Underwater sound produced by individual drop impacts and rainfall

An experimental study of the underwater sound produced by water drop impacts on the surface is described. It is found that sound may be produced in two ways: first when the drop strikes the surface and, second, when a bubble is created in the water. The first process occurs for every drop; the second occurs for some impacts but not others.

A range of conditions is described in which a bubble is produced for every drop impact, and it is shown that these conditions are likely to be met by a significant fraction of the raindrops in a typical shower. Underwater sound produced by artificial as well as real rain is reported. A comparison between artificial and real rain noise power spectra shows some deviations due to different drop-size distributions.

Addition of surface tension reducing liquids to the water in the test tank caused a disappearance of the characteristic spectral peaks in the frequency range 14–16 kHz. These peaks have been observed by several scientists during measurements of real rain. Our findings provide evidence for the theory that the 14- to 16-kHz spectral peak is caused by the ringing of bubbles entrained in the water by the drop impact process.