Sound propagation through vegetation

The propagation of sound through a large number of scatterers (i.e., trees) is treated in a similar way to a classical diffusion problem. A general differential equation governing the sound intensity is derived which is valid under certain conditions, notably that the depth of the belt of vegetation is large, and absorption small. The predictions of this theory are compared with results derived from a small scale model study, and with some field measurements. They are also compared with published field data. The implications of some of the conclusions reached for the practical achievement of effective sound attenuation are pointed out. In general, it would appear that significant noise reductions may be achieved for a predominantly high frequency source if the existing ground cover is acoustically hard, or if there is no “ground effect” attenuation between source and receiver for some other reason. In other cases, the noise reduction will be much lower and may be negative.     

A linearized Eulerian sound propagation model for studies of complex meteorological effects

Outdoor sound propagation is significantly affected by the topography (including ground characteristics) and the state of the atmosphere. The atmosphere on its part is also influenced by the topography. A sound propagation model and a flow model based on a numerical integration of the linearized Euler equations have been developed to take these interactions into account. The output of the flow model enables the calculation of the sound propagation in a three-dimensionally inhomogeneous atmosphere. Rigid, partly reflective, or fully absorptive ground can be considered. The linearized Eulerian (LE) sound propagation model has been validated by means of four different scenarios. Calculations of sound fields above rigid and grass-covered ground including a homogeneous atmosphere deviate from analytic solutions by < or = 1 dB in most parts of the computed domain. Calculations of sound propagation including wind and temperature gradients above rigid ground agree well with measured scale model data. Calculations of sound propagation over a screen including ground of finite impedance show little deviations to measured scale model data which are probably caused by an insufficient representation of the complex ground impedance. Further calculations included the effect of wind on shading by a screen. The results agree well with the measured scale model data.     

Noise abatement in plant monocultures and plant communities

Three types of vegetation can be discerned with regard to the acoustic climate investigated in 4 different plantations, 2 plant communities, and a grass field. In the first type of vegetation, i.e. beech and ashtree forests, the excess attenuation was at least 10 dB/100 m with the receiver at the same height as the source, 1·2 m, and at least 5 dB/100 m with the receiver at 3·9 m and even more in most $\text{1}\text{3}$-octave bands studied. In the second type of vegetation, i.e. the mixed poplar forest and the Stellario carpinetum, a so-called ‘sound window’ could be detected around 2 kHz and the ground effect was more extended towards the high frequency range compared with the first type of vegetation. In the third type of vegetation consisting of evergreen sprucefir, best excess attenuation was found in the closed forest: at least 10 dB/100 m with the receiver at 1·2 m, and 7 dB/100 m with the receiver at 3·9 m; in the belts the excess attenuation was at least 7 dB/100 m and 4 dB/100 m respectively. Best attenuation was, therefore, found in the closed forest, and not in the belts. It was concluded that the sound attenuating capacity of planted vegetations can be used to abate noise pollution in town and landscape planning if the plantations are at least 12 m wide. To obtain the best effect the rows of trees have to be planted perpendicular to the direction of the sound field.     

Evaluation of parameters influencing the lightning performance of communication towers by numerical modeling and experimental tests

This paper analyzes the effective parameters on lightning performance. The effects of tower height, breakdown electric field threshold, the ground slope of installation place, and the effect of the trees around the tower are investigated. A 3-D numerical analysis model is proposed to determine the number of direct lightning strokes to antennas. The communication tower, lightning rod, downward descending leader and upward leaders are modeled by different shapes of charges. Finally, a small-scale communication tower was built and tested in a high voltage laboratory. The experimental tests were consistent with the simulation results verifying the merits of the proposed method.     

Air-ground interaction in long range propagation of low frequency sound and vibration—field tests and model verification

An extensive program of intermediate and long range impulsive sound propagation field tests have been conducted. The test program and the performed measurements are presented. Particular focus is given on the air-ground interaction and its effect on low frequency sound and vibration propagation. It is found that the pressure wave interaction with the viscoelastic Rayleigh wave in the ground may have a significant effect on the ground impedance and the sound and vibration propagation. This introduces an important mechanism not covered in commonly used ground impedance models. Numerical simulation models have been developed and verified against the test data. The ground impedance does not only effect the sound pressure propagation. If either acoustically induced ground vibration, or ground to building transmitted vibration, is to be considered, the acousto-seismic impedance has a dramatic effect on the level of ground vibration induced by a given sound pressure. For a site where Rayleigh wave interaction appears at the dominant frequencies of the sound pressure, the ground vibration may be greater than a factor 100 (40 dB) than at a site with ground conditions not making the interaction happen.     

