HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



ranges the paths begin to drop off in intensity at significantly dif- 

 ferent rates. Thus, the total loss estimated from a signle path will 

 be different at low grazing angles, depending upon which of the four 

 paths is used. 



The second assumption above can lead to more serious difficulties than 

 the first. Based upon the earlier considerations of this paper, whether 

 any set of paths is combined in the signal processing on an rms basis 

 or not depends upon the details of the processing. In the experiments 

 of interest here the received signal was filtered in a 1/3-octave band 

 at several center frequencies. To assess whether assumption 2) is 

 reasonable, estimates of the spectrum of the impulse response for the 

 four paths of Figure 9 were made; these estimates were based upon the 

 computed amplitudes and arrival times for the paths. The relative 

 arrival times as a function of range are shown in Figure 10 for the 

 paths which refract through the sedimentary layer in the sound speed 

 profile shown earlier. (The minimum range corresponds to a path 

 incident upon the bottom at an angJ.e of 20° with respect to the 

 horizontal.) Figure 11 shows the computed spectrum for a range of 

 14 nm; the 9 Hz variation is caused by the up-and-down-going pair of 

 paths at 800 feet, while the 27 Hz variation is caused by the up-and- 

 down-going pair of paths at 300 feet. At 35 Hz, for example, a 1/3- 

 octave filter is about 8 Hz wide at its 3 dB down points; a filter of 

 this width clearly will not yield the rms sum of the features of this 

 figure. Figure 12 shows the computed spectrum at a range of 29.5 nm; 

 all the travel time differences have decreased with corresponding 

 increases in the frequencies of the variations in the spectrum. Again, 

 the filter at 35 Hz will not yield the rms sum of these variations. 



Figure 13 compares the transmission loss for these four paths 

 based upon 1) the rms sum, 2) a 1/3-octave result at 35 Hz, and 

 3) a 1/3-octave result at 100 Hz. In the portion of the figure above 



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