# Acoustic Shell Optimization in Opera Houses for Concert Halls
## 2 Methodology to Transform the Sound Field from Opera to Concerts
A maximum rise of 20 per cent at 125 Hz relative to the mid-frequency reverberation time is likely to be acceptable. At the other extreme of the frequency range, the presence of the'singing formant' suggests that reverberation times at 2 and 4 kHz should be maintained as long as possible. One aspect that applies to both speech and music requirements is the importance of early sound compared to late reverberant sound. Intelligible speech relies on a prominent early sound relative to the later reverberation, whereas for music, the emphasis is on the later sound, often achieved through a longer reverberation time. In the case of music, enhanced early reflections contribute to clarity and a sense of envelopment, especially when the reflections come from the sides. However, strong overhead reflections are considered undesirable for music. On the other hand, the direction of reflections is not a concern for speech. Additional lateral reflections that arrive early enough to improve speech intelligibility can be seen as beneficial for both speech and music applications, as they enhance the overall listening experience in a multipurpose hall. According to the characteristics of the room space, the literature for the optimal reverberation time follows [1] (Table 2):  
For the average values of the other acoustic parameters, their main optimal range is also indicated (Table 3): \begin{table}
\begin{tabular}{c c} \hline
**Configurations** & **T30 [s] Optimal Range** \\ \hline Organic music & T30 \textgreater{} 2.5 \\ Romantic classical music & 1.8 \textless{} T30 \textless{} 2.2 \\ Early classical music & 1.6 \textless{} T30 \textless{} 1.8 \\ Opera & 1.3 \textless{} T30 \textless{} 1.8 \\ Chamber music & 1.4 \textless{} T30 \textless{} 1.7 \\ Drama theatre & 0.7 \textless{} T30 \textless{} 1 \\ \hline \end{tabular}
\end{table} 
Table 2: Reverberation time optimal range suggested by the literature. The optimal interval for baroque theatres is confirmed in most Italian ones [35; 36; 37] both with measurements and/or prediction analysis using acoustic software. For opera houses, some suggestions regarding the way of propagating the sound energy between the hall and the fly tower are found in the literature [38; 39; 40], even if they are still a subject of research [41]. In many cases, for acoustic simulations, the virtual model is considered to be at one single volume, including the hall and the stage, surrounded by heavy absorbing draperies. Three different source positions are usually set [42]. If an acoustic shell is installed on stage, with frames in contact with the proscenium opening, the set configuration increases the volume of the hall, without adding a significant quantity to the total equivalent sound absorption area. Usually, acoustic shells are made of reflecting wood panels, which naturally help to increase the reverberation time in the hall. In particular, the volume within the acoustic shell makes it possible to increase the acoustic active volume of the hall significantly, according to the dimensions of the acoustic shell itself, and the diffusive coefficients of its panels. _From Opera to Concerts: The Hall Active Volume Increasing_  
In the above-cited literature, the optimal range for the ratio between the volume of the hall and the number of spectators is described (Table 4) according to different activities. According to the seating capacity, a theatre can be described as a small, medium, large or grand opera. The referred volume range follows (Table 5):  
According to the hall volume, the proscenium opening dimensions can be estimated, as suggested by the following table (Table 6) [43]: \begin{table}
\begin{tabular}{c c} \hline
**Configuration** & **Ratio Volume/Seats** \\ \hline Concert Hall & 10–12 \\ Opera House & 7–9 \\ \hline \end{tabular}
\end{table} 
Table 4: Optimal range of the ratio between hall volume and spectators [39; 40; 41].