Another source of self-noise is radiated noise from 

 machinery on board the platform. This noise must be 

 reduced in order to obtain valid ambient-noise measure- 

 ments. It is also desirable to place the hydrophone as far 

 from the platform as possible when making these 

 measurements. 



FIELD TESTS 



Various hydrophone suspension systems and cable 

 systems were tested at sea to attempt to devise a system or 

 systems which have low self-noise, are simple to use, and 

 which are adaptable for measurements in deep or shallow 

 water. Tests were run using various cable combinations of 

 buoyant and nonbuoyant cable and suspension systems to 

 attempt to determine the self-noise of the cable systems 

 for a taut and for a slack cable, and also to determine which 

 system of those tested was most suitable. Descriptions of 

 the suspension systems and of the cable systems follow. 



Hydrophone Suspensions 



The purpose of a hydrophone suspension is to isolate 

 the hydrophone from vertical motions which are induced by 

 surface waves and swells. The isolation should be sufficient 

 to prevent "blocking" of the system which is caused when 

 excessively high noise voltages are applied to the preampli- 

 fier input. 



As a starting point for the development of a suspen- 

 sion system, a system based on one developed by General 

 Dynamics/Electric Boat was used 3 (fig. 12A). It consists 

 of a hydrophone -preamplifier unit, a 50-foot cable, a buoy 

 (a flexible inflated plastic hose), a 50 -foot shock cord 



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