HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



For reasons of economy, or for the sake of real-time processing, 

 it may be desirable to do the necessary frequency average by band- 

 pass filtering the received signal before processing. The factors 

 affecting this choice are the same as those mentioned above. However, 

 the penalty for error is higher in this case: in the absence of 

 permanent broadband recordings, the measurement cannot be redone. 



An example of an experiment in which the source and signal 

 processing choices proved well matched to the desired measurement 

 concerns the measured spectrum of an impulse response as shown in 

 Figure 4 . The event was a 3-pound charge dropped at a range of 

 300 nm and detonated at a depth of 60 feet. At this range the total 

 received signal consisted of the arrivals from a single convergence 

 zone. The 5 to 6 Hz variation is caused by the bubble pulses of the 

 shot and the 220 Hz variation by the interference of the direct and 

 surface-reflected paths at the source. The received signals were 

 filtered through 1/3-octave filters at 25, 50 and 100 Hz; these 

 filters were wide enough to average out the bubble pulse effect, but 

 narrow enough to properly sample the surface image effect. 



It is clear from Figure 4 that the received level will be about 

 10 dB lower (and, thus, the transmission loss will be 10 dB higher) 

 at 25 Hz than at 100 Hz. This expectation is borne out in Figure 5 

 which compares the measured transmission losses at these two fre- 

 quencies over the 500 nm range of the event. 



Limitations of CE Sources 



So far the discussion has been limited to the consideration of 

 broadband sources; these sources are well suited to measuring the 



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