Calculation of Reflections in an Urban Environment

Abstract

Sound propagation in an urban environment is often significantly influenced by reflections at building façades. The façades are typically structured in depth in the order of magnitude of decimeters up to meters. Low frequency reflections tend thus to be mirror like while at high frequencies they get a more and more diffuse or scattering character. A model is presented to calculate the effects of both type of reflections. After a discretization of the boundaries into grid cells, the sound pressure and energy exchange between the patches is calculated step by step. A coherent model is established to handle specular reflections. It solves a Kirchhoff-Helmholtz formulation by a time iterative process. The incoherent model for diffuse reflections assumes energy conservation and Lambert distribution for the reflections. The advantage of the model - which is part of the new Swiss railway noise calculation scheme sonRAIL - is its continuous behavior and the ability to take into account the reflector size. The model for specular reflections was validated extensively and proofed to reproduce scale model experiments as well as numerical FDTD simulations with good accuracy.