Fig. 2, which results from tests conduct- 
ed by the Ordnance Research Laboratory 
(University of Pennsylvania) 8 and the Applied 
Physics Laboratory (University of Washington) 
off Key West, Florida, and Dabob Bay near 
Seattle, shows the effect of wind speed and 
grazing angle on surface backscattering 
strength. Surface reverberation is little 
affected by frequency, but, as shown, it is 
strongly dependent upon grazing angle and more 
strongly so at low wind speeds when the grazing 
angle is high. Although not shown on the curves, 
volume reverberation probably provides the 
dominant noise at very low grazing angles 
which indicates that surface reverberation, 
for most sound pathes, is troublesome only 
at relatively short ranges, 
Hoover and Urick have offered an ex- 
planation for the shape of the curves in Fig. 2. 
The flatness of the curves at low grazing angles 
is thought to be due to a near surface scattering 
layer, possibly bubbles, which was observed 
during the ORL tests when the wind speed ex- 
ceeded 10 knots. It might be added here that 
this scattering layer was not observed during 
the APL tests in Dabob Bay. The gradual rise 
at intermediate grazing angles is thought to be 
due to actual surface roughness such as ripples 
and wavelets, whereas, at higher angles 
(greater than 70 degrees) some sort of specular 
reflector is indicated. This is due, possibly, 
to small wave facets oriented normal to the 
sound beam. 
An interesting feature of the curves 
shown in Fig, 2 is that, in the APL tests, 
a plateau occurs at a wind speed of 14 knots 
with no further increase in backscattering 
strengthto the 30 knot limit of the test. The 
ORL curves do not show this plateau evenat 
17.5 knots, but the increase in backscattering 
strength over the 2.5 knot increment falls 
off with increasing wind speed indicating that 
it may be approaching a plateau. World War 
II 24 kc data using a grazing angle of 3 degrees 
shows a plateau at a wind speed of 23 knots 
228 
(see inset, Fig.2) which lends support to this 
idea, The difference in the wind speeds at 
which the World War II and the APL rever- 
beration levels reach a plateau may be due, 
at least in part, to the fact that Dabob Bay is 
almost completely surrounded by mountains 
limiting the wind to either a 5 or a 25 mile 
fetch. Six inch waves under a 20 knot wind 
have been observed in the Bay indicating a 
considerable departure from open sea con- 
ditions, 
The reason for this plateau may be due 
to the fact that the amount of small scale 
roughness that can be superimposed ona 
wave form of given dimensions is finite, and 
in fully developed seas at higher and higher 
wind speeds the tendency is towards longer 
period waves, In other words, as seen by a 
sound beam, the number of surfaces oriented 
normal to it reaches a point of diminishing 
return as wind speed or sea state increases 
beyond a given point. In any event, it is 
important that the position of this plateau be 
firmly established under open sea conditions 
and the reason for its existence determined. 
Much of the effect of surface reverber- 
ation can be circumvented by divorcing the 
sonar system or sound path from the surface, 
but hull mounted systems and active acoustic 
homing torpedoes must continue to face this 
background noise problem at short ranges, 
in the case of ambient noise, backscattering 
levels can be forecast through a prior know- 
ledge of wind speeds or sea state, but with 
considerably less accuracy. An adequate 
correlation with sea state has not been made, 
extensive measurements in Sea states greater 
than 3 have not been taken, and the scatterers 
responsible for the various portions of the 
curve have not been clearly identified. 
As 
Hydrodynamic Noise 
The noise produced directly or indirectly 
by the motion of a ship through the water is 
