PROPAGATION OF SOUND IN THE SEA 



temperature change, the sound paths are bent in the 

 direction of lower velocity of propagation, in other 

 words, in the direction of lower temperature. Even 

 though the changes in sound velocity are small (about 

 1 per cent for a temperature drop of 10 F), the result- 

 ant bending of the sound path becomes appreciable 

 over a distance of a few hundred yards. If, for in- 

 stance, the drop in temperature begins directly at the 

 surface of the water, and totals a degree or more 

 in 30 ft of depth, most of the sound energy will travel 

 along paths bent downward and will miss a shallow 

 target at a range of 1,000 yd. 



Because of this bending of sound by temperature 

 gradients, some departure of sound intensity from the 

 inverse square law is to be expected. The amount of 

 this departure can be calculated if the temperature 

 distribution in the ocean is known. However, even 

 this more complicated process for computing the in- 

 tensity is too simple. The effects of the boundaries of 

 the medium (ocean bottom and surface), and of the 

 absorption and scattering of sound in the body of 

 the ocean must, also, be considered. 



Both the sea surface and the sea bottom affect the 

 sound field intensity. Some of the sound energy 

 strikes these boundaries and is then partly reflected 

 back into the ocean, partly permitted to pass into the 

 adjoining medium (air or sea bottom). The portion of 

 the energy which is reflected will return into the in- 

 terior in a variety of directions. Also, little under- 



stood processes in the body of the ocean affect sound 

 intensity. In some way, a certain amount of the pass- 

 ing sound energy is converted into heat (absorption 

 of sound); and chance impurities such as fish, sea- 

 weed, plankton, and gas bubbles, tend to scatter a 

 small amount of the passing sound energy in all direc- 

 tions out of its principal path. 



For all these reasons, the propagation of under- 

 water sound presents, at first, a rather confusing 

 picture. Considerable progress has been made, how- 

 ever, in understanding the behavior of underwater 

 sound and in utiUzing this partial understanding in 

 the design and tactical use of sound gear. The results 

 which have been achieved are due to a combination 

 of theoretical and experimental investigations. Chap- 

 ters 2 through 10 discuss the background and progress 

 of these investigations. Chapters 2 and 3 lay the 

 theoretical groundwork for the physics of underwater 

 sound. Chapter 4 leads toward the experimental re- 

 sults by reporting on the equipment and procedures 

 employed in the experiments. Chapters 5 and 6 re- 

 port experimental results on the propagation of 

 sound, primarily sound generated by transducers. 

 Chapter 7 is concerned with the observed short-term 

 fluctuation of underwater sound intensity. Chapters 

 8 and 9 deal with the formation and transmission of 

 explosive sound. Finally, Chapter 10 summarizes the 

 results obtained to date and discusses possibilities for 

 future research. 



