FUTURE RESEARCH 



339 



For a grazing angle of about 10 degrees, a typical 

 value for the angle at whieli sound in the main beam 

 strikes the bottom, average values for 10 log m" are 

 -20 db for ROCK, -27 db for MUD, -28 db for 

 SAND-AND-MUD, and -32 db for SAND. Over 

 individual bottoms of a given type, deviations of 

 + 5 db from these average values may be expected. 



There is not much information concerning the de- 

 pendence of m" on grazing angle. It appears that for 

 angles between 10 and 30 degrees m" is roughly 

 proportional to the square of the grazing angle. 



17.3.6 Bottom Reverberation Levels 

 with Horizontal 24-kc Beams 



Figure 8 of Chapter 15 shows the expected rever- 

 beration level at the bottom reverberation peak, as a 

 function of bottom type and of bottom depth below 

 the projector. The height of the peak is a significant 

 quantity in assessing the importance of bottom rever- 

 beration in any given situation. For detailed predic- 

 tion of the levels at ranges past the peak, accurate 

 knowledge is needed of the transmission of sound 

 along the various ray paths to the bottom. 



17.4 FLUCTUATION AND FREQUENCY 

 CHARACTERISTICS 



17.4.1 



Fluctuation 



The measured reverberation is probably the result- 

 ant of a combination of a large number of small 

 amplitudes of random phase. If so, the probability P 

 that the reverberation intensity will exceed the value 

 / is given by the formula 



P = e-^'m (9) 



where I is the average intensity. For the distribution 

 defined by equation (9) , the variance defined by equa- 

 tion (4) is 1} Measurements indicate that equation 

 (9) is a fairly good description of the distribution of 

 reverberation intensities. However, the observed fluc- 

 tuation of reverberation intensity must, in some part, 

 be due to variabiUty in such factors as transmission 

 loss and transducer orientation. 



17.4.2 



Coherence 



Analysis of reverberation records shows that the 

 reverberation tends to occur in the form of pulses or 

 "blobs" of about the length of the ping. For square- 



topped pings and the intensity distribution defined 

 by equation (9), the correlation coefficient in equa- 

 tion (5) has the value given by 



P = 



((l-^y for«^ 

 lo for a ^ 



(10) 



where t is the ping length, and a = \ti — k]. 



17.4.3 



Frequency Spread 



For many purposes it is desirable to know the 

 frequency spectrum of reverberation, which gives, as 

 a function of frequency, the energy in each 1-c band. 

 If the reverberation is simply the combination of a 

 large number of individual echoes, each with the same 

 frequency spectrum as the emitted ping, then the 

 resultant reverberation should also have the same 

 spectrum as the ping. This conclusion is probably 

 not far wrong, although precise measurements of the 

 frequency spectrum of reverberation have not often 

 been attempted. 



The distribution of the instantaneous frequencies 

 of the reverberation (defined in Section 16.3) is also 

 useful information. This distribution can be measured 

 by an instrument known as the "periodmeter." 

 Periodmeter measurements indicate, among other 

 things, that the spread of instantaneous frequencies 

 in the heterodyned reverberation depends on the 

 audio output frequency and the pulse length, but 

 does not depend on the frequency of the outgoing 

 ping. 



17.4.4 Wide-Band Pings 



The fluctuation of the reverberation with wide- 

 band pings is probably not very much different in 

 magnitude from the fluctuation with narrow- band 

 pings. However, the rapidity of the fluctuation is in- 

 creased as the frequency band is widened. In general, 

 the average reverberation levels are not affected by 

 widening the frequency band of the outgoing ping. 



17.5 



FUTURE RESEARCH 



Reverberation studies are a powerful tool in the 

 investigation of properties of the ocean. Information 

 from such studies is necessary to determine definitely 

 the nature of the scatterers, is useful in evaluating 

 theories of transmission loss, and can cast light on the 

 temperature microstructure of the ocean. Also, these 



