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

Scattering by a single bubble near a flat air-water interface is investigated theoretically and experimentally. A ray-acoustic interpretation is used to describe the four scattering paths, from source to bubble to receiver, that determine the response of the bubble. Multiple scattering effects are accounted for using a closed-form solution derived from the multiple scattering series. Experiments are performed by placing a bubble with radius a approximately 425 microm on a fine nylon thread, which is approximately 100 microm in diameter and practically transparent to sound, at a distance d from the interface. The primary variable is d and it ranges from 1 a to 100 a. The bubble 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-range 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 resonant frequency. The dependence of scattering on transducer arrangement, range to bubble, grazing angle, and phase relation among the four paths, vis-à-vis monostatic and bistatic scattering, is discussed.