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Table 1. Acoustic raindrop sizes and corresponding types of bubbles generated Drop Size Diameter Sound Frequency Range, kHz Bubbles Generated Splash Character Tiny <0.8 mm Silent No Gentle Small 0.8-1.2 mm Loud bubble 13-25 Type I Gentle Bubbles every splash Medium 1.2-2.0 mm Weak impact 1-30 No Gentle No bubbles Large 2.0-3.5 mm Impact Loud bubbles 1-35 2-35 Type II, III Turbulent Irregular bubble entrainment Very large >3.5 mm Loud impact Loud bubbles 1-50 1-50 Type II, III Turbulent Irregular bubble entrainment Penetrating jet Raindrop sizes are identified by different physical mechanisms associated with the drop splashes. Table from Ma and Nystuen (2005b). rain were to consist of drops all the same size, those drops would all form bubbles of identical size, and the spectrum of the sound from such a hypothetical rainfall would have a peak at the resonance frequency of the bub- bles. Conversely, if the frequencies of the spectral peaks in the sound of rain can be determined, the associated raindrop sizes can also be determined. Three types of bubbles generated from raindrops and their sound char- acteristics have been identified (Table 1). Type I bubbles are generated from small raindrops. The impact component of the small raindrops is very quiet, and each such raindrop predictably generates a small type I bubble (Pumphrey et al., 1989). The frequency range of these bubbles is a high resonant frequency that Medwin (1990) called a “screaming infant.” Type II bubbles are generated by large and very large raindrops. A large raindrop creates a large, primary, type II bubble that occurs about 50 ms after the drop impact (Medwin et al., 1992). Type III bubbles are also generated by large and very large raindrops. Large drops form an impact crater in the surface so large that tiny droplets are ejected into the air (Nystuen and Medwin, 1995). The reentry splashes of these tiny droplets produce secondary bubbles. These secondary, type III bubbles are delayed, occurring more than 100 ms after the initial drop impact. Because of the wide range in bubble size formed by the droplets, the frequency range of these bubbles is wide. Drizzle (light rain) has a unique signal because it con- sists of small 1-mm raindrops and no large raindrops. Heavier rainfall, containing both large and small drops, produces very loud sound levels across a wide frequency range (Nystuen et al., 1993). Breaking waves also produce sound from bubbles. The distributions of bubble sizes produced by breaking waves and raindrops are quite dif- ferent, however, so that the sound source, whether wind, waves, rain, or drizzle, can be distinguished by the dis- tinctive spectral characteristics of the recorded sound. The Effect of Wind on the Sounds of Rainfall The presence of wind during rain can affect the sound sig- nals created by the drops, depending on wind speed and drop size. Wind causes the rain to slant as it approaches to the sea surface, and it affects the splash of the interaction at the surface. The effect has been studied in laboratory experiments that assessed the chances that an individual drop blown by wind will produce a bubble, creating a measurable sound. Those chances decrease linearly from 100% for normal incidence (rain falling straight down) to 10% for oblique incidence (rain falling at a 20° angle from the vertical) (Nystuen, 1993). The wind naturally has a Summer 2022 • Acoustics Today 65