INTRODUCTION 



1 2 



The classic curves of Knudsen (fig. 1) and Wenz (fig. 2) 

 show the relationship between ambient noise in the ocean and 

 storm or wind activity in terms of wind speed or wave height 

 measured directly, or very nearly directly, above the area in 

 which the sound measurements are made. The Knudsen curves, 

 which treat the frequency range from about 100 c/s to 10 kc/s, 

 describe a consistent dependence of noise level on wind speed and 

 wave height. The Wenz curves treat frequencies down to 1 c/s, 

 and reflect the lack of such dependence below 100 c/s in much of 

 the experimental data. 



It is not well known what relationships, if any, exist between 

 ambient-noise levels in the very-low frequencies and storm activ- 

 ity occurring at great distances from the hydrophone. The hydro- 

 phones that provided data utilized for this report were located in 

 deep ocean areas providing conditions favorable for the reception 

 of any low-frequency noise propagated from distant storm centers. 



In a pilot study 3 ambient -noise levels were considered to be 

 related to the action of winds at the interface. Wind fields over 

 increasing areas to as far as 1400 nautical miles from the hydro- 

 phone were considered. 



In general, the trends suggested a dependence of ambient 

 noise on wind speed in both the high and low ends of the frequency 

 range considered. The dependence decreases with range, but the 

 decrease is smaller at frequencies below 25 c/s. Anomalous 

 results were noted at 20 c/s. 



This study examines the dependence of ambient noise in the 

 frequency range of 10 to 400 c/s on wave height. Wave height is 

 measured both in very near proximity to the hydrophone and at 

 great distances. In addition, changes in noise level are compared 

 with changes in wind speed measured in moderate proximity to the 

 hydrophone . 



