An experimental investigation of the propagation of sound over a curved, rough, rigid surface

Abstract

Small-scale laboratory experiments have been conducted to study the propagation of sound over a curved, rough, rigid, convex surface in the range kR=275–550, and kε=0.09–0.18, where k is the wave number, R the radius of curvature, and ε the characteristic roughness length scale of steep-sloped, densely packed scatterers. Experimental results are presented at 6, 8, and 12 kHz, with the receiver successively on the surface, along the line of sight behind the surface, and along a vertical axis in the shadow zone. At low frequencies (f=6 kHz, kε=0.09), a boundary wave caused by coherent multiple scattering develops near the surface, creeps into the deep shadow zone, and increases the sound-pressure levels measured in the deep shadow zone by up to 8 dB as compared to the levels measured in the absence of surface roughness. At high frequencies (f=12 kHz, kε=0.18), surface roughness causes incoherent scattering and decreases the measured sound-pressure levels by about 10 dB on the surface, in the shadow zone, and by 10 to 20 dB along the line of sight behind the apex. It is shown that incoherent scattering occurs mostly in the bright zone, before the apex, whereas coherent scattering (boundary wave growth) occurs all along the surface, even in the deep shadow.