## 5 Acoustic Calibration  
Before analyzing the effect of different types of scenery across the audience areas, the 3D model was simplified from the complexity of the architectural features, as shown in Figure 6. The layers were grouped and therefore the 3D model was exported in DXF format, to be ready for the acoustic calibration of the absorption coefficients related to the different materials. The software Ramsete makes calculations based on the principles of ray-tracing geometrical acoustics characterized by a triangular-base pyramid spreading [13]. This principle characterizes Ramsete with other software used for acoustic simulations,  
Figure 5: Scheme of the equipment location during the acoustic survey. since many of them are based on conical spreading, with the risk of having bugs but at high degree of reflections. The other advantage of Ramsete is that it does not suffer the calculations time of up to 4,000 receivers inserted into the model, since the traceable time is invariant. Rather, the high number of receivers is linked to the calculation time involved in the spatial distribution of the acoustic parameters. The source on the stage and the microphone positions were reproduced at the same location of the real measurements, with the addition of 362 receivers created in Ramsete, homogeneously distributed across the seats of the main hall [14]. Since Ramsete can uneventfully calculate the response of up to 4000 microphones, the time computation by the software does not suffer for the number of receivers inserted into the model. Table 1 indicates the absorption and scattering coefficients for all the materials considered in the digital model, considered to be the result of both calibration process and extensive experience on acoustic simulations carried out for other Opera houses [15]. The measured and calibrated results were compared and tuned until the difference between the two types of values is minimized to not exceed 5% across all the octave bands. Figure 7 shows the comparison of the main acoustic parameters, where the values shall be considered the average of all the receivers [16]. The spectrum bandwidth herein analyzed \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & \multirow{2}{*}{
\begin{tabular}{c} **Area** \\ **(m2)** \\ \end{tabular} } & \multicolumn{4}{c}{**Absorption Coefficients—Octave Frequency Bands (Hz)**} & \multirow{2}{*}{**Scattering**} \\ \cline{3-3} \cline{5-8}  & & **125** & **250** & **500** & **1 k** & **2 k** & **4 k** & \\ \hline Solid wood—Boxes perimetral walls & 2766 & 0.11 & 0.12 & 0.12 & 0.12 & 0.10 & 0.10 & 0.05 \\ Wood—Boxes party walls & 247 & 0.29 & 0.29 & 0.73 & 0.88 & 1.00 & 1.00 & 0.05 \\ Timber wood—Boxes floor & 303 & 0.05 & 0.05 & 0.10 & 0.20 & 0.30 & 0.59 & 0.05 \\ Floating floor—Stalls & 200 & 0.29 & 0.29 & 0.73 & 0.88 & 1.00 & 1.00 & 0.05 \\ Floating floor—Stage & 878 & 0.64 & 0.71 & 0.58 & 0.52 & 0.50 & 0.72 & 0.05 \\ Solid wood—Paparets & 302 & 0.04 & 0.05 & 0.06 & 0.07 & 0.07 & 0.06 & 0.05 \\ Upholstery—Seats & 192 & 0.65 & 0.80 & 0.90 & 1.00 & 0.99 & 0.94 & 0.05 \\ Light Wood—Ceiling & 1140 & 0.23 & 0.30 & 0.29 & 0.30 & 0.35 & 0.43 & 0.35 \\ Doors & 55 & 0.11 & 0.12 & 0.12 & 0.12 & 0.10 & 0.10 & 0.05 \\ Wood—Stairs & 223 & 0.53 & 0.59 & 0.39 & 0.35 & 0.50 & 0.72 & 0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface, absorption and scattering coefficients of the materials considered in the model of the National Theater of Zagreb. Figure 6: Digital model of the National Theater of Zagreb: perspectival view of the main hall. in octaes ranges between 125 Hz and 4 kHz. This methodology complies with a good practice since the very low (e.g., 16 Hz, 31.5 Hz) or very high (10 kHz, 16 kHz) frequency bands can be affected by distortion, as they are microphone dependent. Hence, the results at these latest octaves cannot be trusted and can be excluded by the acoustic analysis.