CAVANAGH: AMBIENT-NOISE MODELS 



We have identified three submodels that make up an ambient-noise 

 model: source, transmission, and summation. Focus now on the first 

 of these. Ambient noise sources include such things as biologies, 

 rain, seismic activity, oil drills, oil explorations, distant shipping, 

 and wind action. Here we concentrate on what most of the existing 

 models predict: ambient noise resulting from distant shipping and 

 wind action on the sea surface. These two cover the prevailing 

 sources in the regime from 20 Hz up to several kHz as shown in the 

 Wenz-type curves of Figure 1 (after Wenz , 1952). 



Consider first the wind noise sources and wind-dependent ambient 

 noise. Our experience has been that wind-dependent noise can be pre- 

 dicted with good accuracy, provided that the wind speed is known and 

 the acoustic frequency is above 200 Hz. Almost every model that we 

 have reviewed either adds a Wenz level as an isotropic component to 

 the noise or performs a ray calculation of the noise level in the 

 following way : 



The source is considered to be a continuous distribution of 

 point sources located on or just below the ocean surface. If trans- 

 mission ray paths are added incoherently, then each source has dipole 

 directivity. Source spectrum levels (dBs referred to 1 yPa per unit 

 area of surface at unit distance) will be constant below about 500 Hz 

 and then fall off at 3 dB per octave. Typical values are 70 dB for 

 a 40-knot wind at 100 to 500 Hz and 40 dB for a 1-knot wind at 

 5 kHz. In an isospeed ocean, these will yield levels consistent with 

 Wenz (1962) or Knudsen (1948) wind noise. • 



If the model is to estimate depth dependence and vertical direc- 

 tionality, then propagation from each source to the receiver can be 

 predicted by ray theory. Range-averaged transmission loss will 



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