TRANSDUCER HORIZONTAL 



305 



3 



RANGE IN YARDS 

 Figure 31. Surface and volume reverberation levels. 



by equation (25) of Chapter 12. The dots shovs^ the 

 lovv'est reported reverberation levels at each range, 

 and the circles the highest levels. Median values of 

 the data are shovpn by triangles. At ranges less than 

 1,500 yd, the lower triangles are the median values 

 for wind speeds less than or equal to 8 mph, and the 

 upper triangles are the median values for wind speeds 

 greater than or equal to 20 mph. 



These data are neither an adequate nor a random 

 sample, and detailed analysis of them is not justi- 

 fiable. However, some interesting inferences, which 

 should be reasonably reUable, can be drawn from the 

 data. The lower solid line in Figure 31 is a plot of 

 equation (26) of Chapter 12 against range with 

 10 log m set equal to —80 db, A set equal to 3 db 

 per kyd, and Ai set equal to zero. The upper solid line 

 is drawn with 10 log m set equal to — 60 db, with A 

 set equal to 1.5 db per kyd, and Ai set equal to zero. 

 Both lines were plotted with J^ set equal to —25 db.'' 

 If all reverberation at ranges greater than or equal 

 to 1,500 yd is volume reverberation, and if the lower 

 levels at shorter ranges are volume reverberation, 

 then the upper and lower solid curves would represent 

 estimated upper and lower limits to 24-kc volume- 

 reverberation levels. 



The middle solid curve is dravm with 10 log m set 

 equal to — 60 db, and with A set equal to 4 db per 

 kyd. This curve fits the median values of reverbera- 



tion for wind speeds less than or equal to 8 mph 

 surprisingly well at ranges from 500 to 3,500 yd; it 

 can probably be assumed that these median values 

 for wind speeds less than or equal to 8 mph represent 

 volume reverberation. The 4 db per kyd value of A 

 is gratifyingly close to the average value measured in 

 transmission studies under good conditions (see Sec- 

 tion 5.2.2). These results apparently indicate that a 

 scattering coefficient 10 log m equal to —60 db is a 

 typical value for the volume reverberation from 

 horizontally projected 24-kc soimd beams. Oc- 

 casionally, however, the scattering coefficient be- 

 comes very small, 10 log m becoming as low as 

 -80 db. 



It does not seem legitimate to attempt any con- 

 clusions based on these differences in the values of A 

 required to fit the curves of Figure 31. Both A and m 

 are highly variable quantities, and are known to be a 

 function of depth in the ocean. However, comparison 

 of the upper and median curves at ranges past 

 1,500 yd suggests that differences in the long-range 

 reverberation levels may be frequently due merely to 

 variations in A. 



It has already been pointed out that the deep 

 scattering layers discussed in the first portion of this 

 chapter will tend to increase the reverberation at 

 long range above the levels otherwise expected.^ 

 Studies of bottom "-everberation (see Chapter 15) 



