Low-frequency wind-generated noise produced by the impact of spray with the ocean’s surface

Ambient noise measurements for frequencies from 50 to 1000 Hz indicate that wind-generated noise is a significant source of the total ambient noise field during high-wind/sea-state conditions. The acoustic source spectrum levels for the physical mechanism involving the impact of water droplets from ocean spray, streaks, and white caps were derived as a function of wind speed. An empirical approach using the results of Franz and measurements of Morris were used in this analysis. A full analytical derivation of the wind-generated source level using Franz’s results was not possible because knowlege of certain parameters such as size and impact velocity of ocean spray as a function of wind speed is currently unknown. Recent measurements of the ’’white capping index’’ as a function of wind speed were used in conjunction with Morris measurements and Franz’s results to derive the source spectrum levels for wind-generated noise in this frequency interval. The acoustic noise from bubble oscillation in white caps was not addressed in detail in this paper since bubble size distributions and the results of Strasberg indicate that this source is significant only above 1000 Hz. Rain noise was also not included in this paper because knowledge of raindrop size distribution as a function of wind speed and geographical extent of rain within the storm area is currently not known or predictable enough to model. The wind-generated source levels from 5 to 50 Hz from a previous analysis by the author were combined with the results of this paper to yield an estimate of wind-generated source spectrum levels as a function of wind speed for 5 to 1000 Hz. A definite change of wind-speed dependence from 5- to 50-Hz interval to the 150- to 1000-Hz interval is predicted by these results since the dominant physical mechanisms are different in these two intervals. Noise measurements by Perrone, Piggott, and Morris seem to substantiate this prediction, but more measurements are needed to prove this conclusively.