Chapter 17 

 SUMMARY 



17.1 



17.1.1 



DEFINITIONS 



Reverberation 



REVERBERATION IS A COMPONENT of background 

 heard in echo-ranging gear, and is distinguished 

 from the general noise background by the fact that 

 it is directly due to the pulse put into the water by 

 the gear. The traveling ping meets not only the 

 wanted target, but also myriad small scattering 

 centers or other inhomogeneities, each of which re- 

 turns a tiny echo to the transducer. These tiny un- 

 wanted echoes combine to make up reverberation. 

 Thus reverberation, like the echo, is a sound whose 

 pitch is definite and is determined by the frequency 

 of the projected pulse. Often echoes which would be 

 audible over the remainder of the noise background 

 are masked by reverberation. To the operator of echo- 

 ranging gear, reverberation is evident as a quavering 

 ring which sets in as soon as the period of sound 

 emission is finished. 



The scatterers producing reverberation may be 

 located near the sea surface, in the main ocean 

 volume, or in the sea bottom. The reverberations 

 produced by these three types of scatterers are called, 

 respectively, volume reverberation, surface rever- 

 beration, and bottom reverberation. This distinction 

 is physically meaningful, since these three types of 

 reverberation apparently have different properties 

 and can be experimentally differentiated from each 

 other. 



17.1.2 Reverberation Intensity 



The strength of the sound heard or recorded as 

 reverberation depends not only on the intensity of 

 the backward scattered sound in the water near the 

 receiver, but also on the nature of the receiving gear. 

 The intensity of the reverberation actually heard or 

 recorded, after the sound in the water has been con- 

 verted to electrical energy by the receiver, amplified, 



and passed to the ear or recording scheme, is called 

 the "reverberation intensity" and is given the symbol 

 G. As so defined, G equals the watts output across the 

 terminals of the receiving gear. In general, the rever- 

 beration intensity G is a function of time and is re- 

 lated to the sound intensity in the water by such 

 parameters of the receiver system as receiver directiv- 

 ity and receiver gain. Since the quantity G depends 

 on the gear parameters, the absolute magnitude of G 

 is usually not of great significance in studies of the 

 intrinsic character of reverberation or of the mech- 

 anisms producing reverberation. In these studies the 

 reverberation level, defined under the next heading, 

 is ordinarily employed. 



17.1.3 



Reverberation Level 



The reverberation level is the decibel equivalent of 

 the reverberation intensity defined in Section 17.1.2, 

 expressed relative to a standard which makes rever- 

 berations measured with different gear exactly com- 

 parable. Specifically, the reverberation level R' is 

 defined as 



R' = 10 log G - 10 log (/^-F') (1) 



where F is the projector output at 1 yd in decibels 

 above 1 dyne per sq cm, and F' is the receiver 

 sensitivity in watts of output for a received rms 

 sound pressure of 1 dyne per sq cm. Reverberation 

 levels are much more useful than reverberation in- 

 tensities for comparing measurements made with dif- 

 ferent systems, since under identical external condi- 

 tions two different systems sending out pings of the 

 same length should in principle give the same rever- 

 beration level if correction is made for transducer 

 directivity. Reverberation levels usually refer to the 

 average reverberation intensity G found in a suc- 

 cession of pings. 



Reverberation intensities are proportional to the 

 ping duration. Often it is desirable to convert rever- 

 beration levels to the levels which would be received 



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