Effects of a multi-layered poro-elastic ground on attenuation of acoustic waves and ground vibration

Ground conditions affect the propagation of outdoor sound and vibration. This paper focuses on the interaction between air pressure and porous ground at low frequencies- where mechanisms other than the rigid porous effects used in locally reacting models may be important. A 2-D analytical model has been developed in this study for the calculation of acoustic and acousto-seismic admittances in a multi-layered poro-elastic ground. The model can be used as a prediction tool both for the ground effect on the sound and the generation of ground vibration by the sound. The modelled acoustic admittance is validated successfully against established rigid frame admittance models over a frequency range of 1 Hz-3 kHz. Moreover, the acousto-seismic impedance is verified against full scale airblast field test data measured during the Norwegian Trials full scale field test program. For certain ground types, the predicted acoustic admittance illustrates a different behaviour compared with predictions from the traditional rigid frame acoustic impedance models. This emphasises the importance of including a deformable frame in the model to obtain realistic results for these conditions.     

On the sound radiation of a rolling tyre

The sound radiation from rolling tyres is still not very well understood. Although details such as horn effect or directivity during rolling have been investigated, it is not clear which vibrational modes of the tyre structure are responsible for the radiated sound power. In this work an advanced tyre model based on Wave Guide Finite Elements is used in connection with a contact model validated in previous work. With these tools the tyre vibrations during rolling on an ISO surface are simulated. Starting from the calculated contact forces in time the amplitudes of the modes excited during rolling are determined as function of frequency. A boundary element model also validated in previous work is applied to predict the sound pressure level on a reference surface around a tyre placed on rigid ground as function of the modal composition of the tyre vibrations. Taking into account different modes when calculating the vibrational field as input into the boundary element calculations, it is possible to identify individual modes or groups of modes of special relevance for the radiated sound power. The results show that mainly low-order modes with relative low amplitudes but high radiation efficiency in the frequency range around 1 kHz are responsible for the radiated sound power at these frequencies, while those modes which are most strongly excited in that frequency range during rolling are irrelevant for the radiated sound power. This fact is very essential when focusing on the design of quieter tyres.     

STUDY OF SOUND PROPAGATION OVER RANGE-DEPENDENT AND TOPOGRAPHIC GROUND BY FINITE DIFFERENCE MODEL

This paper extends an earlier study on sound propagation over poro-elastic layered ground to range-dependent and topographic ground. The model is based on a pressure-velocity finite-difference formulation and is coded in the computer program PORAC. To highlight the influence of range-dependent parameters and topographic features on sound propagation, four cases are considered: (i) a homogeneous poro-elastic half-space (the base case); (ii) the base case with a zone in the ground with a higher permeability; (iii) the base case with a zone of higher stiffness; (iv) the base case with a rectangular hill. The paper presents typical results of sound propagation in these cases comprising synthetic time histories of overpressure in the atmosphere and ground vibration as well as snapshots of the response of the atmosphere-ground system at selected times. Comparative results in these cases serve to highlight the effect of the various non-homogeneities considered in this study.     

Rainforests as concert halls for birds: are reverberations improving sound transmission of long song elements?

In forests reverberations have probably detrimental and beneficial effects on avian communication. They constrain signal discrimination by masking fast repetitive sounds and they improve signal detection by elongating sounds. This ambivalence of reflections for animal signals in forests is similar to the influence of reverberations on speech or music in indoor sound transmission. Since comparisons of sound fields of forests and concert halls have demonstrated that reflections can contribute in both environments a considerable part to the energy of a received sound, it is here assumed that reverberations enforce also birdsong in forests. Song elements have to be long enough to be superimposed by reflections and therefore longer signals should be louder than shorter ones. An analysis of the influence of signal length on pure tones and on song elements of two sympatric rainforest thrush species demonstrates that longer sounds are less attenuated. The results indicate that higher sound pressure level is caused by superimposing reflections. It is suggested that this beneficial effect of reverberations explains interspecific birdsong differences in element length. Transmission paths with stronger reverberations in relation to direct sound should favor the use of longer signals for better propagation.     

