Optimisation of ground attenuation for moving sound sources

In an earlier study, Attenborough and Li [Attenborough K, Li, KM. Ground effect for A-weighted noise in the presence of turbulence and refraction. J Acoust Soc Am 102:1997;1013-22] derived a closed form analytical formula to calculate optimum ground parameters for reducing the A-weighted noise due to a stationary point source. This paper extends this earlier study by deriving an expression to calculate the sound field due to a source moving at a constant speed above a ground surface. An A-weighted mean-square sound pressure has been derived that enables one to estimate the sound exposure levels of a moving source. Numerical calculations for a realistic range of speeds show the influence of the source motion on the noise levels. Although the predicted effects on the ground attenuation are moderate, they are significant at a relatively low Mach number. In addition, the sensitivity to the atmospheric turbulence and the geometrical parameters tends to be altered by the source motion. On the other hand, it is demonstrated that the optimum ground parameters are similar for stationary and moving sources.     

An acoustic head simulator for hearing protector evaluation. II: Measurements in steady-state and impulse noise environments

The attenuation characteristics of hearing protection devices (HPDs) were measured using a modular acoustic head simulator. The effect in changes in the head configuration was assessed in a steady-state diffuse sound field. The use of artificial circumaural skin had a relatively small influence on the insertion loss of earmuffs (max. 6-7 dB at low frequencies). This contrasts to the very large effects found for the artificial intraaural skin on the insertion loss of earplugs (in excess of 40 dB at low frequencies for some devices). Results were also compared with real-ear attenuation at threshold (REAT) data (ANSI S3.19-1974). In general, there is good agreement between the two methods, especially for earmuffs. Design improvements are proposed for earplugs. The result of an exploratory study aimed at measuring the complex (amplitude and phase) insertion loss of HPDs using an impulse noise source are also reported.     

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.     

NUMERICAL MODELLING OF THE SOUND FIELDS IN URBAN STREETS WITH DIFFUSELY REFLECTING BOUNDARIES

A radiosity-based theoretical/computer model has been developed to study the fundamental characteristics of the sound fields in urban streets resulting from diffusely reflecting boundaries, and to investigate the effectiveness of architectural changes and urban design options on noise reduction. Comparison between the theoretical prediction and the measurement in a scale model of an urban street shows very good agreement. Computations using the model in hypothetical rectangular streets demonstrate that though the boundaries are diffusely reflective, the sound attenuation along the length is significant, typically at 20-30 dB/100 m. The sound distribution in a cross-section is generally even unless the cross-section is very close to the source. In terms of the effectiveness of architectural changes and urban design options, it has been shown that over 2-4 dB extra attenuation can be obtained either by increasing boundary absorption evenly or by adding absorbent patches on the façades or the ground. Reducing building height has a similar effect. A gap between buildings can provide about 2-3 dB extra sound attenuation, especially in the vicinity of the gap. The effectiveness of air absorption on increasing sound attenuation along the length could be 3-9 dB at high frequencies. If a treatment is effective with a single source, it is also effective with multiple sources. In addition, it has been demonstrated that if the façades in a street are diffusely reflective, the sound field of the street does not change significantly whether the ground is diffusely or geometrically reflective.     

A note on the effect of ground absorption in the measurement of aircraft noise

It is shown that results of available theoretical analyses of the far-field sound pressure due to a simple source above an absorbing plane are capable of simple interpretation, and that certain published experimental values of excess attenuation obtained from measurements near the ground are consistent with such results. The effects of changing various parameters are demonstrated and estimates made of the errors likely to be incurred if effects of ground absorption are ignored when determining normalized values of the noise of aircraft.     

计入地面附加衰减的声屏障插入损失估算方法*

基于理论推导和计算,给出了公路声屏障声学设计中,在考虑地面附加衰减情况下计算插入损失的方法。该方法综合考虑了有限长线声源无限长声屏障绕射声衰减量、有限长线声源地面衰减量及遮蔽角对插入损失的影响。通过与《声屏障声学设计和测量规范》（HJ/T90-2004）的计算结果的对比,验证了本文所给方法的精确性及可行性,并对规范所给地面衰减修正量进行了商榷。最后,给出了当预测点位于有限长路段中央法线上时,通过计算线声源地面衰减量得到计算插入损失所需参数值,再计算插入损失的简便方法。本研究为存在地面附加衰减情况下有限长声屏障插入损失计算提供了一个新的参考方法。     

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.     

