The Influence of Bubbles on Ambient Noise in the Ocean at High Wind Speeds

Observations of ambient noise in the ocean at high wind speeds reveal significant departures in spectral shape from previously reported values at lower wind speeds. The observations were made in open ocean conditions in Queen Charlotte Sound, British Columbia, in bands centered at frequencies of 4.3, 8.0, 14.5 and 25.0 kHz; surface wind speeds up to 25 m s−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 frequencies (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 m s−1 the noise levels actually decreased with increasing wind speed. Similar though less extreme behavior was observed at 8.0 kHz. We attribute 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 surface and the sound is both scattered and absorbed by a thin layer of bubbles. Inferred bubble population densities are found to be consistent with previous estimates based on photographic observations. However the wide range of wind speeds encountered allows us to derive a more complete relationship between population density and wind speed and the use of different frequencies provides information on the bubble size distribution. The attenuating effect of bubbles has implications for the use of ambient noise for remote sensing of processes at the air-sea interface.