## 5 Conclusions  
The effects of two modeling alternatives of diffusive surfaces in geometrical acoustics (GA)-based software have been investigated, both objectively and perceptually, in a small variable-acoustics concert hall. This research aimed to isolate the independent effects that a single diffusive surface has on the simulated objective acoustic parameters and perceptual experience based on simulated impulse responses. Thus, two different conditions of the hall, where one of the lateral walls assumed reflective (RF) and diffusive characteristics (DF and DM), have been considered. The two modeling alternatives of the diffusive condition were built based on the simulations state of the art and considering the limits of a possible increase of the modeling level of detail. In the first model (DF) diffusive prisms have been modeled as a flat surface to which a scattering coefficient equal to the scattering coefficient of the diffusive surface they represent (0.75 at 707 Hz) was assigned. In the second model (DM) the diffusive prisms have been modeled as 3D panels with their geometrical characteristics, and to each surface of the prisms a scattering coefficient equal to that of a flat surface (0.05 at 707 Hz) was assigned. The objective evaluation has been carried out by analyzing the variation of the ISO 3382-1 acoustic parameters \(T_{30}\), \(EDT\), \(C_{80}\), \(D_{50}\), and \(IACC\) in each of the three models (RF, DF, and DM). The perceptual evaluation aimed at determining the maximum distance from the wall at which the listeners could no longer perceive the difference between the reflective and diffusive condition. Furthermore, it aimed to investigate if this distance is influenced by the different modeling alternatives of the diffusive surfaces in GA-based software. The psychometric functions have been evaluated and statistically analyzed using the Psignifit toolbox (version 2.5.6). The main conclusions of this work can be summarized as follows:  
* A good match and similar trends can be achieved between simulated and measured results regardless the geometric modeling of the diffusive surfaces. However, this requires different material calibrations with respect to the diffusive properties. * Listeners in a simulated performance space can perceive the presence of different acoustic scattering properties, and it was found that this perception is related to the distance from the diffusive surface and to their geometric modeling. * A more realistic geometrical modeling of the diffusive surfaces leads to more realistic reflections directivities than when vector mixing model is used. This might be the reason for the better audibility of diffuse reflections and better agreement with measurements. * Distance thresholds in simulations (7.50 m) are higher than in real measurements (2.15 m) due to the scattered sound model approximations: reflections and diffraction. * No correlation between the subjective and the objective parameters results could be found, thus there is a need for further investigation on new single measurable parameter of a more practical use. Most of the listeners were sensitive to the presence of acoustic scattering also at distant positions from the diffusive surfaces in simulations. Therefore, the use of these surfaces in large concert halls might affect the uniformity distribution of the acoustic quality in a large group of listeners and influence the design goals. Future work could involve objective and perceptual investigations concerning the comparison of different simulation algorithms, by considering different locations and extensions of the diffusive surfaces in a broader number of hall volumes and shapes. Many musicians in the stage area are frequently located at distances from the stage walls that are comparable or shorter than the distance threshold found here, thus they are expected to perceive the differences between different modeling alternatives and surface scattering properties assigned in simulations.