Time-domain simulations of outdoor sound propagation with suitable impedance boundary conditions

In the context of transportation noise, acoustic sources are usually broadband and in motion, and the propagation environment is complex, with various types of ground, wind fluctuations and temperature fluctuations, and topographic effects. Finite-difference time-domain methods are particularly well suited to deal with these different aspects. A method recently proposed to obtain an impedance boundary condition is implemented in a linearized Euler equations solver, which enables to study long range sound propagation over an impedance ground. Several calculations are performed to show the effectiveness of the method and its ability to take into account relatively complex phenomenons. A long range configuration is studied in homogeneous conditions and in downward-refracting conditions. Numerical results are compared to an analytical solution for homogeneous conditions and to a ray-tracing code for downward-refracting conditions. Surfaces waves are detected in the two cases. We also considered sound propagation in an upward-refracting medium: occurrence of surfaces waves and creeping waves in the acoustic shadow zone is discussed.