JSTENING AND ECHO RANGING 



489 



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Figure 6. Fathometer record of wake echoes from Coast Guard cutter Evnng. 



This type of recording, which has been used widely 

 in other sound studies, has usually been applied only 

 to the analysis of wake echoes rather than to signals 

 transmitted through wakes. The linear distance on 

 the film from mid-signal to mid-echo provides a con- 

 venient record of the range from which the echo was 

 returned, since the distance on the horizontal scale is 

 the product of sound velocity times the time. A 

 number of oscillograms of wake echoes are repro- 

 duced below on the scale of the originals. Figure 7 

 shows three sets of three successive signals, each 3 

 msec long, and the corresponding echoes both from a 

 wake, laid by the E. W. Scripps, and from a target 

 sphere 3 ft in diameter suspended behind the wake at 

 a center depth of 6 ft. The oscillograms were obtained 

 with 24-kc sound during Run 1 of the experiments 

 summarized in Figures 8 and 9 of Chapter 31 and in 

 Table 2 of Chapter 32, which should be consulted for 

 a detailed description of the plan of observations. 



The numerical evaluation of wake oscillograms has 



so far been restricted to the visual measurement of 

 peak amplitudes, described in Section 21.3.1, which 

 generally have been held to be sufficiently repre- 

 sentative of the echo as a whole. A more satisfactory 

 though very time-consuming method would be to 

 measure the amplitudes along the entire echo profile, 

 square the amplitudes and integrate them over the 

 time. This integral would be proportional to the total 

 energy contained in the echo. It is possible to design a 

 mechanism which would perform automatically this 

 sequence of procedures. In any event, it would be 

 desirable to supplement and check fundamental wake 

 studies based upon measurement of peak amplitudes 

 by investigating the total energy of echoes. 



Current procedure is to place the processed film on 

 an illuminated viewer, read the peak amplitude of 

 the echo with the aid of a transparent scale, and cor- 

 rect the measured amplitude, if necessary, for the 

 finite width of the luminous spot on the oscillograph 

 screen. Averages over five successive echoes are 



