STRUCTURE AND ORIGIN OF ECHOES 



417 



BOW 



BEAM 



Figure 31. Effect of conning tower on optical meas- 

 urements on USS Sand Lance (SS381). 



an S-boat at beam aspect for signals from 0.5 msec 

 long resulted in echo oscillograms showing two .dis- 

 crete "blobs" of about equal mean amplitudes with a 

 range separation of about 4 yd.^^'^^ Although the finer 

 details of the echo envelopes and the relative values 

 of the peak amplitudes did not repeat from echo to 

 echo, the main features consistently suggested the 

 presence of two distinct reflecting surfaces on the 

 submarine at beam aspect. For signals longer than 

 4 yd (5 msec), the echo envelopes were almost al- 

 ways resolved into three distinct segments, with the 

 central portion corresponding to the overlap or addi- 

 tion of the two primary echoes foimd for shorter sig- 

 nals. The amplitude of this central portion presum- 

 ably varied according to the initial phase difference 

 and amplitude of the two component signals; the 

 difference between the phases changed little during 

 the period of reflection. Typical oscillograms are il- 



BEAMf 



Figure 32. Effect of conning tower on optical meas- 

 urements on U570 (HMS/M Graph). 



lustrated in Figure 30; the weak echo following the 

 main one is sound reflected from the submarine up to 

 the surface, back to the submarine and then back to 

 the projector, as discussed in Section 21.5.4. Because 

 the individual echo components in Figure 30 appear 

 to be coherent, at beam aspects the echo is probably 

 specular. Nonspecular or diffuse reflection is appar- 

 ently unimportant at these aspects, although other 

 surfaces besides the hull and ballast tanks may con- 

 tribute to the echo. 



23.8.2 



OfF-Beam Echoes 



Echoes from aspects other than beam are generally 

 very different from beam echoes. Not only is the echo 

 weaker, but it is also less well defined; examples are 

 shown in Figures 25 and 28. If a system of high re- 

 solving power is used, such as the oscilloscope and 



