Abstract: In this paper the physical acoustic method (the Kirchhoff's approximation) is extended to treat the scattering of a nonrigid surface in order to estimate the target strength of tar-gets with absorbing coatings. Using the local plane wave approximation, the relationship between the sound pressure and its normal derivative on the surface can be represented by the reflection coefficient of the plane wave and the acoustic imp dance of the surface. The resulting modified Kirchhoff's approximation involves the plane wave reflection coefficient. For the impedance sphere, a comparison between the physical acoustic method and the exact solution shows that the physical acoustic method is still a good approximation for the higher ka values.