AN EXPERIMENTAL INVESTIGATION OF THE PROPAGATION OF SOUND OVER A CURVED, ROUGH, RIGID SURFACE

Small-scale laboratory experiments have been conducted to study the pr opagation of sound over a curved, rough, rigid, convex surface in the range kR = 275-550, and k epsilon = 0.09-0.18, where k is the wave num ber, R the radius of curvature, and epsilon the characteristic roughne ss length scale of steep-sloped, densely packed scatterers. Experiment al results are presented at 6, 8, and 12 kHz, with the receiver succes sively 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 k Hz, k epsilon = 0.09), a boundary wave caused by coherent multiple sca ttering develops near the surface, creeps into the deep shadow zone, a nd increases the sound-pressure levels measured in the deep shadow zon e by up to 8 dB as compared to the levels measured in the absence of s urface roughness. At high frequencies (f = 12 kHz, k epsilon = 0.18), surface roughness causes incoherent scattering and decreases the measu red 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) oc curs all along the surface, even in the deep shadow. (C) 1997 Acoustic al Society of America.