Ch. 35] UNDERWATER SOUND 657 



Underwater Sound 



The extensive use of underwater sound (sonar) equipment in under- 

 sea warfare during World War II demonstrated conclusively the 

 profound effect of the medium on the results obtained. In deep water, 

 detection ranges with echo-ranging equipment depend on tempera- 

 ture gradients in the water and on the sea state, as well as on type of 

 equipment, ability of personnel, and type and speed of ship. In shal- 

 low water, the character of the bottom has proved to be more important 

 than other oceanographic factors. A hard, smooth bottom, such as 

 smooth sand, reflects the sound beam forward with comparatively 

 little distortion and backward scattering, whereas a hard, rough bot- 

 tom, such as rock, scatters so much sound back to an echo-ranging 

 ship that the target echo may be masked, even at relatively short 

 ranges, by the din of reverberation. So little sound is reflected from a 

 soft mud bottom, on the other hand, that sound conditions are very 

 similar to those in deep water. In listening, transmission conditions 

 tend to be best over a smooth sand bottom and are successively poorer 

 for rock, sand and mud, and mud. As reverberation is not involved, 

 listening ranges are therefore usually longest over a smooth sand bot- 

 tom, fairly long over rock or a firm bottom of mud and sand, and 

 poorest over mud. Echo ranges, because of reverberation, tend to be 

 shortest over coral, rock, and boulder-strewn bottoms, intermediate 

 over mixed mud and sand, about the same over soft mud as in deep 

 water, and longest over smooth sand. 



Another factor that influences the range of detection by sonar gear 

 is the background noise heard in the receiver. The character of the 

 bottom affects the distribution of certain biological noise makers, 

 particularly "snapping" shrimp. Snapping shrimp furnish one of the 

 best examples of "pure" science suddenly becoming of operational im- 

 portance in naval warfare. 



There are several species of small shrimp (not to be confused with 

 the edible one), which make a loud click by suddenly snapping a 

 pincer. These animals are widely distributed throughout the world in 

 tropical and subtropical waters (up to about the 50° winter isotherm), 

 in depths less than 30 fathoms (180 feet). They are chiefly confined 

 to rocky and coral bottoms, or those covered with cobbles and boul- 

 ders, where their colonies may be so abundant that the continuous noise 

 produced, as heard in a hydrophone, sounds like frying fat, burning 

 twigs, or the static from a radio receiver during a storm (Johnson et at., 

 1947) . Under such conditions they seriously interfere with the use of 

 listening equipment. Fish, particularly certain members of the croaker 



