SHORT-RANGE PROPAGATION IN DEEP WATER 



203 



1.0 



"-> 0.5 



: -0.5 



-1.5 



-2.0 



so 100 ISO 200 250 300 350 



TIME IN )U.SEC 



400 



450 



500 



550 



600 



Curves A through C are computed from diffraction theory, assuming the explosive pulse near the source to have the form 

 p = Po exp ( — 3 X lO't sec) shown in the curve labeled initial. The conditions assumed in the calculations are compared 

 with those obtaining in the experiment in the following table: 



Figure 8. Observed and computed pressure-time curves in the shadow zone. 



ary and continues to get larger and larger the farther 

 one goes into the shadow zone. These features were 

 observed on all occasions when shots were made in a 

 shadow zone produced by downward refraction, and 

 occasional repeat shots showed that the first cycle or 

 so of the oscillatory pulse observed in the shadow 

 zone was quite reproducible (see Figure 16C). It is 

 interesting to compare these oscillograms with theo- 

 retical pressure-time curves for sound diffracted by 

 an ideal medium which has a plane surface and in 

 which the velocity of sound depends only on the 

 depth. Such theoretical pressure-time curves for ex- 

 plosive sound in the shadow zone have been com- 



puted by CUDWR;" because of mathematical com- 

 plications in the theory, however, the theoretical cal- 

 culations have not been made for exactly the same 

 conditions as any of the shots of Figure 7. Figure 8 

 shows the comparison with shot 21 of Figure 7. The 

 agreement is good as regards time of rise, but the 

 amplitude of the negative part of the pulse is much 

 greater for the observed than the theoretical case, 

 and the observed wavelength is much shorter. The 

 discrepancy would probably be reduced if the calcu- 

 lation could be carried through for a velocity distri- 

 bution approximating the observed one more closely. 

 However, it is quite possible that diffraction theories 



