WEINSTEIN: EXPLOSIVE SOUND-SOURCE STANDARDS 



PROCESSING PROCEDURES 



Another source of uncertainty results from processing procedures. 

 This is a broad subject in itself, which we touch upon only briefly. 



Any processing system has an inherent limit on reproducibility 

 which may stem from the tape-playback system, the processing equip- 

 ment, or human factors. To illustrate the effect on propagation- 

 loss measurements, assume a repeatability of ± 0.2 dB, uncorrelated 

 for both the noise energy and the signal-plus-noise energy. 



The uncertainty in propagation loss, as a function of the S/N 

 ratio, is shown in Figure 11. For a signal-to-noise ratio of - 3 dB, 

 the uncertainty is then + 1.0 to - 1.3 dB, about equal to the total 

 accuracy goal. Also note that because of the asymmetry, a small 

 fixed bias can be expected. 



The results of a repeatability experiment using five 10-second 

 noise samples from a direct-record ACODAC system are shown at the 

 top of Figure 12. The operator exercised considerable care in tape 

 handling to ensure proper lay up of the tape and drive-speed 

 stabilization. The differences are plotted for 1/3-octave bands. 

 Analog filters were used. The results range from + 1.0 to - 1.2 dB, 

 with zero bias and a standard deviation of 0.36 dB. The lower curve 

 shows similar results without care. Fast forward and reverse were 

 used to find the data segment of interest. The results are con- 

 siderably worse over the entire band, and get completely out of hand 

 at the higher frequencies. This particular data set typifies the 

 human-factors problem when direct record is used. 



Repeatability measurements with an FFT processing system for 

 the 50-Hz band yielded similar results; a bias shift of about 0.25 dB 

 and a standard deviation of about 0.25 dB. However, the comparison 



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