Measurement of underwater sound produced by a hydrophobic sphere entering water.

A hydrophobic sphere, freely released from a certain height, entering water makes a cavity behind it. This paper visually demonstrates underwater sound produced by the water entry with the use of a hydrophone synchronized with a high-speed camera. The acoustic signal of the underwater sound recorded can be found to include two predominant frequencies. A remarkable finding is the fact that the higher predominant frequency sound can be excited by the breathing mode of a residual bubble behind the sphere after the cavity pinching off. In addition, the lower frequency sound is predicted to be driven by the oscillation of the free surface caused by the impact of the sphere. [ABSTRACT FROM AUTHOR]