Sound propagation phenomena relevant to prediction schemes

Abstract

Interference between direct and reflected waves can produce a shadow region near the ground because most ground surfaces have a detectably finite acoustic impedance. In a steady isotropic atmosphere this region is penetrated at low frequencies by a ground wave, especially at short ranges, or by a trapped surface wave, significant at longer ranges. The thickness and flow properties of surface layers, (e.g., snow) also affect the propagation of low frequencies near the surface. For ground having impedance discontinuities (e.g., asphalt/grass) diffracted waves originated at the interface. Diffraction theory allows prediction of sound pressures in the shadow region behind thin barriers and has been applied to thick barriers, multiple barriers, and ground surfaces of various shapes. In a downward‐refracting atmosphere, such as propagation downwind or in a temperature inversion, the shadow region is diminished: conversely, in upward refraction, a refractive shadow reinforces the ground impedance shadow region. Wind and temperature fluctuations (turbulence) cause sound energy to be redirected by scattering or variable refraction: this occurs most noticeably at high frequencies and where coherent theory would otherwise predict reduced sound pressure levels. For most of these phenomena quantitative agreement has been achieved between measurements and predictions of the relevant theory: however, the situation in real life is complicated by the fact that several effects usually coexist.