TRANSDUCER HORIZONTAL 



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 o CHARLIE 



10 



15 20 25 



WIND SPEED IN MPH 



30 



35 



40 



Figure 21. Standard reverberation level at 1,500 yards as a function of wind speed. 



there are times when the surface scattering coefficient 

 m' is independent of the angle of incidence of the rays 

 on the surface. However, it is not usually possible to 

 fit the observed reverberation intensities with equa- 

 tion (39) of Chapter 12, if m' is assumed independent 

 of range. Thus, while the basic assumptions leading 

 to that equation are probably correct, it cannot be 

 said that the factors involved in surface reverbera- 

 tion are completely understood. 



More illustrations of the dependence of surface 

 reverberation on range may be obtained from a 

 memorandum issued by UCDWR,^ where extensive 

 measurements of observed deep-water reverberation 

 levels at 24 kc are summarized. This summary is 

 based on data obtained on 6 cruises in the period from 

 November 26, 1943 to September 1, 1944. About 110 

 reverberation curves were obtained, each an average 

 of five successive pings. The ping lengths used varied 

 from 16 to 80 yd, but in the following curves all data 

 have been corrected to the standard 80-yd length; in 

 other words, the following graphs are all plotted in 

 terms of the standard reverberation level. All the 

 data were obtained with the JK projector at a depth 

 of 16 ft on the USS Jasper with the transducer axis 

 horizontal. 



Figure 20 is a plot against wind speed of all the 

 reverberation levels measured at a range of 100 yd. 

 All seasons of the year are represented. Thermal pat- 

 terns were of MIKE, CHARLIE, and NAN types 

 (see Chapter 5), represented respectively by dots, 

 circles, and triangles. In Figure 20 a systematic in- 



crease in reverberation level of about 35 db is ob- 

 served as the wind increases in velocity from zero to 

 20 mph. At wind speeds of 8 mph or less, there is 

 little systematic dependence on wind speed. At 

 greater speeds the level rises sharply, up to speeds 

 of 20 mph or more. Increase of wind speed beyond 

 20 mph has little systematic effect. This dependence 

 on wind speed is correlated with the roughness of the 

 sea. At 8 mph the wind is strong enough to roughen 

 the surface appreciably; occasionally wavelets may 

 slough over, but no well-developed whitecaps are 

 observed. At about 10 mph small whitecaps begin to 

 appear. When the wind has reached 20 mph the sea 

 is liberally covered with whitecaps. The detailed de- 

 pendence of the appearances of the sea on wind force 

 is described in a Navy manual.' Apparently, as the 

 wind speed increases beyond 20 mph, the resulting 

 increase of whitecaps causes little, if any, additional 

 increase in reverberation. 



The median values of the standard reverberation 

 level at 100 yd, as a function of wind speed up to 

 20 mph, are roughly described by the equation 



R= - 118 -h 10 log (1 -(- 2.5 X 10- V) (6) 



where u is the wind speed in miles per hour. This 

 equation is represented by the solid line in Figure 20. 

 Beyond 20 mph, the fimction is assumed to be con- 

 stant at i? = -83 db. 



The reverberation level at long range has a 

 markedly different wind-speed dependence from that 

 at short range. The data in Figure 21 are taken from 



