For the purposes of this report, these two expressions will be used 

 to provide two estimates of sound transmission to bracket roughly the upper 

 and lower ranges of expected sound transmission. Spherical spreading provides 

 a low limit, fo>' conditions where sound propagation is not enhanced by sound 

 reflections from the sea surface and ocean bottom. This occurs for water so 

 deep that reflections from it may be ignored and for shallow water where the 

 bottom and/or surface are highly absorptive. This occurs where the bottom is 

 soft and muddy, and where the surface is covered with old ice which may have a 

 mushy, rough underside. Normally the ocean surface is reflective, except in 

 the case of a highly agitated sea state, with many breaking waves. 

 Cylindrical spreading gives an upper limit for sound propagation. It is used 

 for cases where sound is propagated efficiently, as between a reflective 

 bottom and surface. Hard, smooth bottoms, such as sand and relatively uniform 

 rock tend to be reflective, and as mentioned above, the sea surface is 

 normally reflective at low and moderate sea states (less than state 3). This 

 report will give estimates for detection range under both spherical and 

 cylindrical propagation conditions. Figure 3 shows the transmission loss for 

 both cylindrical and spherical spreading out to a range of 10,000 kiloyards. 

 The effect of normal attenuation loss is clearly evident by the fall-off 

 beyond 1000 yards. Note that this effect is greatest for high frequencies and 

 is nearly negligible at the lowest frequencies of 16 and 32 hertz. 



G-18 



