The evaluation of room acoustics characteristics in rooms has been thoroughly described in several papers since 60-ies. Moreover, the ISO 3382 standard describes several acoustic parameters and their measurements. However, there is only a few information about the methods of pre-processing the impulse responses that are required before calculating those acoustic parameters. If the final goal of measuring the acoustical behaviour of a room is to compare it to others, or to create databases, reliable and comparable data must be obtained, no matter the way they have been measured. At the Engineering Department of the University of Bologna, several tests, with the same starting data set, have been performed to compare the results given by the most commonly used PC applications, in order to identify and quantify any possible incongruity. In this paper, the main processing methods (based on Lundeby, Chu) are analyzed. Moreover, they are compared with the Schroeder (backward integration) methods. In a further step, these methods are applied in some acoustic measurements employed in some opera houses in Italy.