Estimation of directivity and sound power levels emitted by aircrafts during taxiing, for outdoor noise prediction purpose

Integrated noise model (INM) is the most internationally used software to calculate noise levels near airports. Take off, landing or pass by operations can be modeled by INM, but it does not consider aircrafts taxiing, which, in some cases, can be important to accurately evaluate and reduce airports’ noise assessment.Aircraft taxiing noise emission can be predicted using other prediction tools based on standards that describe sound attenuation during propagation outdoors. But these tools require data inputs that are not known: directivity and sound power levels emitted by aircraft during taxiing.This paper describes methods used to calculate directivity indexes and sound power levels, based on field measurements made in Madrid-Barajas Airport (Spain). Obtained results can be used as inputs for general purpose outdoor sound prediction software, which will be able to evaluate noise at airports vicinity as industrial noise.Directivity and sound power levels have been estimated in octave and third octave band terms, for several aircraft families.     

内部体积源作用下的圆柱壳内外声场特性

圆柱壳内各型体积源辐射噪声特性研究是声场建模和声场预报的前提.为了研究具有指向性的大尺度体积源特性对水下航行器结构内外声场的影响,本文结合薄壳理论、等效源和柱腔Green函数构造了体积源激励下的壳体振动耦合方程,研究了体积源表面声散射作用和指向性强弱对圆柱壳内外声场的影响.数值计算结果表明,体积源构造的准确性与其等效源位置有关,等效源配置在体积源几何中心与其结构表面之间0.4—0.6时,可以提高声场计算结果的准确性;大尺度体积源表面的声散射作用会导致壳体内部声场结构发生改变,内声场声腔共振峰发生偏移,并且在部分频段引起较强的声透射现象;此外,体积源指向性变化对壳体内外声场强弱影响较小,其显著作用表现在改变了外辐射声场的远场指向性.该研究结果对噪声预报和控制有一定的参考价值.     

Noise attenuation in factories

The cost of reducing noise levels in factories by covering large surfaces with sound absorbents is high. It is therefore important to be able to calculate in advance the effectiveness of absorbents and to determine how absorbents may be chosen and distributed for maximal noise reduction for the invested capital. For this purpose a mathematical model of sound propagation and attenuation in factories has been developed on certain simplifying assumptions. The interrelationship between the different parameters is found to be rather intricate and the mathematical model must be evaluated using a computer program. The influence on the noise levels of sound scattering objects, the shape and size of the factory and absorbents on the room surfaces have been studied. Especially if the ceiling is high, acoustic baffles are found to be more efficient than absorbents on the ceiling in reducing noise levels.     

Point source equalised by inverse filtering for measuring ground impedance

A point source for measuring ground impedance which complies with the omnidirectionality requirements of the ANSI S1.18 standard is presented. The source consists of a small aperture connected to a conventional loudspeaker through a cone. Whilst this source radiates with deviations lesser than ±1 dB within ±45°, its frequency response exhibits strong peaks arising from resonances of air inside the cone. To equalise these resonances, inverse filtering is applied. Inverse filters pre-emphasise the electrical signal driving the sound source so that zero-phase (or minimum-phase) cosine-magnitude signals are radiated. Equalisation by inverse filtering has two main advantages in the ground impedance measurement. On the one hand, flattening the frequency response avoids large fluctuations of the excess attenuation curve, which can make difficult its inversion to ground impedance data. On the other hand, inverse filtering shorten the time response of the sound source, this in turn making easier the positioning of a time window to separate ground reflection from reflections/diffractions coming from nearby objects usually present in the acoustic scenario.     

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.     

预测声屏障插入损失的抛物方程法及其初始声场研究

Salomons建立的抛物方程(CNPE)方法可以预测非均匀环境中的声屏障插入损失。但是该方法在声屏障与声源距离较近时会产生较大误差。文中通过理论分析发现产生该问题的原因在于CNPE方法所使用的Gauss初始场仅适用于小仰角(10°以内)范围内的声波。为解决Gauss初始场引起的问题,推导了可以用于较大仰角声波的更高阶数的Gauss初始场。通过数值仿真对比了不同阶数的初始场在CNPE方法中的效果。结果表明:4阶初始场是最适合CNPE方法的初始场,将该初始场与CNPE方法相结合,可以准确预测当声屏障与声源距离较近时的插入损失.      

