SHORT-RANGE PROPAGATION IN DEEP WATER 



209 



-30 



0.1 



0.5 



10 



50 



FREQUENCY IN KC 



Figure 14. Spectral distribution of imergy in explosive pulses received at various ranges. The curves shown were 

 obtained by smoothing the values calculated by Fourier analysis from a number of shots made on Apr. 3, 1942, at a 

 depth of 100 ft and received by a hydrophone at .54 ft. 



curves except the first three show a maximum, smce 

 in the direct zone the surface reflection makes the 

 spectrum level decrease at low frequencies, and in the 

 shadow zone a similar effect is produced by the 

 oscillatory nature of the pulse, although a separation 

 into direct and reflected parts is no longer po.ssible. 

 For the first three pulses the integration was not ex- 

 tended over a sufficiently long time to include the 

 surface reflection; if it had been, the curves would 

 form a continuous family. The slopes of the curves 

 for the pulses received in the direct zone are about 

 12 or 13 db per octave at 50 kc; in the shadow zone 

 this slope increases to 17 or 18. This change is of 

 course due to the increase in time of rise on entering 

 the .shadow zone. 



As the range increa.ses in the shadow zone there 

 seems to be a slight decrease in the frequency at which 

 U is a maximum. This .shift, although hardly greater 

 than the experimental error, is probably also due to 

 the increase in time of rise. If the curves for the direct 

 zone had been computed with inclusion of the surface 

 reflection, however, they would have shown a trend 



in the opposite direction, since the frequency below 

 which the cancellation effect is felt is one for which a 

 quarter of a cycle is of the .same order as the time 

 interval between the direct and reflected pulses, and 

 this interval decreases with increasing range. 



The frequency at which the spectrum level for 

 shots in the shadow zone is a maximum shows varia- 

 tions from one day to another which are much greater 

 than the variations from shot to shot on a given day. 

 Table 3 gives values, taken from reference 9, of this 

 frequency observed on a number of days for shots 

 well inside the shadow zone. 



Table 3. Frequency of maximum spectrum level in 

 the shadow zone. 



Date, 1942 Frequency of maximum, kc 



An interesting application of this type of analysis 

 is to compute the attenuation which .sound of dif- 



