154 



SHALLOW-WATER TRANSMISSION 



WIHO FORCE (BEAUFORT) 



Figure 12. Rm versus wind force over ROCK. 



depth or bottom depth. (In very shallow water, less 

 than 10 fathoms deep, transmission is inferior to that 

 found in moderately shallow water.) Transmission 

 over MUD differs but little from transmission in 

 deep water; secondary peaks due to bottom-re- 

 flected sound are not likely to raise the level more 

 than 10 db above the level that would be observed 

 in a deep-water shadow zone. In isothermal water or 

 with upward refraction, transmission over all bot- 

 toms is about as good and sometimes sUghtly better 

 than deep water. 



Transmission anomalies with negative gradients 

 over the well-reflecting bottom types are affected ad- 

 versely by heavy seas. For sea state 3, transmission 

 anomalies are likely to be at least 1 db per kyd higher 

 than in calmer seas. 



6.3 



SONIC TRANSMISSION 



Sonic transmission differs from supersonic trans- 

 mission primarily in that dissipative processes within 

 the water are much less important. The probable 

 value of the absorption or attenuation coefficient at 



WIND FORCE (BEAUFORT) 



Figure 13. ^40 versus wind force over SAND. 



sonic frequencies has been discussed in Chapter 5. 

 It has been estimated^ that at the lower sonic 

 frequencies (2,000 c and less) the attenuation of 

 sound in sea water at a depth of several himdred 

 fathoms is less than 1 db in 20,000 yd. While there is 

 reason to believe that close to the surface, absorption 

 at these low frequencies is appreciably higher,^ it is 

 probably no more than about 0.5 db per 1,000 yd. As 

 a result, sonic sound in shallow water may show 

 evidence of a spread less than that predicted by the 

 inverse square law. This section summarizes the re- 

 sults which were obtained by UCDWR,'""^^ and by 

 CUDWR-NLL."'^ In these experiments, CUDWR- 

 NLL used a single-frequency source, with higher 



" If dissipative processes near the surface at low sonic fre- 

 quencies were as low as they were estimated at great depths 

 in reference 10, then listening ranges on noisy surface targets 

 should be of the order of 100 miles in the presence of deep 

 mixed layers; actual listening ranges rarely exceed 20 miles 

 even with the best sonic listening gear available. 



