Accounting for the effect of diffuse reflections and fittings within street canyons, on the sound propagation predicted by ray tracing codes

Diffusion on building facades and fittings within a street can significantly affect sound propagation in urban areas. These phenomena are however not reproduced by the widespread outdoor sound propagation models that are based on ray codes algorithms, because their modeling would induce increased computational costs. In this paper, a set of 32 175 simulations is achieved with a sound particle tracing code to quantify the errors made when neglecting acoustic diffusion within street canyons, according to the street geometry (width, height, distance between the point source and the receiver), the acoustical properties of the street (diffusion and absorption coefficient of the facades, absorption coefficient of the ground), and the acoustical properties of the fittings (mean free path and average absorption coefficient), in the case of a receiver height of 1.5 m. The diffuse reflections can lead to reduction of 2 dB to an increase of 4 dB of sound pressure levels in the absence of fittings, and can lead to an increase of 10 dB of sound pressure levels in the presence of fittings, for the most unfavorable configurations. The influence of the acoustical parameters and the influence of the street geometry on sound attenuation are closely linked to each other. Moreover, acoustic diffusion results in an overall sound level increase if one considers a linear point source distribution. Finally, regressions are proposed that estimate the impact of diffusive reflections and street fittings on sound propagation as a function of the input parameters. These regressions can now be advantageously used to refine sound levels estimations within street canyons, when using classical outdoor sound propagation models, in the range of the parameters tested.