The influence of bubbles on ambient noise in the ocean at high wind speeds.

Abstract

Observations of ambient noise in the ocean at high wind speeds reveal significant departures in spectral shape from proviously reported values at lower wind speeds. The observations were made in open ocean conditions in Queen Charlotte Sound, British Columbia in bands centred at frequencies of 4.3, 8.0, 14.5 and 25.0 kHz; surface wind speeds up to 25 ms SUP - SUP 1 were recorded. Ambient noise at 4.3 kHz displayed a logarithmic relationship with wind speed throughout the observed speed range, similar to that reported previously. However, at the two higher frequences (14.5 and 25.0 kHz), the noise spectrum levels did not increase with increasing wind speed at the same rate and for winds above about 15 ms SUP - SUP 1 the noise levels actually decreased with increasing wind speed. Similar though less extreme behaviour was observed at 8.0 kHz. Attributes this effect to the presence of bubbles which are known to be entrained at the surface of the ocean. The hypothesis is explored with a model in which the noise source is assumed uniformly distributed over the suface and the sound is both scattered and absorbed by a thin layer of bubbles. Use of different frequencies provides information on bubble size distribution. (from authors' abstract)