Field of a parametric focusing source

An analytical description for the field of a parametric focusing source is derived. It is valid for spherically concave sources with small aperture angle and high ka, under conditions of quasilinear interaction (strong shocks precluded). The solution furnishes computations on the phase and amplitude of difference frequency sound along the axis and in the focal plane, as well as on the width of the radiation lobe in the focal region. Underwater experiments conducted with an f/2 lens coupled to a dual, interleaved primary array are discussed. The results support the utility and validity of the analytical model for describing the distribution of sound along the acoustic axis and across the focal plane. The difference frequency radiation was found to be effectively focused, in that the width of the beam became quite narrow in the focal plane.