The performance of sound absorptive,reflective, and T-profile noise barriers in Toronto

A 4 m (13 ft) high highway noise barrier in Toronto was tested first with an absorptive side, second with a reflective side, and finally with a horizontal cap 75 cm (30 in) wide mounted on its top to create a T-profile. Sound measurements in the residential community behind the barrier showed the T-profile barrier to produce a noise reduction 1–1.5 dB(A) greater than the same barrier without the cap. There was no statistically significant difference between the noise reductions produced by the absorptive and the reflective configurations.     

A parametric investigation of the performance of T-profiled highway noise barriers and the identification of a potential predictive approach

Although a considerable amount of research has been undertaken regarding the performance of T-profile noise barriers, the information available to the practicing highway engineer is confusing. For example, there is a widespread belief that the performance of a top edge, expressed as an insertion loss relative to that of the simple barrier on which it is mounted, is constant, irrespective of the relative locations of the source, barrier and receiver. In order to clarify the situation an investigation has been undertaken, using computer modelling, of the performance afforded by highway noise barriers with T-profile tops with different acoustic treatments. The relative insertion loss was found to increase systematically with increasing top width. Although the relative insertion loss afforded by a reflective T-top is small, significant attenuation can be obtained with an absorptive top. Examination of the effect on performance of the locations of source and receiver relative to that of the noise barrier indicated that, for source and receiver locations typical of those experienced for highway noise barriers, the relative insertion loss for a given width of T-top was a function of (a) the path difference between sound travelling to the receiver via the barrier top and direct sound from the source to the receiver and (b) the barrier height. Plots of relative insertion loss versus the path difference, normalised with respect to barrier heights, for a range of T-top widths and absorbent treatment, resulted in a collapse of data around well defined trend lines which offer the potential of being developed into a prediction method.     

Performance of profiled single noise barriers covered with quadratic residue diffusers

The paper describes an investigation about the acoustic performance of noise barriers with quadratic residue diffuser (QRD) tops, and with T-, Arrow-, Cylindrical and Y-shape profiles. A 2D boundary element method (BEM) is used to calculate the barrier insertion loss. The results of rigid and with absorptive coverage are also calculated for comparisons. Using QRD on the top surface of almost all barrier models presented here is found to improve the efficiency of barriers compare with using absorptive coverage at the examined receiver positions. T-shape and Arrow-shape barriers are also found to provide better performance than other shapes of barriers. The best shape of barriers for utilising QRD among the tested models is the T-shape profile barrier. It is found that reducing the design frequency of QRD shifts the performance improvement towards lower frequency, and therefore the most efficient model for traffic noise is a barrier covered with a QRD tuned to around 400 Hz.     

Procedures for determining the acoustic efficiency of edge-modified noise barriers

This paper describes the noise shielding efficiency of barriers with an acoustic device mounted on their top edge for reducing sound diffraction. Diffraction behind the edge-modified barrier is investigated by scale model experiments in which the positions of a source and a receiver are aligned along a circular arc around the barrier top. The result indicates that the acoustic efficiency of the edge device is a function of the angles of the source and receiver and independent of their radii. Based on this finding, a novel procedure for determining the efficiency of manufactured edge devices is established. This procedure is very beneficial for estimating the edge device efficiency by eliminating ground and meteorological effects. The measured efficiency of the device will be quite useful for the prediction of noise propagation behind the edge-modified barriers.     

Durability of sound absorbing materials for highway noise barriers

This paper presents the results of a research study into two properties of sound absorbing materials intended for highway noise barrier applications: their durability and their sound absorption coefficients before and after exposure to adverse weather. After surveying the products of 34 manufacturers, eight materials and one “absorption system” were tested in the field and the laboratory. Even though there is no single, accepted test for weather endurance for these kinds of material, the results of the study provide in ormation on their likely future behaviour when installed on a barrier. The results apply at least partially to other outdoor situations too, and augment the limited information presently available on sound absorptive materials for outdoor use.     

