242 Lecture 13 
The results of these measurements indicated both an azimuthal and a vertical 
variation in the ambient noise. Azimuthal variation was observed principally 
in the outputs of array beams directed in the upper hemisphere. The distri- 
butions normally were elliptical in shape. It was observed that the maximum 
beam output for a given elevation angle usually aligned itself with the wavefronts 
of the swells while minimum outputs were perpendicular to the swells. This fact 
suggests a nonuniform radiation pattern of the whitecap or wavelet source; 
transmission in directions normal to the wavefront is impeded by the troughs, 
while lateral transmission along the ridges is less affected. 
The variation of the ambient in the vertical plane was generally found to be 
much greater than that in azimuth. Except for very low sea states, for which 
the array output was nearly constant for all directions, the array output was 
generally much larger when the beam was directed vertically upward. Figure 
13.5 is one example of the true-field vertical ambient distribution at a depth of 
560 ft in 2000 fathoms of water for a No. 3 sea state. In this figure, corrections 
have been made for the finite beamwidths of the arrays. 
From the nature of the distribution shown in Fig. 13.5 and other similar re- 
sults, the following picture of the mechanism ofnoise generation and distribution 
in the ocean has been constructed. Noise in the deep ocean is presumed to be 
a superposition of two fields. The first is that which exists at sea-state zero 
prior to the development of the wave motion and/or whitecaps. Its distribution 
0° 30° 
60° 
120° 
Fig. 13.5. True-field vertical distribution 
(No. 3 sea state; three-array depth, 560ft). 
