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249 



However, the echo-ranging vessel is usually rolling 

 and pitching; and the scatterers in the ocean, that is, 

 the air bubbles, the suspended sohd matter, the tur- 

 bulent regions, and the regions of temperature fluctu- 

 ation are all free to move. Thus the interference pat- 

 tern will vary widely from one ping to the next. Since 

 the exact distribution of the scatterers in the ocean 

 cannot be predicted, the irregularities of reverbera- 

 tion can be described adequately only by statistical 

 methods. That is, if we are to be reaUstic, we can 

 attempt to assign values only to the average rever- 

 beration intensity, and the average reverberation 

 variabihty. For example, the inverse square law for 

 the decay of reverberation from the volume of the 

 sea, which was developed previously, could only be 

 valid for the average reverberation from a series of 

 pings; it would be nonsense to expect the reverbera- 

 tion from an individual ping to decay smoothly in 

 exact agreement with this law. 



In order to make clear the meaning of reverbera- 

 tion, the scatterers in the water, such as air bubbles, 

 suspended solid particles, and the like, have been 

 assumed to be very nearly uniformly distributed 

 throughout the sea volume. We have really been 

 describing what is known as volume reverberation, 

 that is, reverberation due to scatterers in the body of 

 the water. However, there is also reverberation due 

 to scatterers at the ocean surface and ocean bottom. 

 These three types of reverberation, volume, surface, 

 and bottom reverberation, behave quite differently 

 on the average. For example, they set in at different 

 times. Volume reverberation is evident at the mo- 

 ment the ping is put into the water, and surface and 

 bottom reverberation do not set in until the sound has 

 had time to travel to these bounding surfaces and re- 

 turn to the transducer. There are other differences as 



well, which are discussed in the main body of the 

 text. 



11.3 



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The next six chapters summarize the reverberation 

 studies carried out under the auspices of NDRC 

 through the spring of 1945. Chapter 12 derives theo- 

 retical formulas for the average reverberation in- 

 tensity on the basis of assumptions which are ex- 

 pUcitly stated and whose validity is critically ex- 

 amined. In that chapter, the expected intensities of 

 reverberation from the volume, surface, and bottom 

 are examined separately for their theoretical depend- 

 ence on many other variables besides time; some of 

 these other variables which play a major part in de- 

 termining the reverberation intensity are the direc- 

 tivity characteristics of the transducer, the trans- 

 mission loss between the projector and the scatterers, 

 the intensity and duration of the projected signal, 

 and the scattering power of the portion of the ocean 

 under consideration. Chapter 13 describes the field 

 and laboratory methods which have been developed 

 for the measurement of reverberation intensity and 

 the analysis of the resulting data. Chapters 14 and 15 

 summarize the observational information on volmne, 

 surface, and bottom reverberation which has been 

 obtained by use of these experimental and analytical 

 techniques and compare these results with the 

 theoretical predictions of Chapter 12. In Chapter 16 

 the variability of reverberation is examined, both 

 theoretically and in the light of the observed data, 

 and the frequency characteristics of reverberation 

 are described. Finally, in Chapter 17 the most im- 

 portant results presented in the body of Part II are 

 summarized. 



