CAVANAGH: AMBIENT-NOISE MODELS 



An example of the input to a field model is shown in Figure 3. 

 Entries denote the number of ships per million square miles for ocean 

 regions of 100 nm radial extent. 



There appears to be no easy way to predict noise with a point 

 model: the propagation must be estimated in detail. However, in 

 the case of the field model, Talham (1964) developed an approach 

 which exploits the relaxed requirements on transmission loss pre- 

 dictions. Although its direct application is limited to range- 

 independent environments, the method is outlined next, in anticipation 

 of the extension given later. 



Let a receiver be located at depth Z and range R = 0, and 

 assxame that a small horizontal region near the ocean surface at range 

 R and on bearing ()> radians (re 0°T) radiates sound with an equivalent 

 point source intensity P(R,(});G)f in a one Hertz band, per unit 

 area, at unit distance, in vertical direction 6 (radians measured 

 from the horizontal) and horizontal direction (J) (equal to the bearing 

 in radians) . For the geometry of Figure 4 (a) , the shaded horizontal 

 region has area: 



-) ' it^^y 



arF 



= RARA()), (1) 



so that the directive, point source intensity for the region is 



P(R,(});9)RArA(J). 



Tl^e intensity at the receiver due to the source radiation is the 

 source intensity reduced by an appropriate attenuation from trans- 

 mission. From a geometrical acoustic point of view, sound propagates 

 along many paths from the point source to the receiver. For any one 

 of these, say the path of Figure 4(b), which has initial angle 6 and 



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