Abstract
The parametric interaction of two high frequency radiations in a focusing field produces a nonlinearly generated, difference frequency radiation that is also effectively focused. An analysis of this phenomenon, utilizing the parabolic approximation of the Helmholtz wave equation, has already been developed. The present paper presents anotheranalytical model which makes the numerical computation much easier compared with the solution of the parabolic equation. The primary sound in the model is roughly assumed to be a spherical wave except in the focal region, where it is assumed to bea plane wave. The difference frequency sound pressure is obtained under the quasi-linear assumption by the volume integration of the radiation from the virtual sources, takinginto account the diffraction effect at the difference frequency. The validity of the model is demonstrated through comparison to computed results of the parabolic approximation.
