Localizing impulse sources in an open space by time reversal with very few transducers

Localizing impulse point sources is a problem of major practical importance for numerous applications in security or equipment monitoring. It is difficult to solve when posed in a strongly congested propagation medium. This paper concerns the case where, in an open space, obstructing bodies are of a sufficient size and number to impede reception of certain direct paths from the source to the receivers. They produce reflected or diffracted paths. A low number of point receivers is used, 2-5, depending on the case. This fits practical constraints one meets in the field. The localization principle being time reversal, the aim is therefore to model the time reversed signal propagation from the receivers. From a direct signal obtained from measurements or computer simulations, the reversed propagation computation is made in the frequency domain or in the time domain. Despite the low number of receivers, which we would expect not to give good refocusing of the reversed wave, we are able in each case to localize the source with a conveniently chosen criterion, based on the time shortness of the signal. The advantage of this technique is its simplicity and speed: in a time formulation, a unique computation allows the localization. This result opens the way to economical measurement techniques for localizing impulse sources in congested or disturbed media, as long as a propagation model allowing integration of refraction, diffraction and reflection effects is available.     

Prediction and Limitations of Noise Maps Developed for Heterogeneous Urban Road Traffic Condition: A Case Study of Surat City,India

Road traffic noise pollution has been recognized as a serious issue which affects human health as well as affects urban regions. Noise maps are very beneficial to identify the impact of noise pollution. A noise mapping study performed to study the propagation of noise in tier-II city along with field measurements. The noise maps are developed using a computer simulation model (SoundPLAN essential 4.0 software). The noise prediction models like U.K’s CoRTN, Germany’s RLS-90, and their modified versions, which can be used for homogenous road traffic conditions, cannot be successfully applied in heterogeneous road traffic conditions of India. In developing country like India, road traffic noise pollution depends on the composition of heterogeneous traffic volume, variance in road geometrical, honking conditions, un-authorized parking, and varying density of the building on either side of the road. These traffic compositions contain vehicles, which have different sizes, speeds variations, a different dimension of vehicles. Because of fluctuating speeds, lack of lane disciplines, and un-authorized parking on main road lanes, honking events becomes inevitable, which changes and affects the urban soundscape of nations like India. Analysis of noise maps showed that horn honking due to un-authorized parked vehicles contributed an additional increase up to 11 dB (A) noise, which is quite significant.     

矩形表面波探头声场的高斯声束叠加法

利用矩形压电晶片和有机玻璃楔块折射可激励出超声表面波,广泛用于固体近表面缺陷检测和材料特性测量.由于描述表面波三维声场的理论方法还鲜有报道,因而主要采用简化的表面波二维声场模型来定量分析这类问题.高斯声束模型近些年被广泛应用于解决超声体波传播的各种复杂问题,然而,目前还没有将其扩展应用到超声表面波的声场的计算中.通过结合表面波格林方程和矩形换能器的高斯声束模型,推导出基于高斯声束叠加的表面波三维声场解析解.进一步,将该方法与点源叠加的数值解进行了分析比较,计算结果表明表面波声场的高斯声束叠加方法在具有较好计算精度的同时,还具有更快的计算效率.     

The influence of turbulence on noise propagation from a point source above a flat ground surface

The presence of turbulence in the atmosphere affects the interaction between an acoustic wave and the ground surface. The noise attenuation by the ground in the presence of atmospheric turbulence is smaller than in non-turbulent atmosphere.A simple engineering model of noise propagation above a flat ground surface, for stationary and moving point sources, has been proposed. The model takes into account the air absorption and ground effect in the presence of turbulence.As well as parameters for type of ground and air absorption, the model introduces two adjustable parameters which must be deduced from in situ measurements at two ranges or two heights. The model’s free parameters have been obtained as a function of the resultant sound speed gradient on the basis of the field measurements performed for a stationary noise source. Also, using field data for a vehicle moving at steady speeds up to 100 km/h, the model has been verified for a moving point source.     

抛物方程的多重非均匀网格模型及其应用

提出了抛物方程的多重非均匀网格模型,以准确求解三维空间存在多辐射源的电波传播问题。通过对不同辐射源建立不同的坐标系,并对其仿真空间采用不同的非均匀网格划分,构建了抛物方程的多重非均匀网格模型。在此基础上,实现了三维多辐射源问题的并行计算。实例仿真了空间存在四个辐射源的电波传播特性。结果表明,抛物方程的多重非均匀网格模型能够准确求解多源的空间电磁场分布特性,且在该算例中,并行技术使得抛物方程的计算速度提升了2.41倍,极大地提高了抛物方程对三维多源问题的求解效率。     

