Sound generation by water drop impact on surfaces

Abstract

In this work, we experimentally study the acoustic response of water drop impacts onto a variety of surfaces using both audio recording and high-speed imaging. Audio recordings of the airborne sound emitted by single drop impacts on deep pools, thin liquid films, and dry aluminum and masonry surfaces are captured in the laboratory. Longer audio recordings are also collected for natural rainfall on wet concrete surfaces. These airborne sound pressure versus time measurements are converted to power spectra using the fast Fourier transform. Each impact type results in unique features in the spectra from 1 to 20 kHz that can be related to the impact and subsequent fluid phenomena. For example, in the case of water drops impacting a deep pool of water, we observe two distinct sound events: one as the drop breaks through the free surface of the pool with frequency content centered around 10 kHz and then approximately 50 ms later at the time of air cavity collapse and Worthington jet formation with frequency content around 4 kHz. A Helmholtz resonator model for the collapse of an air cavity describes the observed acoustic signature well. The acoustic spectra of various natural rainfall events on wet surfaces are comparable to those of discrete drops recorded in the laboratory, but upshifted in frequency.