CHRISTIAN: THE ACOUSTIC OUTPUT OF EXPLOSIVE CHARGES 



Let me work up from the bottom of the list and give a few examples of 

 the kinds of variation we have found influencing that desired 1 dB 

 predictability in 1/3-octave bands. 



Processing Methods. The most obvious source of differences from 

 processing methods is, of course, the fact that some people use analog 

 and others use digital processing. Even with digital processing, how- 

 ever, we have been surprised to find how easily some tenths of dB 

 differences crop up. We recently made a joint study with another 

 laboratory of selected data tapes — some we had recorded and some 

 they had recorded. After overcoming the numerous communication 

 problems involved in exploring our two "standard FFT programs" (and 

 this took no small effort) , we still found several computational 

 details that introduced greater differences than one would expect. 

 For example, simply changing the frequency interval of computation 

 by a small amount introduced as much as 0.8 dB difference in the 

 1/3-octave band centered at 25 Hz on some of the records, but not on 

 others. At 25 Hz, the 1/3-octave band is so narrow that the level 

 is quite sensitive to small computational manipulations. This 

 exercise reminded us again of how wary one must be of applying routine 

 analyses without carefully examining their suitability for a par- 

 ticular spectral energy distribution. 



Recording Equipment Characteristics . The importance of this 

 factor is so well known that I will mention only one point that may 

 be of interest. (We are speaking only of equipment that is fully 

 calibrated, of course.) My example has to do with equipment over- 

 load. Overloading is familiar to underwater acousticians using 

 explosion sources. One would like to think that in an overloaded 

 recording the low- frequency content is still usable and that the 

 signal has simply suffered high-frequency clipping. Figure 6 shows 

 an analytical quick-look at this question, using an idealized 



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