depend primarily upon sound about which it 
has often been said, ''the only thing constant 
about sound in the sea is its variability." 
Since sea water is virtually opaque to 
electromagnetic waves, we can expect sound 
to continue to be the cornerstone of our de- 
tection systems for some time to come, We 
do and will continue to depend upon it not only 
for the detection of submarines by hull mount- 
ed, towed, and bottomed systems, but as the 
sensing element in homing torpedoes, in the 
location of mines, and to provide the guidance 
by which future submarines will maneuver 
through the mountains and valleys of the ocean 
bottom. We can expect, then, that the evolu- 
tion in sonar systems will continue towards 
greater sensitivity and range. One of the 
greatest stumbling blocks in the path of this 
evolution is the environment through which 
sound must propagate, and, not the least of 
the parameters involved is sea State. 
The following discussion is designed to 
review the most important effects of sea state 
on naval and merchant marine operations and 
to point out some of the present and future 
problems. Some areas of the discussion will, 
of necessity, be limited either by classifica- 
tion or the lack of adequate data, 
THEORY OF OPERATION 
EFFECTS 
Ambient Noise 
The ''composite noise from all sources 
in a given environment excluding the desired 
signal and noise inherent in the measuring 
equipment and platform"' 4 may be considered 
ambient noise, Although there are many 
sources of this noise ranging from the thermal 
agitation of water molecules to biological and 
man-made noise, the dominant noise, under 
open-sea deep-water conditions, orginates at 
the sea surface and increases with increasing 
sea state. Its importance to the Navy lies in 
the fact that as sonar systems and platforms 
224 
are made quieter it may provide the limiting 
noise background in which a signal must be 
detected. Indeed, under proper conditions 
it serves this purpose now. 
During World War II deep-water ambient 
noise was studied extensively and the results 
of many wartime measurements have been 
summarized in the form of the well-known 
"Knudsen" curves showing the spectrum of 
deep-water noise and its dependence on sea 
state. 9 These curves, reproduced in Fig. 1 
are still accepted as representative of average 
ambient noise levels in the frequency range 1 - 
25 kc. Over this range the curves indicate a 
noise level increase of about 25 db as sea 
state increases from 0 to 6, with a similar 
increase at a given sea state as the frequency 
decreases from 25 tol kc. More recent inves- 
tigations have shown that thermal noise poses 
an absolute limit to the Knudsen curves at about 
50 ke Ge and that at frequencies below 500 cps 
the values deviate somewhat from the extra- 
polated Knudsen curves. 
It is worth emphasizing that the Knudsen 
curves represent the gross averages of many 
measurements made at the various sea states. 
No distinction is made between sea and swell 
nor do the curves indicate the stage of develop- 
ment or decay of the waves under which the 
measurements were made. It is possible that 
the correlation between wave height and am- 
bient noise is such that sea state can be deter- 
mined by measuring the noise level. Far more 
detailed measurements with respect to sea 
conditions will have to be made, however, 
before this can be demonstrated, Regardless 
of this application, the unrelenting war on 
background levels alone requires a better 
distinction between ambient noise levels and 
sea conditions. 
Although the correlation between ambient 
noise level and sea state has been well dem- 
onstrated, the method by which the noise is 
produced at the surface is little understood. 
It has been suggested that such things as 
breaking waves or white caps, bubbles bursting 
at the surface after being entrained by breaking 
