对湿度不同,频率与大气声吸收系数的变化趋势也是一致的。     

典型浅海地声模型下的反射波特性初步研究

本文给出了一种典型浅海地声模型下，波动方程有限差分正演的计算方法，并详细分析了沉积层的顶层速度，厚度，速度梯度，密度梯度，以及震源特性等参数变化时反射波受其影响的一般规律；而且对于航 道底部的浮层作了重点研究，从而得到了浮泥层的容重变化率及其厚度对反射波的影响规律，以及浮泥厚度与反射波特征谱之间的关系。     

几何绕射理论下楔形障碍物衍射

应用几何绕射理论，研究不同角度和不同频率情况下楔形障碍物衍射衰减规律，并结合90°楔形障碍物现实应用，模拟楔形建筑物下声音传播衰减。得出了楔形障碍物插入损失与楔形角度和声波频率的关系，结果与Maekawa实测值比较并进行精度分析。针对现实场景下楔形建筑物的遮挡，应用空间剖分和声线束追踪生成声音路径，模拟楔形建筑物区域的声音衰减。结果显示声音衰减随楔形角度减小和频率增加而增大，几何绕射理论可应用于大区域建筑物群声传播衰减计算中。     

黄海声传播和混响的异常衰减

Ｗｅｓｔｏｎ等早在６０年代就观察到浅海（英国ＢｒｉｓｔｏｌＣｈａｎｎｅｌ）声传播常会在某一有限频率范围内出现异常大的衰减，异常衰减的中心频率和衰减值随季节和昼夜时间而变化。Ｗｅｓｔｏｎ认为这种浅海声传播异常衰减是由鱼的鱼鳔共振所引起的。周纪浔等在我国黄海夏季强负跃层的爆炸声实验中，也曾观察到声传播有时会在某一频率范围内出现异常大的衰减（当声源和接收器都位于跃层下方时）。并发现传播异常衰减与传播方向有关。周纪浔等提出了一种新的假设，由理论计算结果证明可以用内波孤立波波包对声波的共振散射机理来解释这…     

非周期型理想钻柱系统声传播特性研究

基于一维声传播理论推导出了非周期型理想钻柱系统的频散方程和透射波声衰减方程,数值计算了非周期型钻柱系统几何参量与其频散特性、声衰减特性之间的关系,并利用有限元ANSYS对非周期型理想钻柱系统的声传播特性的结果进行了验证。研究结果表明：非周期型理想钻柱系统的频散特性与多个周期性结构系统频散特性相“与”的结果一致,具有通带较窄而阻带较宽,且频率在阻带范围内的声波沿钻柱传播时声衰减较大的特性。     

梯度高分子溶液的声衰减

聚合物在溶解过程中形成聚合物膜、溶胀层、高浓度溶解层及低浓度溶解层,从而形成梯度高分子溶液,此种梯度高分子溶液对声信号有很好的衰减效果．文中详细研究了梯度高分子溶液在不同频率下,聚合物厚度及梯度结构的变化对反射声衰减效果的影响．结果表明,梯度高分子溶液浓度梯度的形成是声衰减效果好坏的关键。溶液浓度梯度很大或很小时,声衰减效果均不好,只有当浓度梯度适当时,其声衰减效果才最佳。     

货物列车声屏障等效频率的确定及其验证和分析

声屏障设计研究一直是国内外广泛关注的课题,在线路两侧正确地设置声屏障不仅提高噪声控制的主动性,也将降低项目的投资。然而,绕射衰减计算是声屏障设计的重要步骤,声屏障的等效频率又是声屏障绕射衰减计算的重要参数之一。为准确、快捷的进行声屏障设计,需针对不同机车牵引类型,不同时速下货物列车的噪声频率特性进行试验研究,通过对实测数据进行统计分析,由此可得到不同工况下货物列车的声屏障等效频率。     

浑水中兆赫频率的声衰减实验研究

文章报道了浑水中的声衰减的实验研究结果，超声波的频率分别为1、2、3、4、5MHz。实验表明：在同一个含沙量M的浑水中，声衰减系数β大致随频率f的增加而线性增加；对同一个频率f，声衰减系数β大致随含沙量M的增加而线性增加；在同一个频率，和相同含沙量M的情况下，声衰减系数β随泥沙粒径的增加而增加。     

