It has been found that the attenuation of sound In sea 

 water is 40 to 300 times greater than it should be on the basis 

 of simple theory., This is one of the outstanding problems of 

 underwater sound which still demands a scientific explanation,, 



Determination of an attenuation coefficient becomes much 

 more difficult when temperature gradients are present in the 

 top 100 feet of the ocean When, for instance, there are 

 gradients within 50 feet of the surface, the average attenua- 

 tion in the shallow isothermal layer is generally about twice 

 as great as it is when the Isothermal layer is deeper, but it 

 is also much more variable,. Finally, when there is a strong 

 negative temperature gradient beginning at the surface, trans- 

 mission anomaly does not show a simple proportionate increase 

 with range o Temperature gradients cause the sound rays to 

 curve, so that the spreading of the sound obeys more compli- 

 cated laws and obscures the effects of attenuation 



The researchers also undertook to study further the re- 

 fraction theory which had been enunciated as the result of 

 earlier work at Woods Hole, and which had also been developed 

 independently before the war, both at San Diego and In Britain, 

 to explain the observed variability of underwater sound trans- 

 mission Careful observations of what was called the "after- 

 noon effect" had enabled Woods Hole scientists to establish 

 the fact that it was produced by shallow negative temperature 

 gradients,, 



The explanation was simple Since the water near the 

 surface was warmer than any below it, and since sound veloc- 

 ity increases with Increasing temperature, any part of a wave 

 front which was nearest the surface traveled faster than the 

 part which was deeper and in colder water „ Therefore, the 

 whole wave front curved toward the region of lowest tempera- 

 ture, much as a twin-screw ship turns in the direction of its 

 slower propeller,. The sound beam was "bent" toward the bottom,, 



To put it in another way, all sound rays leaving the pro- 

 jector were bent downward,, The one which left the projector 

 at an angle, enough above the axis to become horizontal at 

 the surface was the one which traveled farthest „ It was the 

 "limiting ray c " Rays leaving at higher angles were reflected 

 or lost at the surface, Rays leaving at smaller angles never 

 reached the surface, since they were bent toward the bottom 

 before reaching the top., Beyond the limiting ray was the 

 "shadow zone," a region into which no sound could penetrate „ 



27 



