The study examines some acoustical objective indexes in eleven Italian theatres. The influence of the diffuse-reflection factor on the acoustic modelling was particularly analyzed. A room acoustic prediction and auralization software (CATT-Acoustic®) has been used, to simulate the sound field within the modelled rooms. In addition, the results of acoustical simulations were compared with experimental measurements to test the model’s accuracy. The proposed approach allows the evaluation of the acoustic quality of the sound field in concert halls as a function of their shape or the diffusing material’s properties.