On scattering from a bubble located near a flat air-water interface: Laboratory measurements and modeling

Scattering by a single bubble near a fiat air-water interface is investigat ed theoretically and experimentally. A ray-acoustic interpretation is used to describe the four scattering paths, from source to bubble to receiver, t hat determine the response of the bubble. Multiple scattering effects are a ccounted for using a closed-form solution derived from the multiple scatter ing series. Experiments are performed by placing a bubble with radius a app roximate to 425 mum on a fine nylon thread, which is approximately 100 mum in diameter and practically transparent to sound, at a distance d from the interface. The primary variable is d and it ranges from 1a to 100a. The bub ble is excited by tone bursts with a center frequency of 120 kHz, with the transducers arranged in both bistatic and monostatic configurations. Theory and experiment are in good agreement, verifying the dominant effect of the four paths in the response of the bubble, with multiple scattering playing a role for kd < 1, where k is the wave number of the medium. In the long-r ange limit our simulations agree with those of Ye and Feuillade [J. Acoust. Soc. Am. 102, 798-805 (1997)] including the shifting of the bubble's reson ant frequency. The dependence of scattering on transducer arrangement, rang e to bubble, grazing angle, and phase relation among the four paths, vis-a- vis monostatic and bistatic scattering, is discussed. (C) 2001 Acoustical S ociety of America.