Low-frequency Sound from a Bubble Plume

摘要

A significant wind-driven component of the ambient noise in the ocean exists at frequencies below 1 kHz. Bubble plumes formed by breaking waves may be important contributors to this low-frequency sound. To investigate the acoustic field radiated by a bubble plume, a series of idealized experiments was performed with a vertical water jet plunging into a pool of water to form a bubble cloud immediately beneath the surface. Fresh water and salt were both investigated. Resonance peaks were observed in the spectrum of the sound field radiated by the plume, with the resonance frequencies following fractional power-law dependencies on the jet velocity and the volume fraction of air in the jet, both of which were carefully controlled. A theoretical model of the sound generating process, in which the plume was treated as a resonant cavity, was also developed. Theoretically, two factors were found to be important in characterizing the resonance frequencies: the cone-like symmetry of the bubble cloud, and the sound speed profile within the plume, which was derived from a momentum conservation argument. For the conditions of the experiments, the theoretically predicted resonance frequencies fall below about 600 Hz, and they show almost exactly the same dependencies on jet velocity and air entrainment ratio as observed in the experiments.