HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



range is grossly underdetermined. Second, consider the comparison of 

 Figure 7 which also presents both measured CW data and a parabolic 

 equation calculation. In this case the calculation was done with a 

 resolution of 150 feet in range while the data were taken with a 

 time average corresponding to a range interval of 30 feet. Although 

 different in detail, both curves have comparable structure; the 

 important point is not the level of agreement or disagreement between 

 the curves, but that both represent adequate spatial sampling of the 

 underlying transmission loss and that the model calculation could 

 have been used to set the experimental sampling intervals. 



The chief advantage of a CW source is that it permits experi- 

 mental determination of the behavior of narrowband signals. A 

 significant disadvantage is that it seldom permits a path-by-path 

 analysis of the transmission loss. When properly used, the CW and 

 impulsive sources can provide information on complementary questions 

 regarding the nature of propagation. The impulsive source is suited 

 for measurements of the spectrum of the impulse response of the 

 medium for fixed source and receiver locations (with a frequency 

 average imposed by the nature of the source and, perhaps, even the 

 medium) . The CW source is suited for measurements of the spectrum 

 of the impulse response at one frequency for continuously varying 

 source and receiver locations. 



The Message 



The central point in the above discussion is that proper design 

 of a transmission loss experiment demands a priori estimation of the 

 nature of the loss characteristic to be measured. The present state 

 of acoustic models, both ray and wave, certainly permits making these 

 estimates with high confidence in many cases. Historical precedent 

 is no longer a sufficient (or even necessary) reason for using any 

 signal source or processing technique. 



521 



