Relationship between material parameters and target strength of fluid-filled spherical shells in water: calculations and observations

Thin fluid-filled spherical shells have been used as passive sonar targets for many years. They possess a large target strength which is highly dependent on the sound-speed mismatch between the fluid contained within the shell and the exterior fluid surrounding the shell. In the past, to obtain the appropriate mismatch, the interior fluid mixture contained chlorofluorocarbons (CFCs). Due to a recent production ban on CFCs, it is necessary to choose alternative fluids. The present research analyzes the backscattering target strength of a fluid-filled spherical shell as a function of several material parameters as a guide to choosing alternative fluids and shell materials. Calculations over a broad range of material values display the target strength dependence on the interior fluid parameters as well as the parameters defining the metallic shell. The range of material values presented here is far larger than any previous study addressing the focusing effects of fluid-filled spherical shells. The results should aid in determining liquid fillers and shell materials which yield the maximum possible backscattered returns. Also, several experiments were conducted with stainless steel shells containing a hydrochlorofluorocarbon (HCFC), dichlorofluoroethane. The results are compared with results found from calculations as well as from other experiments involving shells containing a previously used CFC mixture.