Chapter 4— PHYSICS OF SOUND 



and experience in recognition are the best in- 

 surance in countering these artificial targets. 

 Some of the self-propelled kind receive the 

 ping of the echo ranging sonar, amplify it and 

 give it a change in doppler, then retransmit it. 

 They also may be programed to simulate sub- 

 marine movements. Usually the returning signal 

 is stronger than the returning echo from the 

 submarine, so it is easy for the inexperienced 

 sonar operator to shift from the actual target 

 to the artificial one. 



The stationary type also returns a strong 

 echo, which often is more pronounced than that 

 from a submarine. This type of target comes 

 in several forms. 



One form of stationary target is a "pill" 

 of chemicals, wMch reacts with water to form 

 bubbles of carbon dioxide gas. The action is 

 much the same as bicarbonate of soda in water. 

 The pill dissolves, creating millions of bubbles 

 of the gas, which, because they ai'e grouped 

 together, return a strong echo. 



Another target of the bubble type is pro- 

 duced when the submarine releases a quantity 

 of air. A few moments are required for the air 

 to rise to the surface, and while It is doing so, 

 it provides excellent sound-reflective character- 

 istics. Like the pill, the echo returned is often 

 stronger than that obtainable from a submarine. 



Similar to the air bubble released by a sub- 

 m.arine is the knuckle, wMch is a mixture of 

 air and water created by a submarine making 

 a sharp turn. Although it is not in general use 

 by a submarine commander during an attack, 

 because it slows him down, he may have to 

 make a hard, high-speed turn in an attempt 

 to avoid a weapon. The knuckle can produce 

 a clear, sharp signal upon which an inexperi- 

 enced operator may concentrate while the sub- 

 marine escapes. 



Stationary false targets do not affect the 

 pitch of the echo. This characteristic means 

 that a stationary target does not give a doppler 

 change except under very unusual conditions. 

 This unchanged doppler characteristic is an 

 important clue In recognition of a false target. 



When tracking a target, for example, an 

 abrupt change from doppler up (or down) to no 

 doppler may well be an indication that the 

 target has ejected a bubble to throw you off 

 guard, which may result in your making an 

 attack on the false target. You must, therefore, 

 be suspicious of any drastic change In doppler. 



EVERYTHING 



SUBMARINE 

 SUBMERGED 



Figure 4-31. 



71.45 

 Report everything you hear. 



Furthermore, you must be suspicious of echoes 

 that change suddenly from weak or mushy to 

 clear and sharp. Such an echo may be a self- 

 propelled tai'get leading you away from your 

 quarry. 



Electronic and mechanical jammers are also 

 used by submarines. The electronic type usually 

 operates on one of several selectable sonar 

 frequencies, acting as a screen by overloading 

 the ship's sonar receiver. The mechanical type 

 of jammer releases a series of explosions that 

 jam the sonar receiver because of their wide 

 bandspread of noise. The mechanical type Is 

 especially effective In shallow water because 

 of repeated reverberations from the bottom. 



You must remember that doppler can tell 

 you considerably more about target motion than 

 simply whether it Is going toward or away from 

 you. You must practice to achieve the near- 

 perfect discrimination demanded of a good Sonar 

 Teclinlcian. To achieve this goal, you must 

 listen and concentrate on echoes. 



55 



