STRUCTURE AND ORIGIN OF ECHOES 



413 



RANGE 



t~ 





ASPECT ANGLE 

 IN DEGREES 



PORT 

 BEAM 

 270 



;e= 



■BOW 

 



STARBOARD 

 -BEAM 

 90 



of 20 db per kyd for the attenuation coefficient, al- 

 though assumed in the San Diego target strength 

 computations, is not substantiated by tests made at 

 Fort Lauderdale which gave results of 9 to 10 db per 

 kyd. These latter measurements were made by plot- 

 ting the echo level, corrected for inverse square loss, 

 against the range, as the range continuously de- 

 creased from about 600 to 150 yd; the submarine was 

 at stern aspect throughout the run. This discrepancy 

 was mentioned in Section 23.6.2. The high value as- 

 sumed for the attenuation coefficient at San Diego 

 was suggested by early transmission measurements; 

 in these tests, the presence of shallow thermoclines off 

 the coast of California was partly responsible for the 

 high values measured. For deep, mixed water, lower 

 values are more common ^* (see Chapter 5). 



Figure 26. Sound range recorder records of submarine 

 echoes from 30-millisecond pulses. 



Figure 27. Oscillograms of submarine echoes at beam 

 aspect for 10-miUisecond pulses. 



23.8 STRUCTURE AND ORIGIN OF 

 ECHOES 



Most target strengths have been measured from 

 echoes recorded oscillographically on 35-mm motion 

 picture fibn. At San Diego, this film was run past the 

 oscilloscope at a speed sufficiently high to record the 

 detail of each echo; depending on the signal length 

 and the exact film speed, these echoes may be from 

 0.2 to 3 or 4 cm long. Accordingly, these echoes have 

 been carefully studied in an attempt to formulate 



