Underwater sound radiation from a spherical bubble cloud encapsulated by a thin spherical shell

Abstract

Measurements of sound transmission versus frequency are performed (in the Rickover Hall Hydrodynamics Tow Tank Facility) for a submerged spherical bubble cloud that is encapsulated by a thin spherical shell. The bubble cloud is driven by a miniature spherical PZT transducer unit that is suspended in the center. A theoretical model has been developed which predicts the resonant frequencies for the case where the shell thickness has negligible effect. Therefore, the resonances will depend on the radius of the cloud and the sound speed in the bubbly media (which is a function of the air volume void fraction). A shell of urethane elastomer is molded as a single piece by using the ‘‘lost wax technique.’’ Hemispherical molds are used. The first set is for making the ‘‘inner’’ spherical wax ball. The second set forms the ‘‘outer’’ boundary of the casting for the spherical shell with filling ports on the north and south poles. The shell’s inner wax material is then melted out to complete the process. The shell is nearly acoustically transparent since ρ=1.03 g/cm3, clong∼1450 m/s and ctran∼70 m/s. The bubbly fluid consists of castor oil that has been whipped in a microblender.