REVERBERATION FROM WIDE-BAND PINGS 



333 



reverberation from frequency-modulated pings should 

 fluctuate more rapidly than does the echo envelope. 

 Observational evidence that the rapidity of rever- 

 beration fluctuation is increased by the use of fre- 

 quency-modulated pings is quoted in reference 24, 

 and similar observations have been reported in other 

 sources. The echo length resulting from a frequency- 

 modulated ping is equal to the effective ping length, 

 just as with unmodulated pings; the effective ping 

 length is itself determined by the pass band of the 

 equipment. 



The only accurate quantitative data on the fluctua- 

 tion of reverberation from frequency-modulated 

 pings are those given in reference 25. In that report 

 it was found that with frequency-modulated pings 

 the mean standard deviation of the reverberation 

 amplitude is 33 per cent of the mean amplitude, 

 significantly smaller than the 52 per cent value pre- 

 dicted for a Rayleigh distribution. These measure- 

 ments were made with pings of length 2, 4, and 8 sec, 

 frequency-modulated from 48 to 36 kc. Some results 

 on the coherence of FM reverberation are also re- 

 ported in reference 25. 



The mechanism by which such a decrease in the 

 magnitude of the fluctuation could be accomplished 

 is difficult to visualize; but the possibility of such 

 an effect must be admitted, because our theoretical 

 understanding of the problems involved is not 



at all complete. Qualitative reports that frequency 

 modulation is effective in reducing the magnitude 

 of the fluctuation cannot be trusted, since they 

 may be based on the results of time averages 

 performed somewhere in the complicated recording 

 equipment. 



Recapitulating, in echo ranging the objectionable 

 features of reverberation are twofold. Reverberation 

 masks the echo; also, reverberation sinmlates the 

 echo, so that false contacts are often obtained. It is 

 apparent from this chapter that a great deal of in- 

 formation still is lacking about such characteristics 

 of reverberation as blob shape, cause and rapidity of 

 fluctuation, and frequency spread. 



Schemes which have been suggested to suppress 

 reverberation may be combined with proposals for 

 decreasing the average level of the reverberation by 

 decreasing the ping length or by increasing the hydro- 

 phone directivity. Of course, the possibilities of de- 

 creasing the ping length or increasing the hydrophone 

 directivity are always limited by other practical con- 

 siderations. Once the ping length has been decreased 

 as much as practical, and the transducer directivity 

 has been increased to its highest practical value, one 

 must resort to devices which do not reduce the 

 average energy received in the hydrophone, but in- 

 stead reduce the instrumental effects of reverberation 

 in the detecting mechanism. 



