P.M. Kendig 241 
The available information indicates that, at a frequency of 10 kc, rain can 
raise the average underwater noise level 15 to 25 db above the level indicated 
by the Knudsen curves for a given sea state. 
13.1.5. Directional Characteristics 
It has been commonly assumed that ambient noise is isotropic; that is, the 
orientation of a directional hydrophone would have no effect on the detected level. 
However, R. J. Urick [9] has shown that for sounds originating at the surface, 
the variation of sound level with tilt angle will depend upon the directional char- 
acteristics of the hydrophone and the laws governing the radiation from the sea 
surface. He assumed the radiation to be distributed according to some power 
n of the cosine of the angle of radiation measured from the vertical. This analy - 
sis showed that for n equal to 0,1, and 2, the ambient levels measured by a 
searchlight hydrophone will vary with tilt angle. 
The directional characteristics of ambient noise at ultrasonic frequencies 
in water depths of 600 ft and hydrophone depths of 100 ft have been studied at 
the Ordnance Research Laboratory of The Pennsylvania State University. Meas - 
urements made with a searchlight hydrophone indicate that the ambient noise 
level measured in a vertical plane has two different characteristic distributions 
that appear to be functions of sea state. The data at sea states 1 and higher show 
the ambient noise level to be highest in the quadrant between the horizontal axis 
and the vertical axis, normal to the surface. The data taken at sea-state zero 
show that the peak in the ambient noise level usually occurs in the horizontal 
plane. A few exceptions to this do show a distribution at zero sea state similar 
to that obtained at the higher sea states. It would appear then that a transition 
between these two characteristic distributions occurs within the range of zero 
sea state. It was found that the distribution in the vertical plane was not strongly 
dependent on hydrophone depth at 60 or 100 ft. 
The maximum levels were sometimes as much as 10 db higher than the 
levels obtained with the hydrophone axis directed horizontally. These measure - 
ments also indicate that the levels are in close agreement when the hydrophone 
is directed either horizontally or vertically upward. This behavior did not fit 
any cosine law very well, but in some cases it tended to agree with the cos"9@ law 
where n has a value somewhere near unity. 
The directional distribution of ambient noise in the ocean has been studied 
recently and more extensively by the Marine Physical Laboratory of the Scripps 
Institute of Oceanography. This study is reported by B.A. Becken [10]. A 32- 
element, three-dimensional receiving array of hydrophones was employed, 
arranged in the form of a great stellated icosahedron and operating in the fre- 
quency range of 750 to 1500 cps. The 9-ft-diameter array was suspended from 
floats in from 1000- to 2000-fathom deep ocean and at array depths between 150 
to 1000 ft. The digital multibeam steering (DIMUS) technique used in conjunction 
with this array provided 32 simultaneous searchlight patterns with principal 
axes distributed uniformly over all possible directions (47sr). All 32 hydro- 
phones were used in forming each beam. More detailed discussions of this sys- 
tem are given by V.C. Anderson [11, 12]. 
