Water fountains produced by intense standing waves in air.

Abstract

An acoustic demonstration found in several science museums involves a horizontal standing wave tube that contains a shallow layer of liquid. When the liquid is water and the sound pressure level of a standing wave in the air above the water exceeds approximately 160 dB (re: 20 μPa) at frequencies below 1 kHz, vigorous fountains occur at the nodes in the pressure field. The introduction of a surfactant in the water causes clusters of fountains to appear near the nodal planes at somewhat lower sound pressure levels. Standard quasilinear theory provides a reasonable description of the acoustic streaming and dc pressure in the air prior to the appearance of fountains. The fountains seem to result from the ejection of droplets by inner streaming vortices that are formed above the air–water interface. The water fountains and streaming patterns are discussed in relation to Andrade’s observations [Proc. R. Soc. London Ser. A 230, 413–445 (1932)] of solid particulate motion produced by intense standing waves in air. [Work supported by a Rockwell Graduate Fellowship, the ONR, and the Packard Foundation.]