## 4 Materials and Methods
### Measured Results  
A summary of the measured results related to the main acoustic parameters is shown in Figure 6, compared between values measured in the stalls and the elevated balconies. In terms of EDT, the values of the stalls are found to be up to 1 s higher than those measured in the boxes. This difference is considerably reduced at high frequencies, where both trendlines tend toward 1.0 s. Considering that the optimal values of EDT in Opera theaters should range between 1 s, related to speech intelligibility, and 2.6 s for music, Figure 6a shows that the EDT values fall within the optimal range in the stalls, while the values in the boxes are slightly below the lower range limit. The measured values of reverberation time show a negligible difference between the results obtained in the boxes and stalls, with the exception of 125 Hz where a difference of 0.2 s has been found. Since the reverberation time is related to the volume size of a room, the optimal value at mid-frequencies for the Ponchielli theater, equal to 1.2 s, is within the optimal range considered for Opera theaters, as indicated in Figure 6c. Considering that the optimal values of the clarity index should be comprised between \(-\)2 dB and +2 dB, according to [PERSON] [21], Figure 6d shows that the values of C\({}_{50}\) are found approximately within the optimal range in the boxes, with an upward peak at 2 kHz and a downward peak at 250 Hz. The trend related to the stalls is more uniform, around 4 dB and, overall, up to 4 dB above the upper range limit. In terms of music, the values of the clarity index for both stalls and boxes are very similar, fluctuating between 4 dB and 8 dB. These results have also been found in other Italian Opera theaters of similar volume size, measured by the authors of this paper and commented on in the following sections. The mean value related to definition (D\({}_{50}\)) is around 0.7 for the stalls and 0.45 for the boxes, as indicated in Figure 6f. A good speech definition is defined to have values higher than 0.5 (i.e., 50%), while the optimal values for music definition should be lower than 0.5 (i.e., 50%) [18]. The D\({}_{50}\) results show that the Ponchielli theater of Cremona is suitable for both speech and music. Another acoustic parameter that is relevant for an Opera theater is the speech transmission index (STI), which considers the level of distortion in a speech signal caused by building factors like reverberation and background noise [22]. The STI parameter can assume values comprised between 0 and 1 and the measured results for the Ponchielli theater are summarized in Table 2, assessed against the background noise level measured over a 30 min period inside the main hall. STI value summarized in Table 2 is the average value of all measurement positions. A spatial distribution of this parameter is given in Figure 7.  
Figure 6: Measured results of the main acoustic parameters: EDT (**a**), T\({}_{20}\) (**b**), C\({}_{50}\) (**d**), C\({}_{80}\) (**e**), and D\({}_{50}\) (**f**). Correlation between reverberation time and volume size of the theater (**c**). The values in Table 2 should be considered the average of all the measuring points. Based on the results, the average STI value of the Ponchielli theater falls into the \"good\" category, as defined by the intelligibility rating according to ISO 9921 [23]. This result represents another factor that contributes to making the overall acoustic quality of the Ponchielli theater one of the best in Northern Italy. Figure 7 indicates the scale of STI values spread across the plan, where the values around 0.82 are closer to the stage, while the representative STI along the minor axis of the hall and at the back of the volume fluctuates around 0.7 and 0.66, respectively.