煤粉气-固两相流的超声衰减特性

以颗粒相在整个检测空间内均匀分布为前提,采用McClements理论和Bouguer-Lambert-Beer定律共同描述煤粉气-固两相流中超声衰减特性,建立超声衰减系数与气-固两相流相关参数的理论关系,通过数值模拟,分析超声衰减随着颗粒相体积分数、超声频率、颗粒粒径变化的规律。结果表明,超声频率越高衰减系数越大;声衰减系数随着颗粒相体积分数的增大线性递增;选用某固定频率检测,测得2个体积分数下的声衰减系数即可确定衰减-体积分数曲线斜率,从而实现任意衰减系数对应的颗粒相体积分数测量;在相同的体积分数下,煤粉颗粒粒径为10～200μm,声衰减系数随着颗粒粒径的增大单调递减,当煤粉颗粒粒径大于200μm时,声衰减系数对煤粉颗粒粒径不再敏感。     

局域共振子对手性覆盖层振动和声辐射抑制的影响

为增强某些频率区域内覆盖层对加筋板的声辐射抑制作用,在手性结构覆盖层的圆环内填充软橡胶层包覆的金属核心局域共振子。本文基于有限元方法及Rayleigh积分公式,建立了敷设手性覆盖层加筋板简化梁模型,分析了手性覆盖层的振动抑制特性及局域共振子对手性覆盖层振动的影响。结果表明:手性结构的变形模式和局域共振子与手性结构之间的耦合作用对覆盖层的禁带频率带宽和振动衰减起到关键的作用;手性结构覆盖层对加筋板声辐射抑制作用主要来自于覆盖层的振动衰减能力,声辐射效率的变化对声辐射的影响较小。     

Evaluation of acoustic and dynamic behaviour of ethylene vinyl acetate panels

Ethylene vinyl acetate panels, with high vinyl acetate content and a closed‐cell structure, were studied through various experimental techniques as a first approach to evaluate the vibrational and acoustic behaviour of ethylene vinyl acetate panels for building applications. Test specimens, with a variety of densities and thicknesses were tested to evaluate the influence of these two parameters on acoustic impedance, sound transmission loss, dynamic stiffness and attenuation of vibrations. The results obtained shows sound transmission loss values for frequencies up to 2500 Hz with a maximum of about 63.7 dB, the dynamic stiffness results presented a wide range, with a maximum value of 350 MN/m³ and a minimum value of 23.3 MN/m³.On the other hand the lack of pore in the surface produce a high acoustic impedance. It can be concluded that ethylene vinyl acetate presents appropriate characteristics, mainly as an acoustic and vibration isolating material, for floorings and light partitions.     

Ultrasonic dispersion and attenuation near the liquid-gas critical point of4He

We describe sound velocity and acoustic attenuation measurements at several frequencies between 0.5 and 5 MHz along the critical isochore of ⁴ He immediately above the critical pointT _c. The dispersion is obtained with respect to the velocity measurements by Barmatz taken at 1.8 kHz along the critical isochore. The dispersion and attenuation results are analyzed following the theory of Kawasaki and using the more recent modifications resulting from the Fixman-Mistura approach as described by Garland and co-workers. We have determined the critical exponent for the correlation length divergence to be v=0.66±0.04 and found good internal consistency between certain constants appearing in the theoretical expressions for both sound dispersion and attenuation. These experimentally determined constants are compared with those calculated from other experiments. Deviations in the observed acoustic properties from the predicted ones are discussed. They are similar to those found in acoustic experiments and in light scattering experiments near the critical point of Xe.     

Accurate measurements of the acoustical physical constants of LiNbO (3) and LiTaO(3) single crystals

The acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercial surface acoustic wave (SAW)-grade LiNbO(3) and LiTaO(3) single crystals were determined by measuring the bulk acoustic wave velocities, dielectric constants, and densities of many plate specimens prepared from the ingots. The maximum probable error in each constant was examined by considering the dependence of each constant on the measured acoustic velocities. By comparing the measured values of longitudinal velocities that were not used to determine the constants with the calculated values using the previously mentioned constants, we found that the differences between the measured and calculated values were 1 m/s or less for both LiNbO(3) and LiTaO(3) crystals. These results suggest that the acoustical physical constants determined in this paper can give the values of bulk acoustic wave velocities with four significant digits.     

Electrochemical etching (ECE) of fission fragment tracks in soda glass detectors

Fission fragment tracks of ²⁵²Cf in soda glass detectors have been subjected to electrochemical etching (ECE) using an alternating voltage ranging from 15 kV/cm to 34 kV/cm at frequencies 0.5 kHz to 10.0 kHz for periods of time ranging from 20 min to 12 h.The etchant used is HF of concentrations varying from 1.25 vol.% to 20 vol.% at temperatures from 22°C to 80°C. Etch pit diameter, etch pit length and bulk etch rate are the primary parameters, which have been measured under different conditions of ECE. Unlike plastics, soda glasses do not exhibit any “treeing phenomenon” on electrochemical etching. Various causes responsible for this important difference in behavior of plastics and soda glasses have been analysed.     