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.     

Vibration damping material as a means to reduce steel noise barrier cost

A preliminary analysis has been made of the use of vibration damping material as a means to reduce the cost of steel noise barriers in primarily highway applications. For cost-effective barriers, the sound transmission through, as well as over, a noise barrier must be considered. The through-barrier sound transmission characteristics of sample panels from a Toronto noise barrier were measured with and without damping material. It was found that a given through-barrier sound transmission performance could be achieved at less cost with the damping material than without it. Further study is recommended.     

平行声屏障的陷波模态及其插入损失优化

为了改善平行声屏障的性能,本文基于有限元仿真的方法对其陷波模态和插入损失进行了研究。其中,陷波模态是平行声屏障的固有性质,与其几何参数有关。当陷波模态处于共振频率时,平行声屏障内部声场的声能量达到峰值,同时声波垂直入射到声屏障的顶端,使声影区的衍射声能也达到峰值,最终导致插入损失显著下降。本文还对3种优化平行声屏障插入损失的方法进行了分析。结果表明,楔形和扩散型声屏障可降低声波的多次反射效应对插入损失的影响,但是对陷波模态的改善较小。而吸声型声屏障有效的抑制了陷波模态对插入损失的不利影响,从而改善了平行声屏障的性能。     

Deriving correction functions to model the efficiency of noise barriers with complex shapes using boundary element simulations

The boundary element method (BEM) is a commonly used method to compute the insertion loss of noise barriers having arbitrary cross-sections. For large scale three-dimensional problems, however, the BEM is not feasible. On the other hand, standardized calculation methods for noise mapping are efficient, but shapes other than the straight barrier cannot be properly calculated. Attempts to merge these two approaches by using BEM to derive correction functions based on geometrical quantities such as source and target angle as well as the path length elongation between source and receiver caused by the barrier were usually focused on a small set of barrier types, dimensions, absorptive configurations, source or receiver positions. The main objective of this study is to investigate which functions based on the most common geometrical parameters are well suited for approximating the efficiency of different types of barriers, dimensions and absorptive configurations. To achieve this, numerous combinations of 7 different barrier types, different heights and widths as well as 3 different absorptive configurations were simulated using the 2D BEM for 8 different source positions. The octave-band-wise efficiency, i.e. the frequency-dependent gain in insertion loss compared to an equally high, fully reflective straight barrier was used as a basis for the correction functions. Linear as well as polynomial models were compared yielding a polynomial of third degree in the source and fourth degree in the target angle as the best model. Effects on the error using uniform sampling in the target angle instead of a uniform receiver grid as a basis for the correction functions are also investigated. Furthermore, wide-band efficiencies based on standardized traffic emission spectra are calculated showing small errors compared to single-band errors, in particular in the high-frequency range. A linear interpolation scheme is suggested to deal with barriers having dimensions not simulated in this work.     

Performance of T-shape barriers with top surface covered with absorptive quadratic residue diffusers

A previous paper [Applied Acoustics 66 (2005) 709-730] has shown that adding a quadratic residue diffuser (QRD) to the top of a T-shape barrier can provide better barrier performance than an equivalent purely absorptive barrier. In here, we extend the study to look at the performance when a QRD is made absorptive. This paper presents an investigation on the acoustic performance of a few welled-diffusers with different absorption ability on top of a T-shape noise barrier. The absorption properties of the diffusers are modified with different sequences, by filling the wells with fiberglass, by covering the well entrance with wire meshes, and by putting perforated sheet either on the top surface or inside the wells. A 2D Boundary Element Method (BEM) is used to calculate the barrier insertion loss. The numerical and experimental results on diffuser barriers with rigid and absorptive covers are compared. Among the tested models the best method of treating diffuser barriers with absorbent agents in the QRD is found to be a perorated sheet on top or inside the diffuser wells. It is found that increasing the absorption ability of QRD by fiberglass or high resistance wire meshes has negative effect on the efficiency of a QRD barrier. It is shown that, if the increase in absorption destroys the effect of resonance in wells, it will also have negative effect on the insertion loss performance of the QRD edge barrier.     

