PROPAGATION OF SOUND ABOVE AN IMPEDANCE PLANE IN A DOWNWARD REFRACTING ATMOSPHERE

An asymptotic analysis has been carried out to study the propagation o f sound at long ranges in an arbitrary downward refracting sound-speed profile. It has been shown that the solution can be expressed in a fo rm of the Weyl-Van der Pol formula and the present theory offers an ex tension for the established theory to the case of a vertically stratif ied medium. The analysis starts from the full wave equation and the de rivation assumes that the ambient properties only depend on the height above an impedance ground and that there is no wind or turbulence in the medium. In addition, the theory also assumes that the sound speed of the atmosphere increases monotonically with height. For such an atm ospheric condition, the medium forms a waveguide that enhances the sou nd levels at long ranges along the ground surface. It is demonstrated that the acoustical path length derived by the asymptotic method is id entical to that derived by the analytical ray trace model. Furthermore , the asymptotic method also allows a proper inclusion of the surface wave term rigorously. (C) 1996 Acoustical Society of America.