Oscillating Microbubbles Created by Water Drops Falling on Fresh and Salt Water:Amplitude, Damping and the Effects of Temperature and Salinity

摘要

Recent studies of underwater sound produced by raindrops have identified trappedbubbles as the principle sound source. Two mechanisms have been described, one for small drops (Type I) ad one for large drops (type II). A study of sound produced by large raindrops (Jacobus,l99l) showed that the underwater sound radiated by raindrops is 45% less in salt water (salinity, 35ppt) than in fresh water. The same studies also showed that bubbles radiate more energy as the magnitude of the temperature difference between the drop and surface increases. These findings are examined in more detail using the pressure decay curve of of both small and large raindrops. Using small raindrops it is shown that bubbles in salt water have a larger damping constant and smaller initial peak pressures than bubbles in fresh water. Reviewing data from Jacobus (1991) for large raindrops, increasing the absolute temperature difference between the drop and bubble showed little effect on the damping constant, but did increase peak pressure. Since the sound energy radiated by a bubble is directly proportional to the peak pressure squared and inversely proportional to the damping constant, the energy radiated by bubbles from raindrops, increases with absolute temperature difference and decreases with salinity.