Measurements of the scattering of sound from laboratory breaking waves

Abstract

The results of a laboratory experiment to characterize the backscattered sound field from the bubble cloud generated by spilling breaking waves are described. Gravity waves were generated by a computer controlled plunging-type wavemaker along the length of a 12.7-m-long channel where they were made to break in a 3.6 m×3.6 m×2.4 anechoic tank. An underwater F42A transducer with a special parabolic reflector was used to generate incident bursts of sound ranging from 15 to 40 kHz. This procedure was conducted in the presence of breaking and nonbreaking gravity waves in order to isolate the acoustic scattering strength of the bubble clouds from surface roughness. Detailed measurements of the average void fraction of the bubble cloud at the instant the sound was incident were made. These observations show that there is a significant increase in backscattering strength from the bubble cloud when compared to surface roughness. The scattering strength of the bubble cloud, with an average void fraction of 0.33%, is also shown to increase as the incident frequency approaches the resonant frequency of the individual bubbles in the cloud. A simple theory will be given to predict some of these observations.