The anatomy of underwater rain noise

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 m...