62 



RAY ACOUSTICS 



TEMPERATURE-F 



TEMPERATURE-F 

 80 



RANGE IN YARDS 

 1000 2000 3000 



A COMPLETELY ISOTHERMAL OCEAN 



150- FOOT ISOTHERMAL LAYER 

 OVER SHARP THERMOCLINE 



80 



50-FOOT ISOTHERMAL LAYER 

 OVER SHARP THERMOCLINE 



1 50- FOOT ISOTHERMAL LAYER 

 OVER WEAK THERMOCLINE 



50-FOOT ISOTHERMAL LAYER 

 OVER WEAK THERMOCLINE 



STRONG NEGATIVE GRADIENT 

 FROM SURFACE DOWN 



Figure 25. 



inside the place where the limiting ray hits it; and so 

 all the surface-reflected rays remain inside the 4.8- 

 degree ray, also. 



If the actual sound intensity obeyed the predic- 

 tions of the ray diagram, no sound at all would pene- 

 trate out to horizontal ranges of more than a couple 

 of thousand yards in the top 500 ft of the ocean. Thus, 

 a submarine further from the projector than 2,000 yd 

 could be almost certain of escaping detection by sonar 

 gear. In practice, as with split-beam patterns, the 

 shadow zone in the strict mathematical sense of a 

 region of zero intensity does not exist. However, un- 

 like split-beam patterns, the transmission anomaly 

 invariably increases sharply at or near the indicated 

 separation of sound from shadow when the down- 

 ward refraction is strong. As discussed in Section 5.4, 

 such zones of weak sound are observed whenever the 

 temperature gradient is strong enough to cause the 

 predicted shadow zone to start nearer the projector 

 than about 1,000 yd; the sound in them is about 

 30-40 db weaker than would be predicted by the in- 

 verse square law. If the negative gradient is weak, 

 however, so that the predicted shadow zone does not 

 begin until a range of about 1,500 yd or more, sound 

 usually decreases gradually and at a more uniform 

 rate ; the shadow zone in such a case can scarcely be 

 said to have any real existence. Possible mechanisms 

 for the penetration of sound into predicted shadow 

 zones are discussed in Section 3.7. 



Sample intensity contour diagrams. 



3.5.3 Intensity Contours 



Not all the characteristics of the sound field becoma 

 apparent from a glance at the ray diagram. Although 

 the distribution of intensity is governed by the 

 spreading of the rays, the degree of spreading cannot 

 be accurately estimated visually, and it is even diffi- 

 cult to judge qualitatively. If a ray diagram is avail- 

 able, the intensity at a" point may be quickly esti- 

 mated by measuring the vertical separation of the 

 rays nearest that point, in accordance with equation 

 (90). Since it is assumed that the ray bending is in a 

 vertical direction only, the predicted sound intensity 

 will be directly proportional to the measured separa- 

 tion of the rays. 



Intensity contours provide a very graphic method 

 for displaying the results of such computations. In 

 practice, the intensity loss is usually reported in 

 decibels below the sound level on the axis at a dis- 

 tance of 1 yd from the projector. The exact value of 

 the spreading loss at maximum echo range depends 

 on many factors, such as the strength and direction- 

 ality of the projector, the efficiency and operating 

 condition of the gear, the intensity of background 

 noise, and the amount of intensity loss due to absorp- 

 tion and scattering. In many cases, it is useful to 

 know at what range the intensity loss due to spread- 

 ing will be 55 db, at what range it will be 60 db, etc. 

 It is clear that the range at which the spreading 

 loss has a specified value will depend on the depth of 



