622 HYDROGRAPHIC MANUAL PaGE 560 



other media. Sound waves in water are confined to a layer between definite bound- 

 aries formed by the surface and the bottom, a condition not found in other media of 

 large dimensions. This layer is of varying thickness, but its vertical dimension is 

 generally small as compared to the horizontal distances usually involved in subaqueous 

 sound ranging. 



622. Propagation in an Ideal Water Medium 



The laws governing the propagation of sound in water can be more clearly explained 

 and understood if the transmitting medium is assumed to be a homogeneous one and 

 if the boundaries are well defined, plane, and parallel. Furthermore, in a simple case 

 of this nature, and assuming the frequency of the sound is low enough not to be atten- 

 uated by the viscosity of the medium, efficient transmission of sound will be obtained. 

 A body of water approaches the ideal case of homogeneity where the temperature and 

 salinity are uniform throughout, and where foreign matter is absent. And, under these 

 conditions, it will afford efficient transmission of sound if the bottom is smooth and 

 level, and presents a good reflecting surface. 



Such a condition is not entirely hypothetical for, during certain seasons, this ideal 

 is approached in certain localities; for example, in certain areas of nearly uniform depth 

 on the Atlantic Continental Shelf during the winter and early spring. In some localities 

 on the Pacific Coast and in Alaska waters, conditions approach this ideal during most 

 of the year. (See fig. 130.) 



From a nondirective source of sound in an elastic medium, waves are radiated in 

 all directions. From the source to the point of reception in a bounded medium a sound 

 wave may travel an almost unlimited number of paths. There may be a direct path, 

 a refracted path, and a multitude of reflected paths. In practice it will be found that 

 all three of these types of paths apply to subaqueous sound ranging in varying degrees, 

 depending on the distance and the condition of the medium. Within a certain range 

 the sound that travels along the direct path (without reflection from the boundaries) 

 will be the first to reach the receiving apparatus, but at a greater distance, where an 

 unreflected wave cannot reach the point of reception, sounds that have been refracted 

 and reflected will be the only ones to arrive. The sound wave that arrives first with 

 sufficient intensity to be detected is, of course, the one that is used in subaqueous sound 

 ranging — any sounds arriving later only serve to prolong the received signal. In 

 6221 and 6222 which follow, the propagation by various paths is considered. 



6221. Reflection of Sound 



The long-distance transmission of sound in water is greatly assisted by reflections. 

 Subaqueous sounds, after having been reflected a number of times, are known to have 

 retained sufficient energy to be recorded at a distance of 400 nautical miles from the 

 source by instruments of only ordinary sensitivity. And a sound wave generated by 

 an electromagnetic oscillator to measure the depth in about 200 fathoms of water has 

 retained sufficient energy to be heard in headphones after having been reflected 23 times 

 alternately from the bottom and the surface. The excellent transmission of sound in 

 water is due, not only to the fact that water is a relatively good medium for the prop- 

 agation of sound waves, but also to the relatively good reflecting surfaces formed by 

 the water surface and bottom. The boundary between air and water, when the latter 

 is smooth, is a good reflecting surface for sound waves; likewise the boundary between 



