Low-frequency noise from a bubble plume formed by a plunging water jet

Abstract

Wind-driven, low-frequency ambient noise in the ocean may be due to bubble clouds created by wave breaking. To investigate this idea, a laboratory experiment was performed in which a vertical jet of fresh water plunged into a fresh water pool, to form a conical plume of bubbles beneath the surface. Both the jet speed, uj, and the air entrainment ratio, q, were carefully controlled and monitored. The power spectra of the sound from the plume, measured with an adjacent hydrophone, exhibit a sequence of nonuniformly spaced peaks below 1 kHz, which are associated with the resonances of the bubble-plume cavity. Each eigenfrequency scales accurately as (1/uj)1/2 and approximately as (1/q)1/4. A theoretical analysis of these scalings, based on a momentum-flux argument, shows that the sound speed profile within the plume varies as the square-root of depth down the axis. Using this square-root profile in the wave equation, an analytical solution for the pressure field within the plume is obtained, from which an explicit expression for the eigenfrequencies is derived. These theoretical eigenfrequencies, predicted from the conical geometry and square-root sound speed profile, exhibit the same nonuniform spacing and inverse-fractional power-law scalings as seen in the data. [Research supported by ONR.]