## 2 Methods
### Numerical Method
#### 2.2.3 Reflection-Based Scattering
Thus, this number was set at 100 K rays in all models and the maximum reflection order was set to 10K. A TO = 2 has been chosen based on the literature and frequently used values for similar environments [49]. Run-to-run variations have not been considered here since, based on the literature investigations, the simulator can be considered sufficiently stable [50; 51]. Post-processing of both measured and simulated data has been carried out using ITA-Toolbox, an open source toolbox for Matlab [52]. Assumptions and Adjustments  
In order to achieve realistic auralizations, it is important to have a well-calibrated GA model. Based on [PERSON] [53], a simulation is well calibrated when the difference between simulation and measurement is less than the JND of each objective acoustical parameter. Based upon this statement, the calibration in this study was made by comparing the simulated objective parameters to the measured ones in real conditions of the hall. Furthermore, some of the indications given in the general procedure described in [PERSON] and [PERSON] [50] have been used. The calibration steps can be summarized as follows:  
1. **Assuming diffuse field conditions:** The acoustic conditions used for this study do not represent a diffuse sound field. Since this might influence the correct estimation of the material properties, a different configuration of the hall has been used, and the diffuse field is assumed as an approximation to estimate the absorption coefficients. The model (DM1) used for the material calibration is presented in Figure 6. This model has been chosen since it was considered to have a sufficiently diffuse field based on the achieved spatial uniformity of the reverberation time. Compared to the DM model, it assumes both long lateral walls and ceiling in diffusive condition, while the two short walls are maintained in the absorptive condition. In this configuration a more diffuse sound field is likely to be generated since larger diffusive surfaces have been used and distributed symmetrically in the room. The diffusive surfaces have been modeled as 3D prisms. This was preferred with respect to a flat surface modeling alternative, since it is a closer geometrical representation of the real room. The difference in volume between the two modeling alternatives is (V3D \(-\) Vflat = 44.32 m\({}^{3}\)), which could lead to differences in reverberation time of about 1%. 2. **Preliminary acoustical properties:** As far as the absorption coefficients are concerned, in order to start the calibration process, preliminary acoustical properties (Table 1) have been assigned to the geometrical model's surfaces based on the data reported in the project reports and standard literature [37; 38]. Based on the recommendations given in [41], the scattering coefficients were set to 0.05 (707 Hz) for all flat surfaces and for the modeled 3D panels, i.e., in this case each prism surface was set 0.05 (707 Hz). Since no data could be found for the absorption coefficient of the structure and for the diffusive configuration of the panels, the same absorption coefficientshave been used for these three types of surfaces based on the similar construction material (metal sheet). 3. **Variation of the acoustical properties:** The acoustical properties of type A and B absorptive surfaces, structure, and diffusive panels have been modified since they have the most significant impact on the overall value of the equivalent sound absorption area, due to high absorption coefficients and surface extension, respectively. The variation has been performed manually and step by step until the overall mean differences (for the source and receiver configuration) between measurement and simulated results for _EDT_, \(T_{30}\), \(C_{80}\), and \(D_{50}\) resulted in less than one JND. However, the variation of the absorption coefficient of these surfaces has been restricted in order not to lose the typical acoustic properties of the material they represent (for e.g., a rockwood surface should not vary into a plastered one).