As the work progressed it was found that certain earlier assump- 

 tions, while useful, were not strictly true, Transmission 

 studies begun at San Diego proved that shadow zones do not al- 

 ways occur when refraction theory predicts them, and that when 

 they do occur they are not utterly soundless Intensities with- 

 in the predicted sound field, similarly, are affected by many 

 things, though refraction nay be extremely important. 



Refraction theory is best borne out by the condition which 

 led to its formula t ion A predicted shadow zone materializes 

 only when there is a strong negative gradient starting at the 

 surface o Then the sound within the shadow zone is more than 

 a thousand times weaker than that in the direct sound field, 

 There is some sound in the shadow zone, it is true, but not 

 enough to produce an audible echo from a submarine „ What 

 little sound there is probably results from the forward scat- 

 tering of sound, perhaps by a deep "scattering layer „" 



There are no other conditions under which shadow zones 

 materialize. With all other types of temperature distributions, 

 no matter what the ray diagram predicts, sound intensity at any 

 ~iven depth diminishes regularly with range , There is no point 

 beyond which the intensity drops sharply,. 



While basic, research expenditures are of the nature of a 

 capital investment, subject to amortization over a long period 

 of time, many immediate returns were realized on the work done 

 at Woods Hole and San Diego , 



The problem of estimating maximum echo ranges is very 

 much like weather forecasting. It is not perfect, but it is 

 far better than guess worko Early methods of range prediction 

 were fairly simple . In fact, they turned out to be too simple. 

 But, the prediction method that finally evolved contains two 

 different procedures, one fcr deep water and one for shallow 

 water. Under certain conditions the method used in deep water 

 may be used in shallow water as well. When these conditions 

 are not found, however, the problem of range prediction in 

 shallow water is so complicated by bottom reflections and 

 reverberations that it must be solved by entirely different 

 means. The range tables now in use are based not only upon 

 transmission measurements, but also upon studies of noise, 

 reverberation and recognition. Even so, it must be admitted 

 that they give only approximate values and it is to be hoped 

 that the studies may be continued and that they will result 

 in new methods which will prove more accurate and less dif- 

 ficult to use- It is particularly desirable that research be 

 continued into a srstem of prediction fcr use in shallow water. 



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