Ambient noise vertical directionality—a useful empirical model

Abstract

Descriptions of the noise vertical directionality which transform into simple spatial correlation functions are particularly useful because directionality measurements are conveniently made in the spatial correlation domain. A useful model for the horizontally concentrated distant shipping noise is the function exp[−(cosθ/2ζ)2], where θ is the angle with the vertical and ζ is a parameter‐controlling beamwidth. For typical beamwidths this expression transforms with negligible error to exp[−(ζkd)2] for the spatial correlation function. Depression or elevation respect to the horizontal can be handled with simple elaboration. Several authors have described the directionality of the noise deriving from surface agitation in terms of cosθ and cos°θ (isotropic), both of which have closed‐form transforms In kd space. These three expressions are used in combination to characterize the noise field in the Labrador Sea as measured in three different wind conditions, from frequencies at which shipping noise dominates through the transition region to frequencies at which surface noise is the major factor.