Combined wave and ray based room acoustic simulations of audio systems in car passenger compartments,Part II: Comparison of simulations and measurements

The present series of papers summarizes the results of a three-year research project on the realistic simulation of sound fields in car passenger compartments using a combined Finite Element (FE) and Geometrical Acoustics (GA) approach. The simulations are conducted for the whole audible frequency range with the loudspeakers of the car audio system as the sound sources. The challenges faced during the project relate to fundamental questions regarding the realistic sound field simulation in small enclosures with strong modal and diffraction effects. While Part I of this series of papers focusses on the determination of the boundary and source conditions for the simulation model of the car compartment, the present paper, denoted here as Part II, presents extensive objective and subjective comparisons of the corresponding room acoustic measurement and simulation results.By applying the FE method to the low frequency part of the room transfer function (RTF) the study aims at the quantification of potential objective and subjective benefits with regard to the simulation quality in small rooms, when compared to a purely geometrical acoustics approach. The main challenges and limitations in the simulation domain are due to the very small volume, the difficult to determine source and boundary conditions and the considerable diffraction effects (especially at the seats) in the car passenger compartments. In order to keep the complexity of the FE simulations at a manageable level, all boundary conditions were described by acoustic surface impedances and no fluid-structural coupling was considered in the FE simulation model.While the results of the study reveal that an overall good agreement regarding the energy distribution in time and frequency domain is generally possible even in such complex enclosures, the results also clearly show the limitations of the impedance boundary approach in the FE domain as well as the strong sensitivity of the simulation results with regard to the uncertainty in the boundary and source conditions in both simulation domains. It can thus be concluded, that possible fields of application of the FE extension in room acoustic simulations lie in the prediction of the modally dominated low frequency part of the RTF of well defined rooms and in the prediction of sound fields that are strongly affected by near-field or diffraction effects as in the car passenger compartment. However, due to the considerable problems in the determination of realistic boundary conditions for the FE model, improved measurement techniques are urgently needed to further improve the overall simulation quality.     

An analytical model for a dodecahedron loudspeaker applied to the design of omni-directional loudspeaker arrays

The dodecahedron (12 sided) loudspeaker is used extensively for acoustic measurements as an approximation to an omni-directional sound source, for example in building acoustics and automotive applications. In many instances its use is favoured above other omni-directional source approximations e.g. open pipe because of its greater power. However, analysis by other authors has shown that the radiation deviates from omni-directional above a certain frequency. In this paper an analytical model of the sound pressure field generated by a dodecahedron loudspeaker is constructed. The sound pressure field generated by the model is then compared with measured data and good agreement is demonstrated. Having established and validated the model, it is adapted to form alternative spherical loudspeaker arrays based on Platonic and Archimedean solid geometry and also a number of spherical packing routines. The aim of the analysis is to determine the optimal geometry for an omni-directional source using pistonic loudspeakers.The analysis shows that constructive interference of the pressure field across the spherical baffle surface and not individual loudspeaker pistonic radiation characteristics is the most significant factor with respect to deviations from omni-directional radiation for a complete spherical array. Consequently, loudspeaker elements which physically occupy the spherical baffle surface to their maximum extent provide the most extended frequency range of omni-directional operation. The most optimal spherical array distribution is shown to be the dodecahedron.     

不确定海洋声场中的检测性能损失环境敏感度度量

现有的检测算法在实际的不确定海洋环境中会出现失配情况, 进而导致检测性能下降, 但是定量分析这种检测性能损失的工作却很少见, 因此本文定义了检测性能损失敏感度函数, 并给出了基于蒙特卡罗方法的计算方法.检测性能损失敏感度是一个表征海洋环境不确定度的物理量, 它反映了海洋参数变动和检测性能损失之间的关系.利用地中海某处海洋环境进行仿真, 针对各个海洋环境参数, 计算了全海域的检测性能敏感度. 结果表明: 1)检测性能损失呈现了很强的空间性, 在20 km处, 水面声速有4 m/s的变化下, 检测损失最小为1%, 而最大达到60%.声信道中的检测性能较为稳定, 其在远距离上更是如此; 2)各个海洋环境参数对检测性能损失有不同的影响, 水体声速剖面和第一层海底介质的声速和厚度是对检测性能影响最大的量; 3)环境参数敏感度呈现很强的频率特性, 海底底质参数包括底质厚度、密度和吸收系数等随着频率的升高对检测性能的损失影响变小. 关键词： 检测性能损失 环境敏感度 不确定海洋环境     

C波段机载合成孔径雷达海面风场反演新方法

针对基于散射计地球物理模型函数的机载合成孔径雷达(SAR)海面风场反演中存在的风向获取依赖于图像风条纹或数值预报、 散射计数据和浮标等背景场资料, 风向与SAR图像时空分辨率不匹配, 进而影响机载SAR海面风场反演精度等问题, 本文根据机载SAR对海探测特点, 研究一种适用于C波段机载SAR的海面风场反演新方法. 利用SAR图像距离向不同入射角的后向散射系数, 依据地球物理模型构造最小代价函数, 通过代价函数的求解直接从机载SAR数据同时反演出海面风速和风向. 利用论文提出的海面风场反演方法分别对仿真SAR数据和实测C波段机载SAR数据进行风向、 风速的反演误差分析及试验验证.研究结果表明, 该方法适用于机载SAR海面风场反演, 可不依赖背景风向直接反演出精度较高的风速和风向; 雷达后向散射系数误差是决定海面风速、风向反演精度的关键因素, 辐射定标精度越高则反演误差越小; 海面风速反演误差随着风速的提高而增大, 当海面风速大于18 m/s时, 风速反演误差显著增加, 而海面风向的反演误差与风速无明显关系. 关键词： 机载合成孔径雷达 海面风场 多入射角     

