In many cases the in situ measurement of the absorption coefficient requires an iterative method for the correct calculation of the surface impedance of a sample. This happens because when spherical waves reflect on a sample's surface the pressure field above it is a function of the sample's surface impedance. As the pressure field involves an integral term, numerical integration is required in the iterative algorithm, which can be time consuming. The aim of this paper is to present an iterative approach based on Pony's method, which instead of numerical integration uses a series expansion with a few terms. Therefore the time of processing is decreased. Besides the description of the method, simulations and measurements (with a p-u probe inside an office room and a car) are presented. A comparison, with numerical integration, regarding the accuracy and the time of processing of Pony's method is also discussed.