ERRONEOUS OBLITERATED RESTRICTED 



DATA INFORMATION DISPLAY 



SIGNAL PROCESSING CIRCUITRY 



T 



INPUT CIRCUITS 



POWER SUPPLY TRANSIENTS 

 THERMAL NOISE 



TRANSDUCER 

 MAGNETIC ELECTRICAL ACOUSTIC 



ELECTRICAL INTERFERENCE J 



SIGNAL 



MAGNETIC 



ELECTRICAL 



ACOUSTICAL 



ACOUSTICAL INTERFERENCE 



"I 



LINE EFFECTS SELF NOISE SEA NOISE RADIATED NOISE 



ELECTROMAGNETIC | | | 



X 



RADIATED COUPLED FEED GROUND 

 BACK LOOPS 



PROPELLER 



r 



MACHINERY 



HYDRODYNAMIC PROPELLER 



' OTHER ' 



ACOUSTIC MACHINERY 

 DEVICES I 



I 



HYDRODYNAMIC 



' OTHER 

 ACOUSTIC 

 DEVICES 



STRUCTURAL AIRBORNE 



STRUCTURAL AIRBORNE 



Fig. 7.39 Origin and effects of acoustical and electrical interference. [From Ref. (45)] 



Acoustic Interference 



Three types of acoustical interference can 

 find their way into signal circuits and conse- 

 quently lower the signal-to-noise ratio: Sea 

 Noise, Self Noise and Radiated Noise. 



a) Sea Noise: 



In an acoustical system free of other 

 noise sources, sea noise is the background 

 against which signals must be detected. The 

 origin of the noise may. be due to thermal 

 agitation of the water molecules, seismic ac- 

 tivity, surface and sub-surface traffic and 

 sea surface motion which is dependent on 

 wind speed. In coastal waters additional con- 

 tributions are received from waves breaking 

 on the shoreline, turbulence around obstruc- 

 tions and noise from marine life. Because sea 

 noise is always present and must be ac- 

 cepted. Figure 7.40 is taken from Haigh's 

 report and indicates the background sea 

 noise for various sea states; these levels are 

 expressed in a 1-Hz bandwidth so they must 

 be increased by 10-log bandwidth to arrive at 

 the interfering pressure level. 



b) Self Noise: 



Self Noise is that produced by the vehi- 

 cle itself (propellers, machinery, hydrody- 

 namic) and picked up in its own transducers 

 or hydrophones. The submersible user 

 should also be aware that noise from its own 

 support ship can also be detrimental to func- 

 tions such as communications and tracking 

 and must also be considered. 



ALUMINAUT 



FREQUENCY VHi 



Fig 7 40 Background sea noise under various sea state [From Ref, (45)] 



363 



