Underwater noise due to rain, hail, and snow

Abstract

The spectra of underwater noise generated by rain, hail, and snow have been measured in a lake at a depth of 35 m, for a variety of atmospheric conditions. Rain noise spectra, for light winds (<1.2 m s−1), have a sharp peak at 13.5 kHz with a steep falloff (∼60 dB/oct) on the low-frequency side and a more gradual falloff (9 dB/oct) on the high-frequency side. A quasi-flat spectral regime exists in the frequency interval 2–10 kHz. Wind, for speeds increasing above 1.2 m s−1, progressively rounds the peak. The spectral level at 15 kHz (i.e., near the peak) shows a linear dependence on the log of the rain rate with wind speed as a parameter. Correlation of the rain noise spectra with raindrop-size distributions suggests that low frequencies are generated by the larger drops, although this aspect of the problem needs further work. Hail noise spectra have rounded maxima appearing between 2 and 5 kHz with an approximately 10-dB falloff on each side. The spectrum of underwater sound generated by gently falling snow shows a linear increase in level, averaging 5 dB/oct, when plotted against the log of frequency.

The Freely-oscillating Bubble

“…Scrimger [110] and Nystuen [99] made the observation, which has since been confirmed [111,112]. Nystuen and Farmer [99, 100] made predictions of the shape of the initial short acoustic pulse, and nonacoustic dynamic pressure resulting from the flow field, through numerical modelling of the drop splash flow field.…”

Drop-Liquid Impact Phenomena

“…The oscillations of bubbles entrapped by falling drops furnish an explanation of the characteristic acoustic signature of light rain falling on the ocean surface: the underwater so und spectrum has a seemingly universal structure with a peak around 15kHz (Scrimger et al 1987;Prosperetti et al 1989;Medwin et al 1992). From (23) one deduces that the entrapped bubbles have an equilibrium radius of about 200 JLffi.…”

Plumbing the depths of Ligeia: Considerations for depth sounding in Titan's hydrocarbon seas

“…It is known that the underwater acoustic signatures of hail and snow on Earth are different from those of rain (e.g., Scrimger et al, 1987). Titan surface conditions do not permit methane to be at equilibrium in the solid phase, although (as sometimes occurs in Earth's warm tropics) fast-falling hail particles can in principle reach Titan's surface before they melt (Graves et al, 2008) so observing undersea hail noise in Ligeia cannot be completely ruled out.…”