For years, the explosives industry has sought a direc- 

 tive explosive charge, with only minor success. The pneu- 

 matic sound source would provide an intense low- frequency 

 point source for use with large parabolic reflectors. At 

 frequencies in the order of 300 c/s the size of a suitable 

 reflector might not be compatible with a shipboard installa- 

 tion, although it might be quite suitable for an ocean- floor 

 installation. As an example, at 300 c/s, if the parabolic 

 reflector were 2 00 feet in diameter, then a 5° beam would 

 be obtained. The directivity index of such an assembly 

 would be approximately 32 db and it is possible that a 

 source level of as much as 138 db above 1 dyne/cm^ at 

 1 meter would be obtainable. This would be a very usable 

 source level at this low frequency. 



Figure 26. Examples of rubber spheres designed by 

 manufacturer for testing: (A) 3- inch OD with 0. 75- inch 

 wall; (B) 1. 375-inch OD with 0. 25-inch wall. 



49 



