Laboratory measurements of thermal noise

Abstract

An experimental study was undertaken of the aspect of underwater ambient noise which normally receives only a cursory reference—namely, thermal noise. A brief survey of the theory and literature, and consideration of how to separate amplifier noise from hydrophone noise, is followed by laboratory measurements on piston and spherical hydrophones. Results are presented of impedance and power spectral density measurements undertaken in water and other fluids. Measurements with the piston hydrophones were also performed as a function of distance from tank and water surface boundaries or from another hydrophone. The cross power spectral densities are presented as a function of separation distance between pairs of hydrophones. All hydrophones had their free field receive sensitivities and beam patterns measured, but the spectral densities have not yet been converted to pressures as the more interesting features are seen near resonances, where phase calibrations will also be required.