The effect on sound propagation in streets of the distribution of acoustic scatterers on facades

Abstract

Until recently street noise propagation models assumed that faade reflections were either totally specular or totally diffuse. However, the characteristics of building facades are such that some energy will be reflected specularly and some diffusely. Many current models, therefore, use a scattering coefficient to characterize the diffuse reflection characteristics and the assumption is made that a fraction of the sound energy falling on any point on the facade will be scattered to the same degree. From observation it can be seen that facades are not homogeneous but consist of surfaces which are planar in nature but feature discontinuities such as windows, doors, ledges, etc. which will tend to act as distributed scattering centers. The effect of distributions of scattering centers on the propagation of sound in streets is examined using a combined image source and randomly scattered sound rays. Computer generation of street models using an object based approach enables the study of many different configurations using RAYNOISE software. Results are compared with those obtained assuming a homogeneous distribution of facade scattering using both RAYNOISE and a technique proposed by the authors. The results of a scale model experiment to investigate the effect of distributed scatters are presented.