CHRISTIAN: THE ACOUSTIC OUTPUT OF EXPLOSIVE CHARGES 



The customary method of getting around the finite amplitude 

 problem is indicated at the bottom of Figure 4. Just go a sufficient 

 distance from the charge so that the nonlinear effects from that 

 point on are negligible for the application of interest, call this a 

 "reference range," and there define an effective source level which 

 can then be used to examine the signals measured at greater ranges. 

 This practice was initiated by Weston, who chose a 100-yard refer- 

 ence range. And for the fairly small charge weights and detonation 

 depths that Weston was treating, most of the finite amplitude effects 

 are, indeed, negligibly small beyond 100 yards range. Unfortunately, 

 such quantities have a way of becoming gospel and being dissociated 

 from the physical facts that led to their selection. 



The 100-yard reference range by now has become a sort of junc- 

 tion through which source levels are shuttled at a rate of 20 log R. 

 If you want to compare different measurements made at various ranges - 

 a half mile, a mile or so — assume spherical spreading and extrapo- 

 late them to 100 yards. If you are enamored of the sonar equation's 

 one-yard reference range, just add 40 dB to the source level. We 

 rarely find experiments with data taken on a closely scaled grid 

 that allow us to see the rate at which finite amplitude effects are 

 varying, and to sort out all effects of the medium. So we often are 

 trapped in the circuit of using our desired information to reduce 

 available data to try to improve our desired information. We must 

 break out of this circuit if we really want to know effective source 

 levels within 1 dB for an assortment of charge weights, depths, and 

 frequency bands. 



The fact is that the appropriate reference range for defining 

 effective source level is itself a function of the charge weight, the 

 charge depth, and the frequency band of interest. I know of no 



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