# Acoustic Shell Optimization in Opera Houses for Concert Halls
## 4 Comparison of Results and Discussion  
The reverberation time parameter is affected by both the dimensions of the acoustic shell and the number of musicians on stage (Figure 16). For each frequency, it decreases both as the number of musicians on stage increases and as the size of the acoustic shell decreases. In detail, the orchestral ensemble increases more based on the presence of the choir, in the OAC3 configuration, and brings significant changes to the overall RT value compared to the OAC2 configuration. The difference between the overall values is equal to 1 JND; at the frequencies of 250 Hz, 2000 Hz and 4000 Hz, this trend was confirmed for receivers in the stalls and in the two lower levels of boxes, while in the upper levels, in some seat's receivers (30% ca. ), the change is smaller than 1 JND. Figure 16: Average T30 comparison for the three main configurations, OAC2, OAC3, and OAC5. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Standard Deviation**} \\ \hline Hz & 125 & 250 & 500 & 1000 & 2000 & 4000 \\ T30 & 0.08 & 0.11 & 0.13 & 0.17 & 0.16 & 0.07 \\ C80 & & & 1.62 & 1.81 & 1.97 & \\ G & 1.52 & 1.92 & 1.97 & 1.96 & 2.04 & 2.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Big acoustic shell: internal orchestra (OAC2). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Standard Deviation**} \\ \hline Hz & 125 & 250 & 500 & 1000 & 2000 & 4000 \\ T30 & 0.09 & 0.06 & 0.07 & 0.08 & 0.07 & 0.05 \\ C80 & & & 1.73 & 1.89 & 2.02 & \\ G & 1.46 & 1.85 & 1.89 & 1.88 & 1.96 & 2.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Small acoustic shell (OAC5). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Standard Deviation**} \\ \hline Hz & 125 & 250 & 500 & 1000 & 2000 & 4000 \\ T30 & 2.00 & 1.84 & 1.84 & 1.83 & 1.75 & 1.45 \\ C80 & & & 2.0 & 2.15 & 2.26 & \\ G & 1.52 & 1.93 & 2.05 & 2.03 & 2.08 & 2.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Small acoustic shell (OAC5). The small shell configuration significantly reduces the reverberation time at all frequencies compared to the OAC2 configuration, with differences approaching 2 JND at frequencies of 125 and 2000 Hz. These differences are spread to 80% of the receivers' positions. Regarding the standard deviation, in the configuration with the choir, the maximum standard deviation is observed only at the low frequency of 125 Hz (OAC3), while at other frequencies the maximum deviation shifts to the configuration with the larger acoustic chamber and orchestra only (OAC2). The minimum standard deviation is observed in the configuration with the smaller acoustic shell (OAC5). This results in greater uniformity of the sound field distribution. As the number of musicians on stage increases, for the same acoustic shell dimensions, the TR is slightly reduced and the uniformity of the parameter distribution in the hall increases. The C80 parameter is very similar in the two configurations, referring to a large acoustic shell (OAC2 and OAC3), significantly higher values are observed for the small acoustic shell OAC5, with an approximate change of 1 JND at all frequencies (Figure 17). These differences are spread in all the receivers' positions. The C80 is more sensitive to a reduction in acoustic shell volume and less sensitive to changes in the number of musicians than the T30. In the OC3 configuration, the mean C80 value of the boxes is higher, with larger standard deviations, than in the stalls. In the OC5 configuration the standard deviation in the stalls does not differ significantly from the boxes, even if the mean value in the stalls is higher than that in the boxes. The G parameter decreases almost significantly from the OAC2 to OAC3 configuration, not reaching 1 JND, while it exceeds 2 JND in the OAC5 configuration compared with both previous configurations (Figure 18). These differences are spread in all the receivers' positions.