UNDERWATER SOUND GENERATION BY RAINFALL

This paper presents experimental findings on the mechanism of underwater sound generation by rainfall. Although using underwater sound to measure the rate of rainfall is a promising technique, conflicting models have been proposed for the spectral contributions of the two rainfall sound sources (raindrop impacts on the water surface and air bubble resonances) and correlating rainfall rate to spectral level has proven difficult. In order to resolve these problems, high-speed data acquisition and processing of underwater sounds recorded in a lake under real rain and artificial raindrop conditions were used. The two rainfall sound sources have been identified in the time domain and their respective contributions to the long-term spectrum have been determined. Bubble resonances were found responsible for the spectral peak around 13-15 kHz and raindrop impacts were found responsible for a broadband spectrum with a negative slope. The poor correlation reported in literature between the rainfall rate and the level at 13-15 kHz is now explained by the sensitivity of bubble generation to raindrop distribution and surface conditions. The better correlation obtained outside this frequency range is explained by the systematic occurrence of raindrop impacts.