Surface wave period modulations in near surface ambient sound

Abstract

Observations of near‐surface, high‐frequency ambient sound in the deep ocean demonstrate modulations at surface wave frequencies. Ambient sound was recorded at frequencies of 2 to 20 kHz at 30 m depth for a total of 65 hours through a variety of sea state and wind conditions. Throughout these observations, modulations in ambient sound level occur at surface wave frequencies and are in phase with the surface displacements directly above the acoustic observations. The modulations are present at all frequencies within the recorded bandwidth but are most obvious at higher frequencies; at 8 kHz, fluctuations in signal level of about 5 dB occur. A model which assumes continuous sound generation along the ocean surface with source levels adjusted in proportion to wave parameters is developed. Results from this model suggest that sound sources must be closely spaced compared to the dominant surface wave lengths to reproduce the observed fluctuations. For the data analyzed, sound sources must be continuous on a scale of 10 m or less to account for the observations. A possible cause for sound generation consistent with these observations is small wave breaking events triggered either by long wave/short wave interactions, or by variations in wind stress over the long waves. Such sound sources should be distinguished from discrete sources that have been associated with larger breaking waves at lower acoustic frequencies.