Ceiling baffles and reflectors for controlling lecture-room sound for speech intelligibility

Reinforcing speech levels and controlling noise and reverberation are the ultimate acoustical goals of lecture-room design to achieve high speech intelligibility. The effects of sound absorption on these factors have opposite consequences for speech intelligibility. Here, novel ceiling baffles and reflectors were evaluated as a sound-control measure, using computer and 1/8-scale models of a lecture room with hard surfaces and excessive reverberation. Parallel ceiling baffles running front to back were investigated. They were expected to absorb reverberation incident on the ceiling from many angles, while leaving speech signals, reflecting from the ceiling to the back of the room, unaffected. Various baffle spacings and absorptions, central and side speaker positions, and receiver positions throughout the room, were considered. Reflective baffles controlled reverberation, with a minimum decrease of sound levels. Absorptive baffles reduced reverberation, but reduced speech levels significantly. Ceiling reflectors, in the form of obstacles of semicircular cross section, suspended below the ceiling, were also tested. These were either 7 m long and in parallel, front-to-back lines, or 0.8 m long and randomly distributed, with flat side up or down, and reflective or absorptive top surfaces. The long reflectors with flat side down and no absorption were somewhat effective; the other configurations were not.     

Measurement of airborne sound insulation of timber noise barriers: Comparison of in situ method CEN/TS 1793-5 with laboratory method EN 1793-2

Recent progress in the development of European standards has allowed the in situ testing of roadside noise barriers. CEN/TS 1793-5 describes a test method using maximum length sequences (MLS) for the characterisation of airborne sound insulation. However, many barriers are tested according to a laboratory standard, EN 1793-2, based on measurements carried out in reverberant chambers and in the case of timber barriers with a relatively low airborne sound insulation it is not clear to what extent the results of the two tests compare. The paper describes the results of tests carried out using both methods. Six samples of timber barrier were compared including single-leaf and double-leaf constructions and single-leaf constructions with an absorptive core. Very good agreement was found especially when account was taken of the valid frequency range in each test method. The results open up the possibility of routinely evaluating the performance of timber barriers at the roadside where build quality can be variable and there are concerns that the acoustic performance may not match that obtained under laboratory test conditions where typically the barrier is more carefully constructed.     

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 use of in-situ test method EN 1793-6 for measuring the airborne sound insulation of noise barriers

The in situ measurement of the airborne sound insulation, as outlined in EN 1793-6:2012, is becoming a common means of quantifying the performance of road traffic noise reducing devices. Newly installed products can be tested to reveal any construction defects and periodic testing can help to identify long term weaknesses in a design. The method permits measurements to be conducted in the presence of background noise from traffic, through the use of impulse response measurement techniques, and is sensitive to sound leakage. Factors influencing the measured airborne sound insulation are discussed, with reference to measurements conducted on a range of traffic noise barriers located around Auckland, New Zealand. These include the influence of sound leakage in the form of hidden defects and visible air gaps, signal-to-noise ratio, and noise barrier height. The measurement results are found to be influenced by the presence of hidden defects and small air gaps, with larger air gaps making the choice of measurement position critical. A signal-to-noise ratio calculation method is proposed, and is used to show how the calculated airborne sound insulation varies with signal-to-noise ratio. It is shown that the measurement results are influenced by barrier height, through the need for reduced length Adrienne temporal windows to remove the diffraction components, prohibiting the direct comparison of results from noise barriers with differing heights.     

