Effects of salt on bubble acoustic radiation in water

Recent measurements of laboratory-generated noise by breaking waves exhibit an increase in sound-pressure levels for salt water compared to fresh water over a broader range of frequencies [A. R. Kolaini, J. Acoust. Soc. Am. 103, 300–308 (1998)]. The surprise increase in sound-pressure levels inspired the study of the effects of salt on the sound radiation by single bubbles released from various size needles. A needle assembly and a false tank (2×2×4 ft), with acoustically transparent walls placed in the middle of an anechoic tank (12×12×8 ft), were used to study the acoustical characteristics of bubbles released from needles. The false tank was filled with water that contained various percentages of sodium chloride (NaCl). In this paper, the results of experiments to examine the variations in radiated sound pressure and the change in the damping coefficient of bubbles as the salinity level increased are reported. The effects of a surfactant agent such as Triton™ 100-X and other mineral acids such as HCl on the bubble acoustic radiation are discussed. The observations show that the change in the local surface tension may not alter the acoustic radiation, whereas the local influence of the ions on water structure, possibly related to the electric repulsion, may play a dominant role in altering the sound pressure and significantly reducing the quality factor of the bubble sound.