## 4 Discussion
This was assumed to indicate that more early sound energy is collected because of reflection and diffraction. The objective acoustic parameters increased (_EDT_ and \(T_{30}\)) and decreased (\(C_{80}\) and \(D_{50}\)) with increasing surface scattering as in the measurement results. This shows that in simulations also the diffusive surfaces tend to disperse reflections in space and time leading to a longer reverberation time compared to the reflective condition. In the reflective condition, a more concentrated spatial and temporal reflection is generated and successively absorbed by the absorptive surfaces that cover the ceiling and the other lateral walls. It should be highlighted that this result might change for different combinations of the diffusive and absorptive surfaces as summarized in [PERSON] at al. [35]. The influence of the different modeling alternatives of the diffusive surfaces was more evident at positions close to the variable wall for the _IACC_ parameter. The values of this parameter at the positions close to the lateral wall were different for the DM and DF conditions. [PERSON] et al. [64] showed that the correlation between the impulse responses at two ears at positions close to any surface is lower due to effects of the surface proximity on the impulse response of the ear oriented towards it. In this way, the binaural impulse response is affected asymmetrically on each ear channel (left and right). In the configuration investigated in this study, the right ear is oriented towards the variable wall, while the left ear is oriented towards the other lateral wall, which is absorptive. Thus, the differences in reflections between the two ears are emphasized also by this asymmetric distribution of the material properties, i.e., the correlation between the two ears decreases. The perceptual investigation highlighted the differences between the scattered sound models shown in Figure 4. Based on the scattering perceptual thresholds [35], a difference in scattering coefficient of 0.70, which is the difference between the scattering coefficient of the diffusive surface modeled flat (0.75) and the reflective surface (0.05), should be easily perceived by the subjects. Despite this, it was not possible to find a distance threshold when the flat modeled diffusive surface (DF) was used. A distance threshold of 7.50 m was found when the DM model was used, i.e., the diffusive surfaces were modeled as 3D panels. As shown in Figure 4g, the more realistic modeling of the diffusive walls in the DM case (including edge diffraction) leads to more realistic directivities than modeling by vector-based scattering, i.e., vector mixing, used in the DF case. In particular, when compared to the BEM simulation polar distribution, it can be noticed that the latter model does not consider the part of the energy that is deflected in the direction of the incidence angle. This might be the reason for the better audibility of diffuse reflections and better agreement with measurements. The distance threshold is higher than the distance of 2.15 m found by performing the same listening test with measured binaural impulse responses. Such different thresholds might be due to the scattered sound model approximations. As reported in [PERSON], [PERSON], and [PERSON] [14], some major limitations of a Lambert-based scattering model are that it neglects phase, thus neglects interference effects between specular and nonspecular scattering, whose sum constitutes the total surface scattering at a receiver position in a room. In the case of the modeled diffusive surface (DM), the scattering algorithm automatically takes into account also edge-diffraction effects, that follow a far-field type of behavior, i.e., are distance dependent [14]. Thus, the differences between the reflective and diffusive condition are evident also at longer distances in simulations with a high degree level of detail.