基于柱坐标系抛物方程和矩量法的电波传播混合算法

针对包含近源障碍物条件下的电波传播问题,提出了一种新颖的电波传播预测混合建模方法:矩量法(MOM)和圆柱坐标系抛物方程法(PEM)混合建模方法(MOM-PEM);MOM用于包含辐射源和近源障碍物的小圆柱区域内的电波传播建模,PEM用于MOM计算空间外的大区域范围内电波传播建模。MOM和PEM的计算过渡区域进行精细化网格剖分处理以避免场强数值传递的不兼容。仿真模拟了三类近源障碍物存在场景下的电波传播问题:有限开窗屏障碍物、立方体障碍物以及包含辐射源的半封闭空间障碍物,并将混合算法计算得到的结果和相同环境下采用全矩量法计算得到的结果进行了数值对比,结果表明混合算法和矩量法在精度上吻合较好。     

LED光学传播的有限元研究

有限时域差分(FDTD)广泛应用于提高发光二极管(LED)芯片外量子效率.这种方法是针对波导结构的一种分析计算,对于激光器是有效的,但是对LED很难模拟其有源层多种模式出射的情况.应用有限元电磁场分析方法对LED进行光学建模并对光子晶体结构的外量子效率进行了计算.特别在对光源的处理上,使用了点光源球面波来进行分析并且考...     

NUMERICAL MODELLING OF MEDIAN ROAD TRAFFIC NOISE BARRIERS

Outdoor sound propagation from road traffic is modelled by solving a boundary integral equation formulation of the wave equation using boundary element techniques in two dimensions. In the first model, the source representing a traffic stream can be considered as a coherent line source of sound. The results can then be transformed to derive a pseudo-three dimensional solution to the problem. In the second model the line source is incoherent. For receivers near the ground, the second model predicted significantly higher values of ground attenuation than the first. The first model generally produced better agreement with ground attenuation results obtained using the U.K. traffic noise prediction model. For conditions when a noise barrier was present and the ground was absorbent, the incoherent line source model generally predicted significantly higher values of attenuation than those from the barrier and ground attenuation calculated separately. Over a range of receiver positions and barrier heights a similar, but less marked effect was observed when the coherent line source model was used. On dual carriageway roads, it is possible to incorporate barriers on the central reservation as a noise control measure. These are “median” noise barriers. The incoherent line source model is used to assess the performance of median barriers in reducing noise when installed alone and also with associated roadside barriers. A sound absorbent median noise barrier 1m in height produced consistent values of insertion loss of between 1 and 2dB over the range of receiver positions and ground conditions considered. When the median barrier was used in conjunction with a roadside barrier it produced a consistent improvement in insertion loss of between 1 and 2 dB over the range of conditions considered.     

Promising source of high-power broadband microwave pulses with radiation frequency variable up to two octaves

Using a numerical model, we propose a source of microwave pulses, based on the interaction of a high-current relativistic electron beam with plasma. The source is an intrinsic noise amplifier with a pulse duration shorter than 3 ns, which does not permit the emergence of feedback and self-excited generation. The wave gain factor depending on the plasma concentration makes it possible to control the radiation frequency in the range 4–17 GHz, within which a spectral width of ～2 GHz, a power of ～150 MW, and an energy efficiency up to 15% are preserved. The possibility of using the available small-size source of high-voltage pulses with a high repetition rate is considered.     

A beam tracing method for interactive architectural acoustics

A difficult challenge in geometrical acoustic modeling is computing propagation paths from sound sources to receivers fast enough for interactive applications. This paper describes a beam tracing method that enables interactive updates of propagation paths from a stationary source to a moving receiver in large building interiors. During a precomputation phase, convex polyhedral beams traced from the location of each sound source are stored in a "beam tree" representing the regions of space reachable by potential sequences of transmissions, diffractions, and specular reflections at surfaces of a 3D polygonal model. Then, during an interactive phase, the precomputed beam tree(s) are used to generate propagation paths from the source(s) to any receiver location at interactive rates. The key features of this beam tracing method are (1) it scales to support large building environments, (2) it models propagation due to edge diffraction, (3) it finds all propagation paths up to a given termination criterion without exhaustive search or risk of under-sampling, and (4) it updates propagation paths at interactive rates. The method has been demonstrated to work effectively in interactive acoustic design and virtual walkthrough applications.     

Application of simulation program to specific urban situation

In the paper, the simulation program predicting the sound level has been applied to one of buildings neighbouring a road. In the simulation are considered: geometry of road surroundings, the shape of the investigated building façade including, road geometry (number of lanes and their positions) and traffic structure (vehicle flow rates and their average speeds). Adjustable accuracy in an urban system modelling allows finding the model which is adequate to the real system. For the model of the accuracy limited by the simulation error due to vehicle speed estimation, by comparison with the field measurement, the model validation range has been established. Based on the simulation adjustable accuracy, possibilities of the validation range extension have been analysed.     

Study of Rayleigh type surface wave in the initial stressed irregular bottom of ocean due to point source

Rayleigh type surface wave propagation in the irregular bottom of ocean model which is the interface of homogeneous liquid layer over laying an irregular boundary of homogeneous orthotropic half space under initial stresses has been discussed in this paper. Three different dispersion equations are obtained in the form of simple equation using and not using Perturbation technique. Some special cases have been considered. The effect of irregularity, initial stressed, point source, and depth of liquid layer on the propagation of Rayleigh waves has been analyzed and results of numerical discussion have been presented graphically for three different dispersion equations. Mainly the graphs are shown the variation of phase velocity with wave number in different cases.     