高声压级时多孔金属板的吸声特性研究

针对高声压级下有限厚度多孔金属板在线性阻抗背衬条件下(背衬表面声压与声质点速度为线性关系)的吸声问题,提出了一个描述不同声压级下材料层法向吸声性能的一维模型,并给出求解材料层内部声质点速度的线化与差分方法,以预测多孔金属板在高声压级下的非线性吸声特性。在阻抗管中对两块多孔金属板进行了声学测试,得到了材料层法向表面阻抗和吸声系数随入射声压级变化的实验结果。研究表明:实验与理论预测符合良好,验证了模型与数值方法的正确性。本文所提原理和方法,可用于一般硬质多孔材料。      

Sound reduction by barriers on the ground

An existing theory on the propagation of spherical sound waves over ground with a finite acoustic impedance is to a large extent verified by field measurements over different types of ground surfaces. It is then shown that it is reasonable to combine this theory with ordinary diffraction theory to get a solution of the mixed problem with a barrier on the ground. Full-scale measurements have been carried out with a 3 m high wall and the results show good agreement with the predicted values. It is also shown that it is generally of minor importance whether a thin screen is sound absorbing or not.     

The impact of the neck material on the sound absorption performance of Helmholtz resonators

Helmholtz resonators with sound absorption materials filling the neck may have an improved sound absorption capacity. In this work, parallel perforated ceramics with different perforation diameters were installed into the neck of a Helmholtz resonator to improve its acoustic impedance to simultaneously achieve a better acoustic absorption coefficient and a wider absorption bandwidth. An experimental system was built to investigate the effect of the perforation diameters on the sound absorption performance of the resonator. It is found that nonlinear effects near the resonance frequency affect the resonator?s neck mouth impedance and further its sound absorption performance significantly. For frequency range 50–500 Hz, a model of the neck mouth impedance is developed based on a revised Forchheimer relationship. The experimental results are in good agreement with the theoretical model.     

Pionisation, relativistic lead-lead collisions, and the nature of average phase space density

The average phase space density (APSD) of the particles produced in high energy nuclear collisions at ultrarelativistic energies has here been theoretically estimated on the basis of some particular models for particle production spectra. The model-based values so obtained have been compared with the very recent experimental results in the field on the relevant observable and also with the calculated results obtained by some other models. Based on such comparisons, the present work indicates very strongly that Hagedorn's model has a sound potentiality to achieve a competitive status in its capability to deal with the data on the APSD factor in heavy-ion collisions. The impact and implications of all this have also been emphasised here in the end. Received: 14 November 2000 / Accepted: 19 April 2001     

The diffraction of sound by an impedance sphere in the vicinity of a ground surface

The problem of sound diffraction by an absorbing sphere due to a monopole point source was investigated. The theoretical models were extended to consider the case of sound diffraction by an absorbing sphere with a locally reacting boundary or an extended reaction boundary placed above an outdoor ground surface of finite impedance. The separation of variables techniques and appropriate wave field expansions were used to derive the analytical solutions. By adopting an image method, the solutions could be formulated to account for the multiple scattering of sound between the sphere and its image near a flat acoustically hard or an impedance ground. The effect of ground on the reflected sound fields was incorporated in the theoretical model by employing an approximate analytical solution known as the Weyl-van der Pol formula. An approximation solution was suggested to determine the scattering coefficients from a set of linearly coupled complex equations for an absorbing sphere not too close to the ground. The approximate method substantially reduced the computational time for calculating the sound field. Preliminary measurements were conducted to characterize the acoustical properties of an absorbing sphere made of open cell polyurethane foam. Subsequent experiments were carried out to demonstrate the validity of the proposed theoretical models for various source/receiver configurations around the sphere above an acoustically hard ground and an impedance ground. Satisfactory comparative results were obtained between the theoretical predictions and experimental data. It was found that the theoretical predictions derived from the approximate solution agreed well with the results obtained by using the exact solutions.     

深海海底山环境下声传播水平折射效应研究

声波在深海海底山环境中传播时,海底山会对声传播产生重要影响.2016年在南海深海进行了一次海底山环境下的声传播实验,观测到了由海底山引起的三维声传播效应,本文利用BELLHOP射线理论解释了海底山环境下的三维声传播机理.结果表明：声波在传播过程中与海底山作用后破坏了深海会聚区结构,导致传播损失增大,在海底山后形成具有明显边界的声水平折射区,利用二维声传播模型无法解释实验现象,海底山后声水平折射区实验测量的声场结构与N×2D模型计算结果存在明显差异,实验的传播损失比N×2D模型计算结果大10 dB.通过三维射线模型分析N×2D模型计算结果与实验结果存在明显差异产生的原因,发现由于声波水平折射作用,部分声线无法到达接收器,使得三维声传播效应对海底山后一定角度范围内声场影响较为明显.因此,深海海底山会引起明显的三维水平折射效应,应在水下目标探测和定位等应用中给予重视.     

