Chapter 11 



INTRODUCTION 



11.1 DEFINITION OF REVERBERATION 



IN ANY ECHO-RANGING or listening device, the 

 wanted signal is always received in the midst of a 

 certain amount of extraneous noise. This background 

 noise is a mixture of noises from many sources, most 

 of which are operative whether the transducer is 

 being used for echo ranging or for listening. Examples 

 of noises which appear in both echo-ranging and lis- 

 tening backgrounds are noise from breaking waves, 

 sounds produced by such marine organisms as snap- 

 ping shrimp, noise from the forming and collapsing 

 of bubbles around the screws of the ship, noise due 

 to the local motion of the water in the vicinity of 

 the moving sound head, and the general din of the 

 motors of the ship and auxiliaries. In both listening 

 and echo ranging these noises, in varying degrees de- 

 pending on conditions, are present to confuse the 

 operator. 



Echo-ranging backgrounds, however, have another 

 component all their own, which is directly due to the 

 pulse put into the water. This component is called 

 "reverberation," and is the topic for discussion in 

 Chapters 11 to 17. 



Reverberation is evident to the operator of echo- 

 ranging gear as a quavering ring, which sets in as 

 soon as the gear is rigged for reception, that is, as 

 soon as the period of sound emission is ended. ^ This 

 rolling sound has about the same pitch as the out- 

 going pulse. Reverberation is very loud a fraction of a 

 second after the ping is sent out, but diminishes 

 rapidly thereafter if the receiver amplification is not 

 changed. However, this rapid decrease does not neces- 

 .sarily mean that echoes from distant targets will al- 

 ways be audible above the reverberation coming in 

 at the same time. Wanted echoes also become fainter 

 as the time interval between ping emission and echo 

 reception increases so that echoes which would be 

 audible over the remainder of the noise background 

 are often masked by reverberation. 



Although reverberation in the sea shows some simi- 



larity to the well-known reverberations in a room, in 

 many respects it is quite different. In a closed room, 

 a sound is reflected with diminishing intensity back 

 and forth between the walls and floor and ceiling 

 imtil it is finally absorbed. Since the absorption of 

 sound in air is relatively small, the sound energy dis- 

 appears in appreciable amounts only at the bound- 

 aries of the room; and the decay of the reverberation 

 is simply a measure of the decay of acoustic energy 

 in the volume of air enclosed by the room. In the sea, 

 there are these important differences. The sea has 

 boundaries similar to a ceiling and floor (sea surface 

 and bottom), but nothing like walls to interrupt the 

 free passage of sound in a horizontal direction. 

 Further, the sound-transmitting properties of sea 

 water differ from those of air. The sea volume both 

 absorbs and scatters sound energy in appreciable 

 amounts. Thus, the behavior of reverberation in the 

 sea is a special problem upon which little light can 

 be cast by the known properties of reverberation in 

 a room. When an echo-ranging pulse is sent into the 

 water, some of its energy does return back to the 

 transducer; but the amount of returning sound de- 

 pends on many factors besides the rate at which 

 sound energy is being removed from the volume of 

 the ocean by the boundaries. 



11.2 ELEMENTARY PROPERTIES OF 

 REVERBERATION 



There is every reason to believe that reverberation 

 is almost always the resultant of a large number of 

 very weak echoes arising from small bodies or irregu- 

 larities in the path of the ping. These tiny targets 

 may be called "scatterers." They may be air bubbles, 

 suspended solid matter, minor irregularities of the 

 ocean surface and bottom, local fluctuations of water 

 temperature, or any other inhomogeneities in the sea. 



Let us suppose, to begin with, that the scatterers 

 producing reverberation are all identical, and are 

 uniformly distributed throughout the volume of the 



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