During free field reciprocity calibration for a convergent spherical acoustic wave of a focusing transducer, the diffraction loss correction coefficient is a key parameter of obtaining the reciprocity coefficient. In this paper, we use the so-called impulse response integral method instead of the Rayleigh integral method to calculate the diffraction loss coefficients of various therapy transducers such as the mechanical-scanning B-Mode transducers and the Doppler transducers. Some numerical graphs with typical parameters are given. Compared to the Rayleigh integral method, the impulse response integral method greatly reduces the computational time while at the same time maintaining the accuracy of the computation results to certain extent.