278 



EXPERIMENTAL PROCEDURES 



Figure 6. Sample record from Woods Hole reverberation camera (reverberation from 12 successive pulses superposed). 



known, the auxiliary projector itself may be cali- 

 brated with the aid of the standard receiver, and then 

 used to calibrate the receiver. The echo received from 

 a sphere of known target strength at a known dis- 

 tance has been used by WHOI to determine the 

 product FF'. Present practice at both laboratories is 

 to make a calibration at least once every working 

 day, whenever possible.' 



Although calibration is simple in principle, experi- 

 ence has shown that there is likely to be considerable 

 inaccuracy in all projector and receiver calibrations. 

 At UCDWR, it was found that the values of F and F' 

 determined by calibration procedures may change 

 unaccountably with time, sometimes changing by 

 nearly 10 db from day to day, and by somewhat 

 lesser amounts during a single day.' Some method 

 for detecting calibration errors in the field is desirable 

 since these errors are reflected as errors in the rever- 

 beration levels inferred from the measured intensities. 



13.1.3 Typical Reverberation Records 



Most of the reverberation data obtained by 

 UCDWR are in the form of oscillograms on 35-mm 

 motion picture fihn. Figure 7 shows a sample record 

 of the reverberation from three successive pings sent 

 out at 8-sec intervals. For convenience in display, 

 each reverberation record was cut into three sec- 

 tions, as shown in the illustration; the three 4 's make 

 up the first record; the three 5's the second; etc. The 

 marks on the upper edge of each record give the time 

 scale; these marks are 2.5 msec apart. The point a 

 represents the emission of the signal; h the onset of 

 reverberation with transients caused by the opera- 

 tion of the changeover relay; and c, d, e, places where 

 the gain was automatically increased by the atten- 

 uators. At /, the reverberation has decreased below 



background noise. The film speed (12.5 in. per sec) is 

 high enough to show considerable fine structure in 

 the reverberation. However, it is not high enough to 

 resolve individual cycles; thus the trace shown in 

 Figure 7 represents the envelope of the received 

 reverberation. 



The records shown in Figure 7 are quite typical and 

 illustrate some of the statements which have been 

 made in this volume about the behavior of reverbera- 

 tion. The recorded amplitude at a given time past 

 midsignal is not constant from record to record, even 

 though these pings were sent out and the reverbera- 

 tion was recorded under the same adjustments of the 

 experimental apparatus. In general, however, when- 

 ever reverberation measurements are made, there 

 are major features which persist from record to rec- 

 ord. One such characteristic is the point of onset of 

 bottom or surface reverberation. Another is the in- 

 variable tendency of reverberation of a given sort 

 (volume, surface, or bottom) to decrease with in- 

 creasing time, as is predicted by the theoretical 

 formulas of Chapter 12. This decrease makes neces- 

 sary the provision in the system of gain changes at 

 points such as c, d, and e; without these gain changes 

 it would be impossible to record all the reverberation 

 at measurable amplitudes. Occasionally, successive 

 reverberation records show a systematic increase at 

 certain points. These increases can usually be cor- 

 related with the calculated increase due to the onset 

 of surface or bottom reverberation; sometimes they 

 are ascribed to the existence of local regions of high 

 scattering strength. 



13.2 



ANALYTICAL PROCEDURES 



After the field work is done, the films containing 

 the received reverberation records are taken to the 