Analysis of one-dimensional wave scattering by the random walk method

The random walk method is applied to a one-dimensional Helmholtz equation with a variable wave number. The solution is represented as a mathematical expectation of a specified functional on paths in a complex space. This solution degenerates to the ray-method approximation in domains where the latter method may be used, but the probabilistic formulas presented describe also backscattered waves whose existence is not explained by the standard asymptotic techniques. The numerical results confirm the efficiency of the random walk approach to the analysis of wave propagation.     

Modelling traffic noise mathematically

In this paper we describe a mathematical model for the prediction of traffic noise levels in an urban or suburban situation. The model is ultimately intended to provide an alternative to existing methods of prediction. At the present time, only noise levels produced by stationary sound sources have been considered.Any point in a chosen area is described by its grid co-ordinates. A detailed plan of the buildings or other structures in the area and the position(s) of the sound source(s) are needed as input to the model. Noise levels at all grid positions in the area are then calculated on the basis of the attenuation of sound due to direct propagation, diffraction and reflection.The results obtained so far are given and since the model is in an early stage of development, and has yet to be proved against measurements in real situations, possible refinements and future developments are discussed in some detail.     

Influence of vehicle noise emission directivity on sound level distribution in a canyon street. Part I: Simulation program test

Simulation programs may be useful tools for controlling an environmental noise. The computer simulation program PROP11 that enables predictions of the time-average sound level within an urban system is used here. A roadway as a noise source is represented by a sum of the sound exposures due to individual vehicle pass-bys. Different representations of equivalent point sources for various classes of vehicles are allowed including directivity characteristics other than omnidirectional. Propagation throughout an urban system contains multi-reflections from the walls and single and double diffraction at their edges. In this paper, the PROP11 program is used to predict the sound level between opposing façades of buildings in a canyon street. A multi-lane road is assumed to contain two classes of vehicles (light and heavy) ones. The equivalent source representing vehicles is defined by the source power spectrum, its position above the ground and directivity characteristics. The consequences of introducing equivalent source directivity are analyzed.     

海洋波导中目标声辐射场的计算方法

针对海洋波导中目标声辐射场的计算问题,提出了一种基于波叠加法并在近、远场采用不同水声传播模型的建模方法,可以将近、远场作为一个统一的系统进行高效地分析。该方法通过给定的已知表面振动速度的结构计算出目标内部虚拟点声源的源强,再配合在相应的水声环境中点声源传播模型的Green函数计算出结构外的声辐射场。以Green函数为纽带,在求源强和计算近场声辐射场时采用镜像虚源法,而在计算远场声辐射场时采用简正波法。通过该方法得到的有限水深波导中声速剖面为正梯度、负梯度、负跃层的3种情况下的脉动球、刚体摆动球的声辐射场计算结果与COMSOL的有限元计算结果进行对比,结果表明了该方法在提高计算效率的同时保证了计算精度。      

HF ground wave propagation over a curved rough sea surface in the presence of islands

For a vertically polarized line source, in the context of HF (3–30?MHz) ground wave propagation over a curved rough sea surface in the presence of islands, this paper adapts the FB-SA (Forward–Backward Spectral Acceleration) method to compute the attenuation function over sea–land (island)–sea mixed paths for different shapes and heights of the islands. The rigorous FB-SA numerical method is based on the method of moments and was originally developed for scattering from rough surfaces and is especially efficient in solving a huge problem, which is required to predict the ground wave propagation over a long surface. In addition, for zero island height, this method is compared with an analytical approach expressed from a (residue) series, in which the roots of a differential equation, depending on the Airy function, must be calculated. In addition, from an intuitive approach and from the work of Barrick and Ishimaru, this analytical approach is extended to include the sea roughness and then validated from the FB-SA.     

Sound focusing in rooms: the time-reversal approach

New perspectives in audible range acoustics, such as virtual sound space creation and active noise control, rely on the ability of the rendering system to recreate precisely a desired sound field. This ability to control sound in a given volume of a room is directly linked to the capacity to focus acoustical energy both in space and time. However, sound focusing in rooms remains a complicated problem, essentially because of the multiple reflections on obstacles and walls occurring during propagation. In this paper, the technique of time-reversal focusing, well known in ultrasound, is experimentally applied to audible range acoustics. Compared to classical focusing techniques such as delay law focusing, time reversal appears to considerably improve quality of both temporal and spatial focusing. This so-called super-resolution phenomenon is due to the ability of time reversal to take into account all of the different sound paths between the emitting antenna and the focal point, thus creating an adaptive spatial and temporal matched filter for the considered propagation medium. Experiments emphasize the strong robustness of time-reversal focusing towards small modifications in the medium, such as people in motion or temperature variations. Sound focusing through walls using the time-reversal approach is also experimentally demonstrated.