# Acoustic Shell Optimization in Opera Houses for Concert Halls
## 2 Methodology to Transform the Sound Field from Opera to Concerts  
Because baroque theatres are used as concert halls or as multipurpose spaces; firstly, it is recommended to follow the EN ISO 3382.1 [28] to check the acoustic field quality. The standard clarifies that \"the reverberation time of a room was once regarded as the predominant indicator of its acoustical properties. While reverberation time continues to be regarded as a significant parameter, there is reasonable agreement that other types of measurements, such as those of relative sound pressure levels, early/late energy ratios, lateral energy fractions, interaural cross-correlation functions and background noise levels, are needed for a more complete evaluation of the acoustic quality of rooms\" [21]. For this reason, the optimal acoustic range for other main acoustic quantities is suggested, considering a generic multipurpose use of the hall, according to the following table (Table 1):  
The standard does not subdivide this interval according to the main possible activities inside the space (concerts, speech, film projections, opera, etc. ), even if it is known that the sound fields of spaces for speech are not optimized for classical music, and vice versa, as the following graph specifies (Figure 3). \begin{table}
\begin{tabular}{c c c c c} \hline
**Quantity** & \multicolumn{4}{c}{**Optimal Range**} \\ \hline
**Subjective** & **Acoustic Quantity** & \begin{tabular}{c} **Single Number** \\ **Frequency** \\ **Averaging [Hz]** \\ \end{tabular} & \begin{tabular}{c} **Just Noticeable** \\ **Difference (JND)** \\ \end{tabular} & **Typical Range** \\ \hline Subjective level of & Strength G [dB] & 500 to 1000 & 1 dB & \(-\)2 dB; +10 dB \\ \hline Perceived & Early decay time EDT [s] & 500 to 1000 & Rel. 5\% & 1.0 s; 3.0 s \\ \hline Perceived clarity of & \begin{tabular}{c} Clarity, C80 [dB] \\ \end{tabular} & 500 to 1000 & 1 dB & \(-\)5 dB; +5 dB \\ \begin{tabular}{c} Position \\ \end{tabular} & \begin{tabular}{c} Definition, D50 \\ Centre time, TS [ms] \\ \end{tabular} & 500 to 1000 & 0.05 & 0.3; 0.7 \\ \hline Apparent source & \begin{tabular}{c} Early lateral energy fraction, \\ width (ASW) \\ \end{tabular} & \
\begin{tabular}{c} 125 to 1000 \\ JLF or JLFC \\ \end{tabular} & 0.05 & 0.05; 0.35 \\ \hline Listener & Late lateral sound level, LJ, &
\begin{tabular}{c} 125 to 1000 \\ \end{tabular} & Not known & \(-\)14 dB; +1 dB \\ \hline \end{tabular}
\end{table} 
Table 1: The optimal acoustic range for the main acoustic quantities for a generic multipurpose use of the hall according to the ISO 3382.1 [28]. Figure 3: Optimum reverberation time as a function of the space volume [29]. To find out the reference acoustic values for each activity, an optimal range of reference must be found in the literature [8; 13; 30; 31; 32]. In operas, both the singer's voice and the musical instruments must be appreciated. Short reverberation times are common in theatres predominantly designed for drama where a reverberation time of only 1 s is suggested. This value is certainly shorter than the ideal for music. A reverberation time of 1.8 s is ideal for orchestral music [33; 34] in concert halls, but it is inappropriate for speech intelligibility. The conflicting acoustic requirements for speech intelligibility and musical performances for Opera beg the question of the appropriate reverberation time for opera houses. A compromise is necessary. Reverberation times for opera between 1.3 and 1.8 s are appropriate; the exact point on this range can be chosen according to the volume, the number of seats and the taste of the audience. The appropriate frequency characteristics for the reverberation time in opera houses are also in between the requirements for speech and music. A longer bass reverberation time is undesirable for speech. Neither is it desirable for the orchestral sound in an opera house in the stalls, since high frequency sounds are suppressed due to absence of sightlines into the pit.