Whether produced by a leaky tap dripping into a sink or by raindrops falling onto a lake, the distinctive and at times irritating plink of a small drop falling into a larger body of liquid has been the subject of scientific curiosity for more than a century. In 1989 Hugh Pumphrey and colleagues determined that the source of the sound is a pulsating air bubble trapped under the surface of the water during the drop impact. However, until recently it was not clear just how the underwater bubble produces the plink heard above water by the human ear. In this Quick Study we describe a project that took advantage of devices unavailable to Pumphrey to better understand how the bubble generates its characteristic sound.

Figure 1 shows a schematic of our experimental setup. A syringe generated 4.0-mm-diameter water drops that fell into a small fish tank filled with water. We,...

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