CHAPTER IV 



UNDERWATER ACOUSTICS 



Submarine Concealment, 



The problem with which the researchers of NDRC's Division 6 

 had to deal was technical and complex They were asked to devel- 

 op equipment which would detect a submerged submarine at a dis- 

 tance away great enough so that the combatant doing the detect- 

 ing could do something about it--either run away,, or attack 

 before the submarine could get home with its torpedo „ Also, 

 the equipment must locate the submarine with sufficient accuracy 

 to give an attack a reasonable chance of success 



Salt water is an excellent medium for concealment. Both 

 theory and experiment show that it 5s almost opaque to all forms 

 of electromagnetic waves--to radio s radar, infrared, ultraviolet 

 waves and to ordinary light„ Even in the clear waters of the 

 mid-Atlantic, objects cannot be seen at any considerable distance „ 

 The magnetic field of a submarine can be used to detect its pre- 

 sence, but again this field is imperceptible beyond a few hun- 

 dred feet„ The same is true of the disturbance in the electrical 

 properties of the sea caused by the presence of the metallic hull 

 of a submarine in the vicinity,, 



Early in their study of the problem the scientists of Divi- 

 sion 6 were convinced that the only form of radiant which could 

 be employed to detect a submerged submarine at distances great 

 enough to be militarily useful was sound o 



A submerged submarine has certain weaknesses „ Except un- 

 der unusual conditions , it must keep under way in order to 

 maintain its equilibrium*, and a submarine moving under water^ 

 even at very low speeds , produces low- frequency noises from 

 the machinery and from movements inside the hullo When the 

 submarine is under way at high speed, as for example when it 

 is trying to escape , high-frequency noises are produced by 

 cavitation about the propellers 



The other weakness of the submarine lies in the fact 

 that, though no leviathan, it is a bulky vessel „ For example. 



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