Characteristics of sound radiation from large raindrops

Abstract

Drop diameters from 2.7 to 4.5 mm are common in heavy rainfall. By using a hydrophone with a flat response up to 300 kHz and A/D conversion up to 1 MHz the components of the sound radiation for these large terminal-velocity drops in the time and frequency domains can be identified. At these high sampling rates it is possible to see notches on the lagging edge of the impulse trace. These notches at 100 kHz and higher frequencies are probably caused by internal drop reflections. Lower frequency oscillations with a range of frequencies less than 10 kHz occur after a time lag of 30 ms or more. Both the frequency of oscillation and the time lag appear to be the functions of drop size. Although the amplitudes of these components are comparable for individual drops the energy radiated by the low-frequency oscillation is greater due to its longer duration. These early results suggest that it may be possible to ascertain the drop spectrum (number of drops as a function of diameter) in rainfall from the spectrum of the underwater sound of the rain. [Research supported by ONR.]