10 



100 1000 



FREQUENCY (HZ) 



10,000 



Figure 11. Frequency-response curves of the portable tape recorder. 



SELF-NOISE SOURCES 



The electronic system noise was discussed in the 

 section on the hydrophone -preamplifier. This source of 

 system noise was shown to be considerably below the noise 

 levels to be expected from the lowest ambient-noise level 

 likely to be encountered. 



Another source of self-noise is due to the motions 

 of the cable and suspension system. In particular, self- 

 noise arises from flow noise, cable vibrations caused by 

 towing the cable, and accelerations and static pressure 

 changes which are caused by vertical motions of the hydro- 

 phone due to the action of surface waves on the system. In 

 the cable systems which are described below the self-noise 

 is reduced by isolating the hydrophone from the motion of 

 the ocean and the motion of the ship. This is done through 

 the use of shock cord in the suspension system and by main- 

 taining slack in the cable during measurements. The acce- 

 leration sensitivity of the hydrophone, as mentioned pre- 

 viously, is minimized by design. The sensitivity of the 

 system decreases below 10 Hz, so that it is relatively in- 

 sensitive at the very low frequencies associated with major 

 wave motion. 



19 