声纳信号宽容性检测指数的初步分析

自上世纪六十年代以来,信号的宽容性(Robust,Robustness)处理就开始在统计数学与信号处理领域受到关注。由于声纳使用的水声环境的特殊复杂性,从事声纳信号处理和声纳设计的学者对环境适应性处理和宽容性水声信号处理开展了大量的研究。主要的目的是探索水声环境效应以及对声纳信号处理的影响,企图寻求一种适应环境的宽容性信号处理方法。本文初步探讨水声信号处理领域宽容性检测的基本概念,给出一种度量宽容性性能的数量指标。同时推导在不同声速剖面下声纳检测宽容性指标和所使用频率的关系。对文中所提出的理论方法与国内外实际海试的某些实例进行了分析比较。     

海洋表面波的3-4-5波共振守恒理论

面对海洋表面完整的两大波要素–-张力波和重力波, 构建出一个确定、丰富、基本的有限水深海洋表面波的“3-4-5波共振守恒理论”. 与以往经典、现代的多种结果相比, 充分保证了该理论的“精确性、对称性、完备性”, 为后继、普适的海洋波湍流统计理论提供了一个必备基础. 关键词： 海洋表面波 Hamilton描述 3-4-5波共振 波湍流     

Transmutation of non-local boundary conditions in ocean acoustics

We employ a transmutation technique to construct a new non-local boundary condition for the paraxial approximation in ocean acoustics. The transmutation operator introduced by De Santo and Polyanskii, when applied to the Helmholtz equation governing the acoustic pressure in the water column, leads to the so-called parabolic equation of Fock and Tappert. This transmutation operator acting on the N-D map at the water–bottom interface yields an intermediate non-local boundary condition for the parabolic equation which eliminates the backscattering terms in the Schwartz kernel of the N-D map. The kernel of the intermediate condition is approximated by a uniform stationary phase formula taking account of the possible coalescence of the brach points of the integrand with the stationary points of the phase, and it leads to a non-local boundary condition of Volterra-type for the parabolic equation. This condition is quite different than similar conditions derived by other approximations, in that the kernel of the Volterra operator is smooth, the smoothing effect coming from the fact that the horizontal range coordinate is scaled with the relative refraction index between the water column and the bottom.     

On heat and moisture exchanges between the sea surface and the atmosphere during the Medalpex

Summary Data collected by a buoy, moored in the Ligurian Sea about 27 nautical miles off the coast during the period 1 March-31 May, 1982, are analysed. The buoy was equipped by the Institute for Naval Automation (IAN) of the Italian National Research Council (CNR) during the Mediterranean Alpine Experiment (Medalpex), joint program of the Alpine Experiment (Alpex). Exchanges of heat and mass across the air-sea interface are computed from the collected data and comparisons with existing values are made. The resulting agreement confirms the strong interaction between the sea and the atmosphere in some peculiar situations, and lends weight to the oceanographic hypothesis for the statistical occurrence of deepening of orographic cyclones in the Liguro-Provencal basin. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome.     

All-optical adaptive scanning acoustic microscope

We have constructed a fast laser-based surface acoustic wave (SAW) microscope, which may be thought of as a non-perturbing scanning acoustic microscope. The instrument is capable of rapid high resolution vector contrast imaging at several discrete frequencies, without any damage to the sample. Tailoring the generating optical distribution using computer-generated holograms allows us to both focus the acoustic waves (increasing their amplitude) and to spread the optical power over the sample surface (preventing damage). Accurate quantitative amplitude and phase (velocity) measurements and unique acoustic contrast mechanisms are possible with our instrument based on this technology due to the non-perturbing nature and the instrument geometries.However, the complexity of the optical generation profile leads to a strong dependence on material properties such as the SAW velocity and material anisotropy. We address these issues in this paper, and demonstrate how a spatial light modulator may be used to adapt the generating optical distribution to compensate for the material properties. This facilitates simpler alignment and velocity matching, and, combined with an acoustic wavefront sensor, will allow real-time adjustment of the generating source to enable imaging on anisotropic materials.     

Ripple damping on water surface covered by a spreading film: Theory and experiment

Summary The results of point-by-point laboratory dampingvs. frequency measurements using mechanically generated ripples on a water surface covered by monomolecular film of oleic alcohol are discussed in terms of a theory that takes into account coupling of Laplace and Marangoni modes.

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Riassunto Una teoria delle onde gravito-capillari alla superficie di acqua ricoperta da un film monomolecolare di spandimento, che tien conto dei modi di Laplace e di Marangoni, e del loro accoppiamento, è usata per guistificare delle misure di smorzamento ondoso effettuate in vasca con film di alcool oleico.

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Резюме В рамках теории, которая учитывает связь мод Лапласа в Марангони, обсуждаются результаты лабораторных измерений затухания в зависимотси от частоты. При измерения использовалась механически возбужденная рябь на поверхности воды, покрытой мономолекулярной пленкой олеинового спирта.