Key points are not available for this paper at this time.
Variational principles are presented for the scattering amplitude in the general acoustic scattering problem, and, for spherically symmetric scatterers, for the phase shifts. Integral equations for the acoustic field are also given and the properties of the scattering matrix are developed. The entire formulation remains valid in the presence of discontinuities of density and/or velocity in the medium.
Gerjuoy et al. (1954) studied this question.