Low Frequency Noise from Breaking Waves

摘要

This document contains an invited paper given at the Conference on NaturalPhysical Sources of Underwater Sound at the University of Cambridge, July 1990. Recent experiments confirm the production of sound by breaking waves at lower frequencies (30 to 500 Hz). Individual breakers produce impact noise as well as a random collection of individual spectral events. Measured ocean ambient noise spectrum levels increase at less than 1 dB per octave toward a broad maximum, which has a weak wind speed dependence between 300-500 Hz. Noise intensities (< 500 Hz) are a function of wind speed (U) to the 2n power with 1.3 < n < 2.5 and a value of n=1.5 at 200 Hz. The production of noise in this region has a dipole characteristic. Breaking waves produce an impact, bubble plume, and bubble cloud. The dynamic evolution of these plumes and clouds provides a mechanism for sound production. Since the initial plume and cloud have appreciable void fractions, compressible resonant behavior of these structures as a whole or as multiply connected regions can be represented as compact acoustic monopoles and dipoles. (jd)