Rigorous justification of a localized approximation to encode off-axis Gaussian acoustical beams

With the model of generalized Lorenz-Mie theory (GLMT) and the extended boundary condition method, describing the interaction between electromagnetic (EM) waves (more specifically laser beams) and particles, an acoustical GLMT-like framework can be used to deal with acoustical wave scattering. The incident acoustical wave may then be encoded in a set of beam shape coefficients (BSCs) similar to the ones used in EM scattering. Following a paper devoted to the evaluation of acoustical BSCs using a localized approximation in the case of on-axis Gaussian acoustical beams, the present paper deals with the evaluation of acoustical BSCs in the case of off-axis Gaussian acoustical beams.