CHRISTIAN: THE ACOUSTIC OUTPUT OF EXPLOSIVE CHARGES 



appreciably for typical high-explosive material such as TNT, pentolite, 

 and RDX. But they are changed significantly when aluminum is added to 

 the explosive mixture. For example, HBX-3, which is a particular 

 mixture of RDX, TNT, and aluminum, has a bubble period that is about 

 25 percent greater than the bubble period of TNT. Consequently, the 

 bubble fundamental frequency, and the frequency spacings of subsequent 

 peaks and nulls in the spectrum, are only about 80 percent as great as 

 those of TNT. This is illustrated in Figure 7, where dashed and solid 

 lines refer to HBX-3 and TNT, respectively. These curves are computed 

 from simplified analytical representation of the pressure waves. The 

 differences in 1/3-octave band spectrum levels of these two materials 

 are tabulated on the right-hand side of the figure. At the lowest 

 frequencies, while the curves are still increasing toward the first 

 peak at the fundamental, the band level for HBX-3 is several dB higher 

 than that of TNT. As one moves up in frequency, however, it becomes 

 a game of catch-can, and which of the two materials has the higher 

 energy level depends on the location of the particular frequency band. 

 In any case, we expect serious trouble if we try to compare narrow- 

 band data from two such different materials without accounting for 

 their different source spectra. 



Charge Weight and Depth. I would like to discuss these two 

 important quantities together for a moment, to describe a method of 

 source level prediction used by Gaspin and Shuler (1971) . The tech- 

 nique involves first generating a quasi-theoretical pressure-time 

 history, such as that shown in Figure 8, and then transforming to the 

 frequency domain. The pressure-time curve is fitted through a series 

 of points (indicated in Figure 8) , the coordinates of which are de- 

 rived from empirical functions as shown in Figure 9 (Slifko, 1967). 

 The curve of Figure 9 allows one to estimate the amplitude of the 



first bubble pulse, P , for selected values of charge depth, Z , 



B o 



47 



