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System (GOOS; see goosocean.org). Recognizing the importance of oceanographic information for society on the one hand (tsunamis, El Niño events, climate) and the difficulty and expense of obtaining oceanic data on the other, the GOOS comprises a variety of shared, sustainable platforms, from autonomous floats to long- term moorings to scientific cabling systems across the sea floor and terminating on shore. The availability of these platforms means that the acoustic rain gauges have the potential to become ubiquitously deployed rain gauges for global ocean coverage. In addition, the measurement of ocean sound is recognized as an essential ocean vari- able for GOOS, and the hydrophones of the rain gauges can be employed to measure sound generally. A Bit of History The nature of splashes of droplets hitting a water sur- face is a surprisingly complicated subject that has long attracted scientific interest. More than a century ago, photographs were used to describe the detailed process by which droplets strike the water surface. In that process, a drop will often create a small, temporary crater, splashes, or waves on the water surface. More importantly about rain-producing sound, a raindrop will often also form a cavity or entrain a small bubble of air when it enters the water. The phenomenon of a drop striking a water surface is controlled by surface tension or the attractive forces of liquid molecules along the water surface. The book by Worthington (1908), A Study of Splashes, has beautiful images of droplet splashes (Figure 1). Medwin and colleagues (1992) used an abandoned ver- tical utilities shaft with an anechoic tank at the bottom to build a unique facility for raindrop sound research. Rainfall could be simulated because the shaft was a 26-m- tall air chamber that allowed falling water drops to reach terminal velocity. Using this shaft, distinctive underwater sounds of different drop sizes and their drop splashes could be examined. They could also identify the different acoustical characteristics of the drop sizes (Medwin et al., 1992). Concurrent field studies developed the use of underwater sound to detect and quantify rainfall (Nyst- uen, 1986). From this work, passive acoustic instruments were developed to use the oceanic ambient sound field to measure rain rate, drop size distribution, and other properties (Nystuen, 2001). Such instruments are called “passive” because they do not require deploying an “active” acoustic source. Extracting the Rain Signal from Noise Ocean ambient sound originates from two basic sources: the activities of humans (anthropogenic sound: shipping, pile driving, construction) and nature (rain, wind, wave breaking, fish, marine mammals, earthquakes). Differ- ent sources of ocean ambient sound contribute to the overall sound level in different frequency ranges; here, we need only concern ourselves with the sound from the wind or rain. Oceanic ambient sound is measured by a hydrophone mounted on a suitable platform that provides power for data sampling, recording, storage, and transmittal. At any given time, the local acoustic pressure fluctuations (in micropascals \[μPa\]) measured by a hydrophone result from the total contributions of the myriad sounds of the ambient environment. For a given frequency, the contributions to the pressure fluc- tuation from different sources cannot be distinguished. But if sources have unique frequency spectra (sound pressure level frequency), then the times when only those sources are Figure 1. A series of photos shows the impact process of a droplet falling into the water at different times starting at time (T) = 0. After the initial impact (1), a crater is formed and the crown- shape ring rises (3 and 4) and falls (6 and 7). Then a central column emerges (8 to 10), with ripples propagating outward (12). Adapted from Worthington (1908). Available at bit.ly/3Cq4aZS. Summer 2022 • Acoustics Today 63