SURFACE-WAVE PERIOD MODULATIONS IN NEAR-SURFACE AMBIENT SOUND

Observations of near-surface, high-frequency ambient sound in the deep ocean demonstrate modulations at surface wave frequencies. Ambient so und was recorded at frequencies of 2 to 20 kHz at 30 m depth for a tot al of 65 hours through a variety of sea state and wind conditions. Thr oughout these observations, modulations in ambient sound level occur a t surface wave frequencies and are in phase with the surface displacem ents directly above the acoustic observations. The modulations are pre sent at all frequencies within the recorded bandwidth but are most obv ious at higher frequencies; at 8 kHz, fluctuations in signal level of about 5 dB occur. A model which assumes continuous sound generation al ong the ocean surface with source levels adjusted in proportion to wav e parameters is developed. Results from this model suggest that sound sources must be closely spaced compared to the dominant surface wave l engths 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 consisten t with these observations is small wave breaking events triggered eith er by long wave/short wave interactions, or by variations in wind stre ss over the long waves. Such sound sources should be distinguished fro m discrete sources that have been associated with larger breaking wave s at lower acoustic frequencies.