Experimental investigation of the diffraction of sound by a curved surface of finite impedance

Abstract

Laboratory scale experiments have been conducted to study the diffraction of sound by a single curved ridge on top of a flat table. The sound source is an electric spark that produces an N‐wave type transient of about 40‐μs duration. Two 14‐in. microphones capture the signal at a reference position (before the ridge), and at a field point, respectively. The signals are gated to eliminate undesirable reflections, digitized (250 kHz/channel), low‐pass filtered (50‐kHz cutoff frequency), and processed by an IMB PC which computes the Fourier transforms of both reference and field pressure waveforms. Regardless of the spark variability, the ratio R of the Fourier transforms is constant to within 0.5 dB for the frequency range of interest (5–40 kHz). Consequently, R represents the insertion loss of the ridge as a function of frequency. Experimental results include, for two different surface impedances (1) the insertion loss on the curved surface, and (2) the diffraction pattern in the transition region between the illuminated and shadow zones (penumbra), measured at a horizontal distance of 0, 0.71, and 2.09 m from the apex of the ridge. Data will be compared with theoretical results presented earlier. [Work supported by NASA.]