OPERATIONS AND MEASUREMENTS 



493 



influence is to take averages over long series of ob- 

 servations. Even these averages may siiowa slow 

 drift with time, sometimes called variation of the 

 transmission loss, but the amplitude of the variations 

 is of a lower order of magnitude than that of the 

 fluctuations. In measuring the transmission loss 

 which sound undergoes while passing across a wake, 

 the fluctuations of the transmission loss in the sur- 

 rounding ocean mask the effect sought after, or 

 even may entirely obscure it for wakes in an ad- 

 vanced stage of decay. In echo ranging, the sound 

 returned by different parts of the wake undergoes 

 destructive and constructive interference, which to- 

 gether with the gradual change of the internal 

 structure of the wake will invariably cause fluctua- 

 tions of the wake echoes that are even more rapid 

 than the fluctuations of the transmission loss. Conse- 

 quently, wake echoes are even more variable in shape 

 than sound signals which have been affected only by 



fluctuation of the transmission loss in the sea. Figure 

 7 shows the irregular character of wake echoes re- 

 sulting from changing interference effects. Fluctua- 

 tions of the transmission loss in the ocean are also 

 conspicuous in Figure 7; note the change in strength 

 between the last two echoes from the target sphere, 

 in the lower strip of the illustration. 



In echo ranging at wakes over short ranges, the 

 reverberation background caused by the scattering 

 of sound in the ocean constitutes an important limit- 

 ing factor. Under conditions giving very high rever- 

 beration, a weak echo may become lost in the back- 

 ground. Strictly speaking, the echo intensities de- 

 rived from measured echo amplitudes, as described 

 above, include the contribution from reverberation 

 and should be corrected for this superimposed inten- 

 sity. In practice, this correction may usually be 

 neglected whenever the wake echo is sufficiently 

 strong to be distinguished from the reverberation. 