Sound propagation in street canyons: comparison between diffusely and geometrically reflecting boundaries

This paper systematically compares the sound fields in street canyons with diffusely and geometrically reflecting boundaries. For diffuse boundaries, a radiosity-based theoretical/computer model has been developed. For geometrical boundaries, the image source method has been used. Computations using the models show that there are considerable differences between the sound fields resulting from the two kinds of boundaries. By replacing diffuse boundaries with geometrical boundaries, the sound attenuation along the length becomes significantly less; the RT30 is considerably longer; and the extra attenuation caused by air or vegetation absorption is reduced. There are also some similarities between the sound fields under the two boundary conditions. For example, in both cases the sound attenuation along the length with a given amount of absorption is the highest if the absorbers are arranged on one boundary and the lowest if they are evenly distributed on all boundaries. Overall, the results suggest that, from the viewpoint of urban noise reduction, it is better to design the street boundaries as diffusely reflective rather than acoustically smooth.     

Determination of the sound energy level of a gunshot and its applications in room acoustics

In some cases an impulsive noise source such as a gunshot can be a preferred alternative when investigating building acoustics, including sound insulation measurements, when compared to conventional steady state noise sources. A gun equipped with blank cartridges is an impulsive noise source that is lightweight and small enough to be easily transported. The differences in the noise characteristics between individual cartridges for the same gun are usually small, so the impulsive source can be replicated to a high degree. This paper is focused on the practical application of the sound exposure levels produced by a gunshot with a known sound energy level in the rooms under investigation. In this way, the equipment and methods required by the conventional method are simplified significantly. Furthermore, reverberation times need not be measured, since the equivalent absorption area can be directly obtained from the measured sound exposure levels. Using Green’s theorem, the roles of the sound source and measuring microphone were exchanged, which simplified the determination of sound insulation as it was easier to change the position of the gun than the microphone. The results obtained using the impulsive noise source were in good agreement with those obtained using the conventional method. Above 100 Hz, their difference in any frequency band of interest was less than 1 dB.     

The acoustic telescope

A system has been developed for real-time sound source location on full-size jet engines. It consists of an array of microphones connected to a small digital computer, via a sequence of preamplifiers, analog filters and analog to digital converters and multiplexer. Microphone signals can be processed on-line to give displays of time varying source distributions or statistical averages with respect to position and frequency, by using a colour television as well as a display screen and printer/plotter. The whole system can be transported in a small estate car and can be used on both model scale and full size engine test rigs.A theoretical analysis of system performance is in terms of a line source of generally correlated omni-directional sound radiators, which shares the measurable far-field properties of a jet engine noise source. The general properties of the system are described, including its use to correlate spatially separated sound sources, application in the presence of ground reflections and use in a moving airstream. The statistical properties of apparent source distributions are also discussed.A series of experiments on a Rolls-Royce/SNECMA Olympus engine is described, in order to illustrate application of the system.     

Possibilities and limits of sound power measurements with a real-time intensity analyzer

The sound power of a number of test objects was determined from spatially averaged intensity measurements. The results show that the influence of room acoustics is insignificant even for rooms of widely different room constants, if the measuring surfaces are exactly defined and if a good space-averaging technique is used. The intensity integrated over a closed surface defining a source-free space compared to the sound pressure integrated over the same surface gives a measure of the capability of a specific intensity measuring system to suppress external noise. For the test arrangements measured with broad band noise, this suppression was found to be 14–18 dB(A). A similar value of 15 dB was found from sound power measurements on a source with high external sound and an analysis of the results in one-third octave bands. From these measurements an analytical function was derived which describes the average error of the spatially averaged intensity as a function of the difference between the external sound level and the source sound level. For practical measurement situations a further analytical function was derived which gives this intensity error as a function of the difference between the measured (spatially averaged) pressure and intensity levels. Thus it is possible to estimate the error of intensity measurements directly from measured intensity and pressure data.     

Reduction of fan noise by annulus boundary layer removal

Much of the noise produced by a fan is due to varying forces caused by the unsteady flow field through which it passes. In the absence of inlet guide vanes, support struts and other mechanical obstructions, the flow irregularities are caused by large scale intake turbulence and temporal and spatial variations in the annulus boundary layer. In many cases the most effective source is the annulus boundary layer, as this interacts with the blade tip which is the fastest part of the blade and is therefore the most effective noise source. This noise can be reduced if the annulus boundary layer is smoothly bled away from the rotor so that the blade tip sees a thin, uniform boundary layer. A one metre diameter ventilation fan has been run in a duct with and without a boundary layer bleed system and a reduction of far field sound power of approximately 5 dB has been obtained for a bleed of 5 % of the main flow with some far field tone reductions of more than 15 dB. Measurements of far field directivities, in-duct acoustic modes and aerodynamic distortions have confirmed the suggested explanation for this phenomenon. The tests have shown that great care must be taken in the design of a bleed system so that residual boundary layer distortions are not in the correct wavelength range to produce propagating acoustic waves at important blade passing harmonic frequencies. If such distortions are present, the far field noise can increase instead of decrease.     

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.