OCEA^OGRAPH^C CONDITIONS 



411 



Section 21.5 introduced the concept of an attenua- 

 tion coefficient to describe more accurately the trans- 

 mission loss. This attenuation coefficient varies both 

 with the oceanographic conditions and the frequency 

 of the echo-ranging sound. From a quantity of trans- 

 mission data at 24 kc two empirical formulas were 

 suggested for estimating the attenuation coeffi- 

 cient,^ equations (5) and (6) in Chapter 21. They 

 are 



170 

 n = 3.5 + — {!) 



for the hydrophone above the thermocline, and 



« = 4.5 + f (2> 



for the hydrophone below the thermocline, where a 

 is the attenuation coefficient in decibels per kiloyard 

 and D the depth of the thermocline in feet. The prob- 

 able error of this estimate is about 2 db per kyd. 



measurements at 24 kc on different S-boats, calcu- 

 lated by assiuning an attenuation coefficient of 5 db 

 per kyd, showed such deviations that a marked de- 

 pendence on the particular submarine used was sug- 

 gested.^' The particular submarines used are desig- 

 nated in the first column of Table 4, while the target 

 strengths, varying from 7 to 25 db, are reproduced 

 in the second cohunn. Investigation of the oceano- 

 graphic conditions prevailing during the different 

 runs showed an unmistakable correlation between 

 the target strengths and the thermocline depths; 

 when the thermocline depth was only 18 ft, the com- 

 puted target strength was only 7.3 db, while it rose 

 to 25 db for a thermocline 160 ft deep. 



Accordingly, equations (1) and (2) were used to 

 calculate new and presumably more reliable attenua- 

 tion coefficients, from the thermocline depths, listed 

 in the third column, and the depths of the submarines 

 during the nms, in the fourth column of Table 4. 



Table 4. Dependence of reported target strengths on attenuation coefficient. 



.Assumed attenuation coefficient, 5 db per kyd. 

 t Measured attenuation coefficient, 5.3 db per kyd. 



23.7.1 Effect on Measurements 



Direct measurements of the transmission loss dur- 

 ing target strength runs on submarines have been 

 difficult and generally unsuccessful, as described in 

 Section 21.5.2. As a result, it has been customary at 

 San Diego to compute the transmission loss from an 

 estimated attenuation coefficient at the particular 

 frequency employed, usually 5 db per kyd at 24 kc 

 and 20 db per kyd at 60 kc. 



Target strengths computed in this way show enor- 

 mous differences. For example, various series of 



These new values appear in the fifth column, the ap- 

 propriate ranges in the sixth column, in the seventh 

 column the corrections resulting from the assumed 

 value of 5 db per kyd of sound travel, not range — 

 assuming the maximum range from the sixth column 

 — and the new target strengths in the last column. 

 Two results are noteworthy: the standard deviation 

 is reduced by a factor of almost two from 4.8 db to 

 2.5 db, and the mean beam target strength for an 

 S-boat is raised from 16.9 db to 19.7 db. The new 

 value agrees more closely with other measurements. 

 At 60 kc fewer data are available. An assumption 



