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FORECASTING  EFFECTS  OF AMBIENT NOISE
EVALUATING THE IMPACT OF BIOLUMINESCENCE

Aerographers Mate 1 & C
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Figure 9-13.-Ambient noise levels. Four or more ships closer than 30 nmi constitute heavy  noise,  with  ships  closer  than  10  nmi driving the noise level up to the individual ship’s target curve (curve G). Where the bulk of the traffic is farther than 40 nmi, the average traffic curves (curves D and E) apply. This does not apply to a carrier task group. Correlations  of  noise  intensity  with  distance  to nearest ship, with all ships present in the shipping lanes, were negative. For areas not immediately in a heavy traffic area, ship concentration and distance became critical. SURFACE-SHIP TRAFFIC NOISE At the lower frequencies the dominant source of ambient  noise  is  the  cumulative  effect  of  ships  that  are too far away to be heard individually. The spectrum of the noise radiated from ships as observed at great distances differs from the spectrum at close range due to  the  effect  of  frequency-dependent  attenuation. Sea-state  noise Sea state is a critical factor in both active and passive detection. Inactive sonobuoy detection, waves 6 feet or greater  will  start  to  produce  a  sea-state-limited  situation. For  shipboard  sonar  systems,  location  of  the  sonar dome,  ship’s  speed,  course,  and  relation  to  the  sea  all have an effect. The limiting situation is generally sea state  4  or  5.  For  passive  detection,  the  noise  level created by wind waves of 10 feet or greater will result in   a   minimum   of   antisubmarine   warfare   (ASW) operational  effectiveness,  depending  on  the  type  of sensor. WIND-GENERATED NOISE.– Sea-state noise generated  by  surface  wave  activity  is  usually  the primary component over a range of frequencies from 300 Hz to 5 kHz. It maybe considered to be one of the most  critical  variables  in  active  and  passive  detection. SEA-STATE   NOISE   LEVELS.–   T h e wind-generated  noise  level  decreases  with  increasing acoustic  frequency  and  increases  with  increasing  sea state (approximately 6 dB for each increase in sea state). It is very important to understand that all sound-sensor ranges are reduced by additional noise, and that there can be a 20-dB spread in background noise between various sea states. 9-12







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