1) Propeller Noise — Noise produced by the 

 collapse of bubbles created by the 

 propeller rupturing (rotating in) the 

 water. The point of inception of cavi- 

 tation depends on propeller speed 

 and depth. Generally, submersibles 

 have small, high speed propellers 

 and cavitation sets in at slow vehicle 

 speeds; however, the effect of depth 

 is to reduce the point of inception 

 and to move the noise spectrum to- 

 ward the higher frequencies. 



2) Machinery Noise — Noise emanating 

 from machinery and transmitted 

 through the structure induces pres- 

 sure waves. Unless precautions are 

 taken reciprocating and rotating ma- 

 chinery of any type will produce vi- 

 brations which can be transmitted to 

 the hull structure. Electrical trans- 

 formers and chokes can also produce 

 structural vibrations. Structural vi- 

 brations can be minimized by mount- 

 ing all rotating and vibrating ma- 

 chinery on rubber mounts and ensur- 

 ing that there is no mechanical cou- 

 pling between the machine and the 

 structure. In hydraulic and pneu- 

 matic systems this entails the use of 

 flexible pipe couplings, while in elec- 

 trical circuits no conduits should be 

 firmly secured to both the machine 

 and hull. 



3) Hydrodynamic Noise — Noise Originating 



from the flow of water across the 

 face of a hydrophone or turbulence 

 around some protuberance on the 

 vessel. At the slow speeds of sub- 

 mersibles this should not be a prob- 

 lem, except perhaps in those vehicles 

 propelled by water jets, 

 c) Radiated Noise: 



The sources of radiated noise are identi- 

 cal to those of self noise and the cure in 

 many cases is the same. If radiated noise is 

 present to any extent it may be assumed 

 that the vessel has self noise problems. 



Electrical Interference 



Electrical interference is of three types: 

 Electromagnetic, line effects and electro- 

 static. All can be induced directly or indi- 

 rectly into signal processing circuits and 



must be suppressed to a level less than that 

 of sea noise, which is considered the practical 

 background against which sensors operate, 

 a) Electromagnetic Interference: 



When currents pass through conduc- 

 tors, magnetic and electrical fields are 

 formed around the conductors as in Figure 

 7.41. The magnitude of interference depends 

 upon field strength, field geometry, the rate 

 of change of field and frequency and the 

 susceptibility of the receiving circuit. Haigh 

 divides electrical interference into radiated 

 and coupled. The difference is slight and 

 resides merely in the physical separation of 

 the transmitting and receiving elements. 

 Coupled interference is characterized in Fig- 

 ure 7.41a as both magnetic and electric ra- 

 diation between two cables running side by 

 side or between cores in a multi-core conduc- 

 tor. Conventional cable shielding methods 

 can contain the electric field as shown in 

 Figure 7.41b, but either exotic shielding or 

 complete separation of power supply cables 



- ELECTRIC FIELD 

 -- MAGNETIC FIELD 



a/FIELDS BETWEEN TWO CONDUCTORS 



ELECTRIC FIELD 



CONTAINED IN 



CONVENTIONAL 



SHIELD 



b/SHIELDING OF FIELDS 



MAGNETIC FIELD 



NOT CONTAINED 



BY SHIELD 



Fig. 7 41 Electromagnelic radiation (a), and the eftecis of conventional shielding (b). 



364 



