P. M. Kendig 237 
~—~THERMAL-NOISE LIMIT FOR AN 
OMNIDIRECTIONAL HYDROPHONE 
OF UNIT EFFICIENCY 
PRESSURE LEVEL IN I-CPS BAND (db vs | dyne/em®) 
“I29 10 100 
FREQUENCY (kc) 
Fig. 13.1. Deep-water ambient noise levels (Knudsen curves). 
noise over this frequency range varies approximately as the 1.8 power of the 
wind speed. This variation indicates that the noise originates at the surface, 
but very little is known about the mechanism by which it is generated. The 
breaking of waves may be the primary contributor to this noise; but, for sea 
states below those at which waves break, some other mechanism for the pro- 
duction of noise must be postulated. In the absence of peculiar occurrences such 
as biological noise cr the noise due to falling rain, this type of noise is dominant. 
At lower frequencies, somewhere below 1 kc, the variation with frequency is 
decreased and the levels are no longer greatly dependent upon wind force. 
However, at still lower frequencies, the ambient levels appear to increase more 
rapidly as the frequency is lowered. 
The variation of ambient levels with depth in deep water was obtained by 
Lomask and Frassetto [3] with the Bathyscaphe Trieste. These measurements, 
which covered only the low frequency range up to about 300 cps, indicated a 
rather strong depth dependence except in calm seas. The level was shown to 
decrease by almost 16 db from the surface to a depth of 3000 m for a sea state 
of 2 and a relatively narrow band centered at 68 cps. The levels decreased with 
depth at other frequencies in this range but to a lesser extent, the decrease 
being only about 6 db at 10 cps. The measurements also indicated some strati- 
fication effects, but it did not seem that these were associated with the sound 
channel. 
Wenz [4] has recently shown a very interesting variation of low-frequency 
acoustic ambient noise levels in the ocean. His studies reveal a periodic varia- 
tion in underwater acoustic ambient noise levels at frequencies between 20 and 
100 cps which has the following characteristics: 
