294 



DEEP-WATER REVERBERATION 



10 



15 20 25 



WIND SPEED IN MPH 



30 



35 



40 



Figure 20. Standard reverberation level at 100 yards as a function of wind speed. 



are corrected by increasing the experimental points 

 by the value of the correction factor. That is, in 

 Figures 17 and 18 the solid points are values of 



R'it) - 10 log 



cos 6 



(4) 



By using equations (41) and (43) of Chapter 12, the 

 expression (4) obviously equals 



10 log I" + 10 log (y) - 30 log r + 10 log ^ 



- 2A. (5) 



In equation (5), if ^ can be neglected and if m' is 

 independent of range, the only dependence on range 

 is contained in the term —30 log r. Thus, with these 

 assumptions the solid points in Figures 17 and 18 

 should He on a straight line of —30 log r slope if re- 

 fraction has no effect. It is seen that the first few solid 

 points in Figures 17 and 18 do he on a straight line of 

 — 30 log r slope, but that at a range close to that when 

 the limiting ray leaves the surface, there is a sharp 



drop in reverberation level. This drop does not con- 

 tradict the theory in Chapter 12. It will be recalled 

 from Section 12.3 that equation (43) is not valid and 

 therefore cannot be expected to agree with measured 

 levels at ranges past that at which the limiting ray 

 leaves the surface. In equation (4) the correction 

 factor approaches imity and its logarithm approaches 

 zero as the range is increased, and in both Figures 17 

 and 18 the correction is practically negligible by the 

 time the sudden drop in intensity occurs. Conse- 

 quently it is not possible to ascribe the position of the 

 experimental points at ranges past the sudden drop 

 in intensity to uncertainty in the value of the correc- 

 tion factor. Thus the conclusion seems inescapable 

 that the sudden drop is due to the sound rays leaving 

 the surface. 



From the evidence in Figures 15 to 19, it appears 

 that our basic assumption, namely that surface rever- 

 beration arises from scattering in a thin layer near 

 the ocean surface, is probably correct. The observed 

 —30 log r slope in Figure 15 and in the long-range 

 portion of Figure 16 also permit the conclusion that 



