Underwater rain noise : sources, spectra and interpretations

An overview over the most important results obtained over the past of noise produced in water by impacts of single droplets, artificial rain and real rain is presented. Two distinct sources form the major contributors to rainfall produced noise in the sea. The raindrop impact and the pulsations of entrained air bubbles. The impact sound is produced by raindrops of all sizes, while the bubble pulsations are connected with regular entrainment of small gas bubbles, leading to the characteristic peak spectral level between 14 and 16 kHz, and by irregular entrainment of larger gas bubbles produced by impact of large raindrops. The noise spectra are discussed and interpretations of their relations to the raindrop size and impact mechanisms are given. Relations between noise spectra and rainfall rates are discussed and some still unsolved problems are pointed out, particularly related to estimation of light and moderate rainfalls using underwater sound measurements.