The anatomy of underwater rain noise

Abstract

When rain falls onto a large body of water it produces dominating underwater sound over a broad range of audio frequencies. Laboratory studies using more than 1000 single drops, covering the complete size range of actual rain drops at their terminal speeds, have now shown that the complete underwater spectrum of rainfall sound can be dissected into the impact and microbubble sounds produced by four acoustically distinctive ranges of drop diameters D. These are defined as "minuscule" drops (D<0.8 mm), "small" drops (0.8 mm<D< 1.1 mm), "mid-size" drops (1.1 <D<2.2 mm), and "large" drops (D>• 2.2 mm). A minuscule raindrop produces only a very weak, almost undetectable, short duration impact noise. A small drop at terminal speed and at local, near-normal incidence, radiates measurable broadband impact sound followed by the very much stronger sound of a "type I" damped microbubble oscillating at frequencies near 15 kHz. A mid-size raindrop radiates only impact sound. Large raindrops, which comprise the major volume of moderate to heavy rainfall, produce an impact sound and a dominating, "type II," "primary" oscillating microbubble of characteristic frequency 2 to 10 kHz depending on the drop diameter. Also, large drops often generate weaker sounds from "secondary" bubbles. The average acoustic energy spectra of large raindrops are distinctive functions of their diameters, the salinity of the surface water, and the temperature difference between the drop and the surface water. When the underwater acoustic intensity spectrum during heavy rain is calculated from the single drop acoustic energy spectra and the drop size distribution, it compares quite well with ocean measurements. The gas injection at the airwater interface is calculated from the probability of bubble formation during a heavy rainfall.

Drop-Liquid Impact Phenomena

“…The analysis beyond tc is somewhat involved, but some general conclusions can be drawn on the basis ofthe following dimensional argument (Oguz and Prosperetti 1991 ). As expected, and as confirmed by the data of Pumphrey and Elmore ( 1990) and Medwin et al ( 1992), the radiated pressure has a dipole character. The total power radiated by an acoustic dipole constituted by a source-sink pair each of strength q and separated by a distance f is of the order of (see e.g.…”

Jet dynamics post drop impact on a deep pool

“…Note that, in this case, the terminal velocity (V i = 8.9 m s −1 ) would be reached after a free fall of around 15 m. At this height the drop impact does not produce a unique jet but rather a complicated splash shaping a canopy [27,28], which is not considered in this paper. The liquid is contained in a transparent tank taken sufficiently large (85 × 105 mm 2 and 45 mm deep) to rule out the confinement effects.…”

“…This canopy meets at 2469-990X/2017/2(2)/023601 (13) 023601-1 ©2017 American Physical Society the center and two opposite jets are ejected upward and downward. In this case, the downward jet stops the rise of the central jet generated from the cavity collapse [27,28]. Droplets are ejected in all these splashing regimes and they would all need to be quantified in order to improve the existing models of sea spray aerosols, for example.…”

Integrated acoustics systems for ocean observatories

“…In the frequency range from 500 Hz to 50 kHz the dominant sources of underwater ambient sound in the ocean are bubbles generated by breaking waves and raindrop splashes 9,10) . Fig.…”