CAVANAGH: AMBIENT-NOISE MODELS 



course, and speed. So this information is in addition to the 5-degree 

 cell data, but represents a small percentage of the total number of 

 ships in an ocean basin. In at least one recent experiment in which 

 there was extensive aircraft surveillance of surface shipping, 5-degree 

 resolution was the best that could be accomplished for a large ocean 

 area. 



Present information has led us to model the ship source as a 

 point source near the surface that radiates isotropically downward — 

 some modelers put the source at 20 feet below the surface, but do not 

 include the effects of image interference there. The paper given 

 earlier by Mel Pedersen on surface decoupling and the knowledge that 

 we have on image interference suggest that this is a particrilarly 

 important question: what does the ship source look like and how 

 does it interact with the surface? 



Models which we surveyed use an average, broadband, radiated 

 noise spectrum for each type of ship, parameterized on speed and 

 size. In nearly every case, the source levels are derived from the 

 spectrum that was published in the paper by Ross and Alvarez (1954) . It 

 shows a maximum at 70 Hz and a falloff of 6 dB per octave above that. 



Returning now to the prediction of ship-generated noise, present 

 applications suggest that it is best, since we have only shipping 

 distributions in large latitude-longitude squares, to use what I call 

 a field model : one that requires only the average transmission loss 

 as a function of range. In contrast, define a point model as one 

 which uses the precise locations of sources and requires a detailed 

 description of the transmission loss. Figure 2 illustrates the point 

 model; every ship is identified, and the resultant noise is simply 

 the sum of the contributions from each source. 



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