Chapter 4 — PHYSICS OF SOUND 



Layer Effect 



Layer effect is the partial protection from 

 echo ranging and listening detection a submarine 

 gains when it submerges below layer depth. 

 Sometimes a submarine, diving through a shai'p 

 thermocline while taking evasive action, loses 

 the screw noises of the enemy ASW ship. Although 

 usually the pinging can still be heard, it is as 

 a low intensity signal. On the ASW ship, ranges 

 on submarines are reduced greatly when the 

 submai'ine dives below a sharp thermocline. 

 Often, the echoes received are weak and mushy. 



Shallow Water Effect 



Echo ranging is difficult in shallow water 

 because the sound is reflected from the bottom. 

 When the ship is in shallow water, and the 

 ocean floor is smooth, the sound bends down 

 from the surface to the bottom, then back up 

 to the surface, and again down to the bottom. 

 When the transmitted sound acts in this manner, 

 there are spaces empty of sonar coverage into 

 which a submarine can pass and be undetected. 

 In figure 4-20 you will notice how a sonar 

 contact on a submarine can be lost when the 

 submarine enters the shaded area. 



The shaded spaces in figure 4-20 are called 

 shadow zones. Contact is regained when the 

 submarine enters the wave path again. As shown 

 in the illustration, contact is made at long 

 range (point A), it is lost (point B), and re- 

 gained at short range (point C). Note, however, 

 that without the reflection, the maximum range 

 would have been the short range at which the 

 contact was regained (point C). 



The reason for the loss of contact at short 

 range is shown at point D, where the submarine 

 passes beneath the sound. The distance at which 

 contact is lost at short range depends on the 

 depth of the submarine. Here is a rule of 

 thumb for estimating a submarine's depth: 



• The range in yards at loss of contact is 

 rougUy equal to the depth of the submarine 

 in feet. A contact lost at 300 yards would thus 

 be presumed to be about 300 feet deep. 



Once the behavior of sound in sea water is 

 known, it is possible to predict sound condi- 

 tions in the sea. From the temperature gradient 

 of the water, an experienced person can judge 

 the maximum range to expect. A new Sonar 

 Technician is not expected to know how to 



71.28 

 Figure 4-20. — Shallow water effect on trans- 

 mitted sound. 



predict sound conditions, but he should under- 

 stamd why results are poor one day and good 

 another day. He should know what is meant by 

 shadow zones, and the reasons why sound does 

 not travel in a straight line. He also should 

 be able to distinguish between poor equipment 

 adjustment and poor sound conditions. 



Following are some general conditions for 

 hearing echoes, 



1. Usually poor near coasts (50 miles) as 

 compared to sea conditions farther out. Condi- 

 tions for hearing are particularly poor at the 

 mouths of rivers. 



2. Better in winter than in summer. 



3. Better at night than in the middle of the 

 day, especially in spring and summer. 



4. Better in morning than in afternoon, in 

 spring and summer, but little change if wiute- 

 caps are present. In many localities, however, 

 conditions are better in the afternoon than in 

 the morning, because of the effect of prevailing 

 winds that freshen in the afternoon. 



DEEP WATER SOUND PROPAGATION 



From the preceding discussions, it can be 

 concluded that the behavior of the sound wave 

 is influenced considerably by the structure of 

 the sea. Most of our discussion so far has 

 pointed out the adverse effects on a sound 

 beam in comparatively shallow water — say 100 

 fathoms or less. Now, let's examine some of 

 the phenomena that take place in very deep 

 water. 



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