Numerical evaluation of tree canopy shape near noise barriers to improve downwind shielding

The screen-induced refraction of sound by wind results in a reduced noise shielding for downwind receivers. Placing a row of trees behind a highway noise barrier modifies the wind field, and this was proven to be an important curing measure in previous studies. In this paper, the wind field modification by the canopy of trees near noise barriers is numerically predicted by using common quantitative tree properties. A realistic range of pressure resistance coefficients are modeled, for two wind speed profiles. As canopy shape influences vertical gradients in the horizontal component of the wind velocity, three typical shapes are simulated. A triangular crown shape, where the pressure resistance coefficient is at maximum at the bottom of the canopy and decreases linearly toward the top, is the most interesting configuration. A canopy with uniform aerodynamic properties with height behaves similarly at low wind speeds. The third crown shape that was modeled is the ellipse form, which has a worse performance than the first two types, but still gives a significant improvement compared to barriers without trees. With increasing wind speed, the optimum pressure resistance coefficient increases. Coniferous trees are more suited than deciduous trees to increase the downwind noise barrier efficiency.     

Quasi-wavelet calculations of sound scattering behind barriers

Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions.     

A model for insertion loss degradation for parallel highway noise barriers

An algorithm is developed and validated for the prediction of the reduction, or degradation, of traffic noise barrier insertion loss when a second barrier is placed on the opposite side of the highway. The algorithm combines the basic emission, propagation, and barrier attenuation features of the Federal Highway Administration traffic noise prediction model with a geometrical acoustics approach for multiple reflections. The resultant model can accommodate any number of source lanes or receivers, three vehicle categories, and independently variable barrier heights and absorption coefficients. The model was validated against mathematical, scale model, and full scale field data received from other researchers, and has proved to be a good predictor of insertion loss degradation.     

Integration of the efficiency of noise barrier caps in a 3D ray tracing method. Case of a T-shaped diffracting device

Road barrier diffracting caps have shown a renewed interest for several years since they give the opportunity of increasing the barrier efficiency without changing its overall height. First investigations on the efficiency of road barrier caps calculated with a boundary element method (BEM) have shown that the efficiency obtained with coherent line sources is underestimated compared to that with incoherent line sources, more representative of road traffic noise. The present work deals with the characterisation of the real performance of a T-shaped absorbing cap with road traffic noise conditions. Two different approaches are compared: on one hand calculations with the help of a BEM program able to achieve 2D and 2D simulations are made; on the other hand outdoor measurements on a test-wall using a maximum length sequence technique are carried out. The goal in the two approaches is to isolate the top edge diffracted sound field in order to determine an extrinsic value of octave band efficiency of the cap for many source-receiver pairs. These results integrated in a ray tracing prediction method enable the integration of air absorption along each ray path and give the real efficiency of such a device in the case of complex and realistic configurations for barriers of finite length.     

Optimisation with genetic algorithm of the acoustic performance of T-shaped noise barriers with a reactive top surface

In this study, the aim is to optimise the acoustical efficiency of T-shaped noise barriers whose top is covered with a series of wells. This research work uses an optimisation method in order to find the best noise barrier profile considering several variable parameters. Numerical simulations of the acoustical propagation are achieved by use of a 2D boundary element method code. The optimisation part is carried out with a global and direct evolutionary optimisation method: a genetic algorithm. The parameters to optimise are the shape of the protection (the depths of the wells on the crowning) and the flow resistivity of absorbing materials considered. The cost function to maximize is defined through a mean value of the acoustical efficiency of the protection compared to a reference configuration, averaged on several receiver points. Final results show significant optimised values of parameters for efficient protections in order to improve classical noise barriers.     

Noise barrier optimization using a simulated annealing algorithm

This paper presents a novel method to design a desired noise barrier using the global optimization of a simulated annealing (SA) algorithm. This method focuses on minimizing the barrier dimensions, which are related to material and construction costs, as well as satisfying the target sound pressure levels at receiver points on the condition of traffic noise. Various examples are presented to evaluate the optimized barrier dimensions according to given traffic noise sources, ground topography, surface conditions, and the influence of different receiver positions. As a result, a robust method is found that can determine optimized barrier configurations and dimensions that yield cost reductions.