The influence of duration and level on human sound localization

The localization of sounds in the vertical plane (elevation) deteriorates for short-duration wideband sounds at moderate to high intensities. The effect is described by a systematic decrease of the elevation gain (slope of stimulus-response relation) at short sound durations. Two hypotheses have been proposed to explain this finding. Either the sound localization system integrates over a time window that is too short to accurately extract the spectral localization cues (neural integration hypothesis), or the effect results from cochlear saturation at high intensities (adaptation hypothesis). While the neural integration model predicts that elevation gain is independent of sound level, the adaptation hypothesis holds that low elevation gains for short-duration sounds are only obtained at high intensities. Here, these predictions are tested over a larger range of stimulus parameters than has been done so far. Subjects responded with rapid head movements to noise bursts in the two-dimensional frontal space. Stimulus durations ranged from 3 to 100 ms; sound levels from 26 to 73 dB SPL. Results show that the elevation gain decreases for short noise bursts at all sound levels, a finding that supports the integration model. On the other hand, the short-duration gain also decreases at high sound levels, which is in line with the adaptation hypothesis. The finding that elevation gain was a nonmonotonic function of sound level for all sound durations, however, is predicted by neither model. It is concluded that both mechanisms underlie the elevation gain effect and a conceptual model is proposed to reconcile these findings.     

Determination of equivalent sound speed profiles for ray tracing in near-ground sound propagation

The determination of appropriate sound speed profiles in the modeling of near-ground propagation using a ray tracing method is investigated using a ray tracing model which is capable of performing axisymmetric calculations of the sound field around an isolated source. Eigenrays are traced using an iterative procedure which integrates the trajectory equations for each ray launched from the source at a specific direction. The calculation of sound energy losses is made by introducing appropriate coefficients to the equations representing the effect of ground and atmospheric absorption and the interaction with the atmospheric turbulence. The model is validated against analytical and numerical predictions of other methodologies for simple cases, as well as against measurements for nonrefractive atmospheric environments. A systematic investigation for near-ground propagation in downward and upward refractive atmosphere is made using experimental data. Guidelines for the suitable simulation of the wind velocity profile are derived by correlating predictions with measurements.     

Influence of scattering, atmospheric refraction, and ground effect on sound propagation through a pine forest

Sound propagation through a forest is affected by the microclimate in the canopy, scattering by trunks and stems, and ground reflection. Each of these effects is such a strong contributor to the attenuation of sound that mutual interactions between the phenomena could become important. A sound propagation model for use in a forest has been developed that incorporates scattering from trunks and branches and atmospheric refraction by modifying the effective wave number in the Green's function parabolic equation model. The ground effect for a hard-backed pine straw layer is approximated as a local reaction impedance condition. Comparisons to experimental data are made for frequencies up to 4,200 Hz. Cumulative influences of the separate phenomena are examined. The method developed in this paper is compared to previously published methods. The overall comparison with spectral transmission data is good, suggesting that the model captures the necessary details.     

超声搅拌磁流变抛光液的声场仿真分析

为了探究超声搅拌磁流变抛光液的制备及优化工艺,利用多物理场数值计算方法,建立了超声搅拌磁流变抛光液的声场仿真模型,并进行了频域分析。研究了不同液位深度、超声变幅杆探入深度,不同功率下磁流变抛光液的声场分布。通过测量磁流变抛光液的声场强度对声场仿真进行了验证。结果表明：随着距变幅杆距离的增加,声强逐渐减弱,高声强区域主要分布在换能器轴线附近。声强在距变幅杆20mm范围内急剧衰减,变幅杆最佳探入深度为10mm,增大功率有助于空化区域的扩大。声场仿真结果与实验测量结果基本一致,对磁流变抛光液的制备提供了数值计算基础。     

双细胞光声信号的有限元分析

为了分析两个相同的细胞在不同距离下光声信号的特性,对细胞及环境进行了有限元分析。创建了不同距离的双细胞模型和相同环境下的单细胞模型,通过双细胞模型,得到了主体细胞在受到不同距离的另一个细胞的影响下,其不同角度的频域声压级曲线。通过单细胞模型,得到了单个细胞未受到影响时,其不同角度的频域声压级曲线。将主体细胞与单细胞频域声压级曲线进行相似度比较。结果表明,在受到不同距离的细胞影响下,主体细胞各角度的频域声压级曲线,存在与单细胞相似度极高的曲线,且距离为26.6微米时,相似度最高。这项研究表明,在两个双凹形红细胞距离适当时,细胞间的光声信号相互影响最小。