Acoustical localized approximation for acoustical fields with axicon angles.

An acoustical field may be expanded over spherical wave functions with the expansion coefficients expressed in terms of beam shape coefficients. Adapting a technique established in the case of electromagnetic beams, a localized approximation has been developed for a large class of acoustical fields, in particular Gaussian beams, allowing one to evaluate efficiently and rapidly these coefficients. This paper discusses the fact that the acoustical localized approximation so established is of limited validity in the case of acoustical fields exhibiting an axicon angle.