Chapter 18 

 INTRODUCTION 



OBJECTS MAY BE DETECTED by the cohoes they re- 

 turn. In water, sound waves are absorbed and 

 scattered very much less than radio or hght waves. 

 Consequently, sound waves are particularly useful in 

 detecting distant objects under water by means of 

 echo-ranging, that is, sending out a sound signal and 

 listening for a returning echo. 



The loudness of an echo depends on how much 

 sound is absorbed and how much sound is reflected. 

 As a signal is sent out, the energy spreads; some of 

 it is immediately absorbed by the water and is dissi- 

 pated as heat energy. The transmission and absorp- 

 tion of underwater sound have been studied exten- 

 sively in subsurface warfare, and are described in 

 Chapters 1 to 10 of this volume. Some of the energy 

 is scattered at random back to the echo-ranging pro- 

 jector, either by particles or other inhomogeneities in 

 the water, or by the ocean surface or bottom. This 

 scattering gives rise to a phenomenon known as re- 

 verberation, which has also been investigated in de- 

 tail and is treated in Chapters 11 to 17 of this volume. 

 The sound distinctly reflected from an obstacle or 

 target in the path of the sound beam — such as a sub- 

 marine or whale — gives rise to an echo. Chapters 

 18 to 25 discuss the reflection of sound from vari- 

 ous underwater targets. 



Many types of targets are encountered in practice. 

 In particular, recognizable echoes have been received 

 from schools of fish, whales, patches of kelp and sea- 

 weed, and from sunken wrecks or prominent irregu- 

 larities on the ocean bottom in shallow water. Certain 

 water conditions give rise to echoes; at very short 

 ranges, echoes from ocean swells have been observed. 

 Wakes, "pillenwerfer," and other types of bubble 

 screens are effective targets. Their acoustic properties 

 have been studied both theoretically and experi- 

 mentally, and are described in Chapters 26 to 35. In 

 addition, icebergs have been detected by echo-rang- 

 ing, although no such echoes have been measured. 



18.1 



TARGET STRENGTH 



gets. The reflection of sound from submarines and 

 surface vessels has been investigated in terms of 

 target strengths, a quantitative measure of their re- 

 flecting characteristics. Submarine target strengths 

 have been studied as a function of the size and shape 

 of the submarine, its orientation with respect to the 

 echo-ranging projector, the distance from the sub- 

 marine to the projector, and the frequency of the 

 echo-ranging sound. Chapters 18 to 25 summarize all 

 available information along these lines. 



Echo-ranging mea.surements on submerged sub- 

 marines under more or less controlled conditions have 

 resulted in a large collection of target strength data. 

 In addition, submarine target strengths have been 

 computed theoretically and measured in experiments 

 with scale models. Unfortunately, very little is known 

 about the reflection of sound from surface vessels, as 

 no exhaustive series of tests has been made with this 

 sole object in mind. The data describing surface 

 vessel target strengths are few and scattered; con- 

 clusions are tentative and uncertain. No measure- 

 ments have been made of the reflecting characteristics 

 of mines. However, spheres of various sizes have been 

 frequent experimental targets, and the results of 

 echo-ranging measurements on spheres are probably 

 applicable to small-object location and the detection 

 of mines. 



18.2 



USES 



In subsurface warfare, submarines, surface vessels, 

 and underwater mines are the most important tar- 



Tactically, knowledge of how submarines and sur- 

 face vessels reflect sound is very important. A quan- 

 titative evaluation of the contribution which the re- 

 flecting characteristics of the target make to the re- 

 ceived echo intensity is necessary in order to predict 

 maximum echo ranges accurately. In submarine 

 operations, the reflecting properties of the submarine 

 should be known so that effective evasive maneuvers 

 may be taken to reduce, as far as possible, the chance 

 of sonar contact by enemy antisubmarine vessels. 

 For example, it is known that the strongest echo is 

 obtained when the submarine presents its beam to 

 the echo-ranging vessel. Therefore, keeping the at- 



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