Development of a submersible, baffled impedance tube for characterization of bubbly liquids

The ability to measure the acoustic properties of bubbly liquids in situ has long been a goal of the underwater acoustics community. An apparatus for characterizing these properties in the 1–16-kHz frequency range capable of submerged deployment in a controlled environment (large tank or test pond) has been developed. A water-filled, 2.5-cm-thick walled, baffled, stainless steel impedance tube was used to measure the complex reflection coefficient at the opening. The classical result for the radiation impedance of a baffled, open tube is verified, as is that for the radiated sound field along the tube axis. The test tank is then filled with air bubbles (order 500 micron mean radius and 0.1% vf) and the measurement repeated at the water/bubbly liquid interface. The acoustic properties of the bubbly liquid were obtained as a function of frequency by an inversion process using classical theory. Because of the high attenuation in bubbly liquids, the finite-sized baffle (61-cm outer diameter) is treated as being infinite in extent. The measured sound speeds and attenuations are compared with established theory. [Work supported by U.S. Navy ONR.]