waves, the pressure effect of capillary wave 
trains, or atmospheric turbulence created by 
wind over the roughened water surface may be 
responsible, Certainly, the first two of these 
produce some noise, but these conditions do 
not become prevalent until about sea state 3. 
Fig. 1 shows that of the 25 db increase in 
spectrum level between sea state 0 and 6, 19 db 
of this occurs between sea state 0 and 3, and 
10 db between sea state 0 and 1 with the maxi- 
mum significant wave height being 4 feet and 
1/3 foot respectively. This indicates that the 
greatest portion of the noise increase must be 
explained by factors other than breaking waves 
and bubbles entrained by such waves. 
The declining increment in noise level 
above sea state 3 itself may be due to the 
filtering effect of a shallow bubble layer which 
scatters the sound back to the surface thereby 
preventing a large portion of it from reaching 
the hydrophone, 
While ambient noise can be produced by 
fluctuations in hydrostatic pressure, it is 
doubtful that capillary waves contain enough 
energy to account for much of the observed 
noise. Wind, on the other hand, certainly 
possesses enough energy and may play a 
significant role in sound production, but more 
investigation is needed before this can be de- 
termined, 
As sonar systems become more sophis- 
ticated, high ambient noise levels will become 
increasingly bothersome unless discriminated 
against or operated around, Too, since the 
noise is virtually independent of depth, sonar 
systems designed to escape surface reverber- 
ation, adverse thermal conditions, and platform 
noise will still have to contend with ambient 
noise, The Knudsen curves coupled with wave 
forecasts will make it possible to do a passable 
job of forecasting ambient noise levels under 
deep-water conditions, Forecast accuracy 
cannot be improved, however, without a more 
detailed knowledge of noise level with respect 
to sea conditions, 
226 
Surface Reverberation 
Reverberation in underwater sound is the 
sound scattered back towards the source by 
various inhomogeneities of the environment. 
It is customarily designated surface, volume, 
or bottom reverberation depending upon the 
source of the scatterers. The irregularities 
of the surface and bottom are primarily re- 
sponsible for the reverberation originating at 
these two surfaces, while biological organisms, 
particulate matter, and possibly turbulent and 
thermal microstructures are considered re- 
sponsible for volume reverberation. 
With observed increases in surface back- 
scattering strength of around 28 to 30 db at 
wind speed as low as 18 knots, providing a 
level of about-26 db for low grazing angles and 
considerably greater for high angles (see Fig. 
2), the importance of surface reverberation to 
the Navy is obvious. In deep water it will often 
provide the limiting noise level, a fact that, as 
in the case of ambient noise, can only become 
more serious as the noise from other sources 
is reduced, 
Although there can be little doubt that 
surface reverberation is caused by surface and 
near-surface irregularities, and increases 
with increasing sea state, the data from most 
investigations have been more easily correlated 
with wind speed than with sea state, Since the 
wind can only influence surface reverberation 
through its effect on the Sea surface the reason 
for this better correlation with wind speed may 
be due to one or more of the following: (1) the 
lesser accuracy to which sea state is usually 
measured as compared to wind speed; (2) the 
present lack of accurate correlation between 
bubble density and sea state; (3) very high 
frequency waves are a more rigid function of 
wind speed than of sea state; and/or, (4) the 
mere determination of the significant wave 
height does not, by itself, give any idea of the 
stage of development of the wave spectrum, 