Nonlinear optical, piezoelectric, and acoustic properties of SrB4O7

We have measured the second-order nonlinear optical susceptibilities of an SrB₄O₇ crystal and have determined its electromechanical coupling coefficients, piezoelectric charge and voltage constants, and bulk acoustic wave velocities by a resonance method using high-Q resonators. In addition, the dielectric permittivity of the crystal has been measured at 1 kHz.     

Ultrasonic Velocity Studies of Drug Parvon-spas in Mixed Alcohol–Water Solvent Systems at 25°C

Ultrasonic velocities and densities of the drug Parvon-spas in binary mixtures of water with methanol (MeOH), ethanol (EtOH), and propan-1-ol (1-PrOH) have been measured over the complete solvent composition range at 10 mol% intervals at 25°C. Various acoustic parameters such as the acoustic impedance (Z), adiabatic compressibility (β), intermolecular free length (L_f), relative association (R.A.), molar volume (V_m), and molar sound velocity (R_m) have been calculated. In addition, excess functions, i.e., excess adiabatic compressibility (β^E), excess intermolecular free length (L_f^E), excess molar volume (V^E), excess ultrasonic velocity (U^E), and excess acoustic impedance (Z^E) for these three solvent mixtures in the absence and presence of the drug have been calculated. A different behavior of these parameters in these alcohol systems has been discussed in terms of the length of the alcohol molecule, the molecular volume, as well as inter/intramolecular interactions of these molecules.     

Acoustic Emission by Quartz Tuning Forks and Other Oscillating Structures in Cryogenic 4He Fluids

We report on experimental investigations of acoustic emission by quartz tuning forks resonating at frequencies 32 kHz, 38 kHz, 77 kHz and 100 kHz immersed in cold gaseous ⁴He and its normal and superfluid liquid phases. Frequency dependence of the observed low-drive-linewidth at 350 mK together with the temperature and pressure dependences (1.3 K < T < 4.2 K, 0 < p < 25 bar) of the observed damping of the high frequency (77 and 100 kHz) resonators measured in normal liquid ⁴He and its superfluid phase provide strong and direct evidence of the importance of sound emission by these tuning forks. Three analytical models of acoustic emission by vibrating tuning forks are developed and compared with the experimental results. We also discuss the importance of sound emission for experiments with the commonly used 32 kHz tuning forks as well as other oscillating structures??spheres, wires, grids and various micromachined sensors. We compare the relative importance of dissipative losses due to laminar viscous/ballistic drag and acoustic emission in liquid and superfluid ⁴He.     

The effect of the superconducting transition on the acoustic losses in audiofrequency niobium resonators

The mechanical Q of Nb disk samples has been measured as a function of temperature with different annealing treatments. A rapid increase in Q is seen on cooling through the superconducting transition temperature. This is shown to be consistent with the predictions of the theory of acoustic attenuation due to conduction electrons, and the measurements are believed to be the first audiofrequency observation of attenuation by this mechanism. Residual losses in the sample limit the possibility of a quantitative comparison with theory in the superconducting state. Maximum Q values obtained were 1.8 × 10⁷ at 690 Hz, 3.8 × 10⁷ at 1.6 kHz, and 4.8 × 10⁷ at 4.2 kHz. Optimization of the annealing treatment may increase the Q still further, thus enabling improved measurements of the electronic attenuation to be made.Work supported in part by the Australian Research Grants Committe.     

Theoretical study of surface acoustic waves in (n11) GaAs-cuts

The propagation characteristics of true and leaky or pseudo surface acoustic waves (TSAW and PSAW=LSAW) on (n11) GaAs-cuts, n=1, 2, 3 and 4, are theoretically calculated as a function of propagation direction. They include phase velocity (V), electromechanical coupling constant (K(2)), and attenuation factor (alpha) of wave propagation on a metallized surface. The results show that PSAW mode velocities are significantly higher than corresponding TSAW velocities, and for certain propagation directions the attenuation factor is extremely small (10(-5) dB/lambda). Highly coupled PSAW modes exist for propagation directions where the TSAW are very poorly coupled. For certain isolated directions, attenuation of the wave is null (alpha=0), PSAW becoming a non-leaky SAW with partial polarization. And in this case the corresponding TSAW are decoupled from the surface electric excitation. Analysis of relations between various modes (TSAW, PSAW and SSBW, surface skimming bulk wave) is made with the help of the effective surface permittivity function and the generalized slowness diagram. A coupling constant definition different from the usual 2DeltaV/V is used, its validity and application conditions are discussed.