Search

This paper reviews ways of reducing surface transport noise by natural means. The noise abatement solutions of interest can be easily (visually) incorporated in the landscape or help with greening the (sub)urban environment. They include vegetated surfaces (applied to faces or tops of noise walls and on buildings’ façades and roofs), caged piles of stones (gabions), vegetation belts (tree belts, shrub zones and hedges), earth berms and various ways of exploiting ground-surface-related effects. The ideas presented in this overview have been tested in the laboratory and/or numerically evaluated in order to assess or enhance the noise abatement they could provide. Some in-situ experiments are discussed as well. When well designed, such natural devices have the potential to abate surface transport noise, possibly by complementing and sometimes improving common (non-natural) noise reducing devices or measures. Their applicability strongly depends on the available space reserved for the noise abatement and the receiver position.

Sound perception in urban public spaces, a current issue in urban planning, is the topic of this study. The research work is based on a qualitative survey conducted among 29 people. Three groups of people were asked to rate 3 urban squares during two seasons. So, the survey is based on 174 questionnaires, 18 focus groups performed in situ , followed by 29 individual in-depth interviews, including 145 sonic mind maps. This allows us to determine some parameters which influence the perception of environmental sounds. The results indicate a link between visual parameters and the perception of sounds. Representative acoustic indicators of the appraisal of the sonic ambiance and the sound level are then presented. The analysis of sonic mind maps reveals, among other things, that vegetation is associated to the ideal sonic ambiance of a square. Finally, environmental sound representations associated to urban typologies are analyzed. The potential uses of these results by urban planners for improving the quality of urban places are then discussed.

In room acoustics, we measure room impulse responses (RIRs) in order to fully describe the hall. RIRs are composed of a succession of arrivals (i.e., some sound rays which have undergone one or more reflections on their way from the source to the receiver). We propose the eXtensible Fourier Transform (XFT) in order to investigate the time evolution of spectral components of RIRs. The phase evolution versus time allows to estimate the mixing time, which is defined as the time it takes for initially adjacent sound rays to spread uniformly across the room. After presenting some properties of the XFT, we show why one must compensate the natural energy decay of the RIR in order to obtain stationary signals. We estimate mixing times for a set of experimental RIRs and several that are synthesized using a stochastic model. Then, we estimate the dependance of mixing time upon the source/receiver distance in all these RIRs. Results are consistent up to the lack of reproducibility of the sound sources, but are strongly dependent on some parameters used for computing the XFT. We finally discuss the relevance of the name mixing time with respect to the theory and in regard to the time we estimate, that we propose to call cross-over time.

This paper is dedicated to acoustic descriptors and their ability to capture urban traffic noise dynamics. Analysis conducted in this article is based on acoustical measurements taken at five points which depict different typical urban traffic situations. The relevant scale for assessing the dynamics appears to be the duration of the traffic signal cycle. Existing descriptors fail to characterize urban traffic noise at this scale. A set of descriptors is therefore proposed to fulfil this shortage. It is based on the analysis of the mean noise pattern and the variations around this pattern. The set of descriptors proposed enables a differentiation between traffic noise environments which was impossible through existing descriptors.

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.

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 and a half 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. (A)

A new analytical method for the prediction of road traffic noise propagation (called MAPB in French) is presented. This method answers the need of a simple calculation tool usable in the new standard that is being elaborated in France. From the Fresnel zone concept and from simplifications of Weyl-van der Pol’s theory and Geometrical Theory of Diffraction (GTD), new expressions of attenuation due to the ground as well as that due to different barrier types are developed. The complete model in a homogeneous atmosphere (MAPB-H) is obtained by combining these expressions using a classical geometric method. For the case of an atmosphere with a small constant sound speed gradient, the use of corrective length and height terms gives the complete formulation of the MAPB-F (for a positive gradient favourable to propagation) and the MAPB-D (for a negative gradient unfavourable to propagation). Turbulence is neglected in this approach. MAPB-H is validated using a numerical Boundary Element Method (BEM). MAPB-F and MAPB-D are validated with the help of a mixed Rasmussen-Brekhovskikh numerical method, as well as with outdoor measurements. Comparisons of results obtained with the MAPB and those obtained from numerical methods and measurements show good agreement.