There are two components to the sound generated by a raindrop splash. These are the splat (impact) of the drop onto the water surface and then the subsequent formation of a bubble under water during the splash. The relative importance of these two components of sound depends on the raindrop size.
 

Surprisingly, for most raindrops, the bubble is by far the loudest source of sound. Bubbles are one of the most important components of underwater sound (Clay and Medwin 1977). They have two stages during their lifetimes: “screaming” infant bubbles and quiet adult bubbles. When a bubble is created, the pressure inside it is not at equilibrium with the pressure of the surrounding water. The water pushes against the bubble, compressing it. As the bubble shrinks, the air trapped inside increases in pressure. This occurs so rapidly that the pressure inside the bubble becomes higher than that of the water, so it expands to equalize, again overshooting. The bubble oscillates between high and low pressure at a high frequency, creating a distinctive and well-quantified sound. The sound radiates energy, so the bubble eventually reaches equilibrium with its surroundings.
 

The frequency of the sound is well defined (Minnaert 1993) and depends on bubble radius, local pressure, local water density, and a geophysical constant. The important observation is that the size of the bubble is inversely proportional to its resonance (ringing) frequency. Larger bubbles ring at lower frequencies and smaller bubbles ring at higher frequencies. The sound radiated is often loud and narrowly tuned in frequency (a pure tone). But quickly, after just tens of milliseconds, a bubble in water becomes a quiet adult bubble and changes its role—it absorbs sound and is especially efficient at absorbing sound at its resonance frequency.

Naturally occurring raindrops range in size from about 300 microns in diameter (a drizzle droplet) to more than 5 millimeters in diameter (often at the beginning of a heavy downpour). As the drop size changes, the shape of the splash changes and so does the subsequent sound production. In laboratory and field studies (Medwin et al. 1992; Nystuen 1996), scientists identified five acoustic raindrop sizes (see Table 1). For tiny drops (diameter less than 0.8 mm), the splash is gentle and no sound is detected. On the other hand, small raindrops (0.8—1.2 mm diameter) are remarkably loud. The impact component of their splash is still very quiet, but the geometry of the splash is such that a bubble is generated by every splash in a very predictable manner (Pumphrey et al. 1989). These bubbles are relatively uniform in size, and therefore frequency, and are very loud underwater. Small raindrops are present in almost all types of rainfall, including light drizzle, and are therefore responsible for the remarkably loud and unique underwater “sound of drizzle” heard between 13—25 kHz, the resonance frequency for these bubbles.
 

Interestingly, the splash of the next larger raindrop size, medium (1.2-2.0 mm diameter), does not trap bubbles underwater and, consequently, medium raindrops are relatively quiet—much quieter than the small raindrops. The only acoustic signal from these drops is a weak impact sound spread over a wide frequency band. For large (2.0-3.5 mm diameter) and very large (greater than 3.5 mm) raindrops, the splash becomes energetic enough that a wide range of bubble sizes are trapped underwater during the splash, producing a loud sound that includes relatively low frequencies (1 - 10 kHz) from the larger bubbles. For very large raindrops, the splat of the impact is also very loud with the sound spread over a wide frequency range (1-50 kHz). Thus, each drop size produces sound underwater with unique spectral features that can be used to acoustically identify the presence of drops of a given size within the rain.

 Detection and Measurement of Rain at Sea
 Listening to Rain

 
An example of the underwater sound field generated by a heavy thunderstorm recorded in Miami, FL, is shown at left.

The variations in the sound field are associated with changes in the drop size distribution. During the heavy convective downpour, with rainfall rates reaching 150 mm/hr, very large raindrops are present and the sound field is loud across the entire spectrum (1–50 kHz). At the end of the convective downpour, a long drizzle begins. This phase of the storm has few large drops. The sound generated by small drops dominates the sound field producing the distinctive 13–25 kHz peak in the sound field associated with drizzle. At the end of the event, a few large drops are again present and once again the sound field becomes elevated below 10 kHz.

Because the sound signatures for each drop size are unique, it is possible to invert the underwater sound field to acoustically estimate the drop size distribution within the rain. Once an acoustic drop size distribution is obtained, a variety of interesting features associated with the rain can be calculated, for example, rainfall rate or median drop size.

The observed drop size distribution in the thunderstorm and the acoustical inversion based on the unique sound signatures for each drop size. Very large raindrops are present during the heavy downpour. During the following drizzle, only small and medium raindrops are present and the sound of drizzle is heard between 13–25 kHz. Still later, a few large raindrops are present and the sound levels below 10 kHz become higher once again.

(Figure by Jeffrey A. Nystuen, University of Washington Applied Physics Lab)