STRUCTURE AND ORIGIN OF ECHOES 



415 



more precise conclusions regarding, the process of 

 reflection of sound from submarines. 



Exammation of hundreds of oscillograms of echoes 

 at San Diego has iimdc possible a separation of 

 echoes into two classes, beam and off-beam, as illus- 

 trated in Figure 25. Each class shows its own char- 

 acteristics and peculiarities, on the basis of which 

 tentative explanations of reflection phenomena have 

 been made. These two types of echoes produce such 

 different traces on the sound range recorder that the 

 appearance of these traces is used tactically to esti- 

 mate the aspect of the target. Typical sound range 

 recorder traces are illustrated in Figure 26. 



f" 



RANGE IN YARDS 



?00 4nn f.nn aoo 



23.8.1 



Beam Echoes 



Beam echoes are always stronger, on the average, 

 than echoes at any other aspect, both according to 

 the theoretical calculations and the direct and indi- 

 rect measurements. There are four lines of evidence 

 which indicate that reflection is specular and arises 

 primarily from the hull of the submarine. 



First, theory predicts strong specular reflection at 

 beam aspect.* The theoretical values derived assum- 

 ing only specular reflection are in excellent agree- 

 ment with other values measured directly and in- 

 directly. The effect of the conning tower appears to 

 be negligible for the U570 at beam aspect, since it 

 contributes orily 0.2 db to the target strength at long 

 ranges. 



Secondly, the oscillograms of beam echoes are 

 clear cut and closely resemble the square-topped 

 pulses sent out; further examples are illustrated in 

 Figure 27, which shows oscillograms of three succes- 

 sive echoes from a submarine at beam aspect. These 

 beam echoes contrast sharply with off-beam echoes, 

 which are illustrated in Figure 28. Occasionally, 

 beam echoes show very sharp and narrow peaks at 

 either end, or a short "tail" of lower intensity, which 

 are attributed to two different types of surface reflec- 

 tions described in Section 21.5.4. A typical sound 

 rknge recorder record of the double echoes at beam 

 aspect is illustrated in Figure 29. Since beam echoes 

 usually equal the signal in length, for signals 10 or 

 more milUseconds long, the effective reflecting sur- 

 face does not appear to be much extended in the 

 direction of the sound beam; typical oscillograms of 

 beam echoes are illustrated in Figure 30. In other 

 words, the relatively flat area on the hull of the sub- 

 marine is responsible for almost all the energy in the 

 echoes received at beam aspect. This same specular 



u.3JT» 



:-.2 



Figure 29. Double echoes recorded on the sound 

 range recorder. 



reflection may be inferred from Figure 24 where a 

 fine interference pattern is conspicuously absent at 

 beam aspect for altitude angles in the neighborhood 

 of degree. 



Thirdly, optical measurements on a model of the 

 Sand Lance as well as both optical and acoustical 

 measurements on models of the U570 gave identical 

 target strength at beam aspect with and without the 

 conning tower, over a sector of about 20 degrees, as 

 Figures 31, 32, and 33 show. The conning tower, 

 although important at other aspects, contributes 

 little to reflection at beam aspects. 



Fourthly, the importance of hull reflections is 

 evident in Figures 1 through 5 of Chapter 22. Al- 

 though these photographs refer only to optical illu- 

 mination of the model and may not apply perfectly to 

 the reflection of supersonic sound from submerged 

 submarines, they may be representative of what hap- 

 pens acoustically. 



However, measurements made with short pulses 

 at beam aspect show a detailed echo structure which 

 suggests that not all the reflected sound comes from 

 the submarine hull or ballast tanks. Measurements on 



