Henry E. Bass
Phys. Acoust. Res. Group, Univ. of Mississippi, University, MS 38677
L. C. Sutherl
27803 Longhill Dr., Rancho Palos Verdes, CA 90274
The propagation of sound outdoors involves spreading, absorption, interaction with the surface, refraction, diffraction, and scattering. Although there is still no widely accepted prediction scheme that includes all of these effects, the understanding of individual phenomena and contribution to transmission loss along the propagation path has steadily advanced. Transmission loss due to atmospheric absorption is standardized. The effects of real ground surfaces are predictable for a wide range of surfaces. Modern numerical techniques described in a different special session allow one to predict refraction and diffraction in an inhomogeneous atmosphere. These predictions are far superior to simple geometrical optics ray tracing at moderate and low frequencies. Diffraction by barriers of regular geometry can be predicted analytically but irregular shapes require further developments. Scattering by atmospheric turbulence as well as foliage requires additional work prior to providing engineering estimates well based upon experiment.