Experimental study of the sound emitted by water drops impacting on a water surface

An undergraduate-level experiment is described for studying the sound emitted as a liquid drop falls into a water bath. The falling drop intercepts a light beam which, after a preset delay, triggers a flash gun and an oscilloscope. This allows simultaneous photographs of the impact and oscilloscope traces of the sounds picked up by a microphone and hydrophone to be obtained. Such studies show that most of the sound emission originates from radial oscillations of an air-filled bubble lying within the bulk of the liquid and created by the impacting drop. The observed dependence of emission frequency on bubble radius agrees with that predicted by Minnaert's formula. In addition a small low-frequency signal is recorded as the drop strikes the surface, before the detachment of the bubble. Most of this latter signal is thought to be a near-field effect.