## 5 Acoustic Measurement Results of Monte Castello Theatre  
In this section, after the IRs were obtained, the acoustic parameters were calculated using Aurora, a plug-in for Audition 3.0. The main acoustic parameters have been analysed, to be considered the average of all source-receiver positions in unoccupied conditions. Different acoustic parameters defined by the international standards ISO 3382-1 have been analysed and commented on, to be included are as follows:  
* Reverberation Time (T\({}_{20}\)): it is one of the most essential parameters to express the acoustic characteristics of a space;
* Early Decay Time (EDT): it combines with the reverberation time to reflect the overall acoustic condition of a room;
* Clarity indexes (C\({}_{80}\) and C\({}_{50}\)): C\({}_{80}\) refers to the measurement of a hall for musical performance; the clarity evaluated for speech performances is C\({}_{50}\). They are the main parameters for assessing the acoustic quality of theatre spaces;
* Definition (D\({}_{50}\)): it is used to evaluate the quality of speech perception in rooms. Unlike concert halls, a large number of opera performances are staged throughout the year in theatres, so the speech intelligibility conveyed to the audience is also an important factor in expressing its acoustic characteristics;
* Lateral Fraction (LF): it represents the ratio of the laterally reflected sound energy in a room over the sound energy arriving from all directions, including the direct sound energy coming from the source. These reflections are responsible for the extensive perception of the sound source. The lateral fraction is a measurement of the spaciousness of a room. Lateral sound energy and sound energy that arrive  
Figure 8: Plan of 1763 theatre: position of microphones for the acoustic measurements. to the listener from all directions should be considered separately, which is why omnidirectional microphones must be used in investigations;
* Interaural Cross-Correlation (IACC): it is called the Interaural Cross-Correlation Coefficient for music and speech. In particular, the IACC represents a value that measures the maximum similarity between waves arriving at the two ears. The main acoustic parameters are reported in the octave bands between 125 Hz and 8 kHz. Figures 9-13 show the graphs of the measurements of the main acoustic parameters. Measurement results relate to a comparison with the stalls and balcony. The sound source has been installed in the centre of the stage. The receivers have been located across the stalls and in some selected boxes. Figure 9: Measurement results of the acoustic parameters: Reverberation Time (**a**), Early Decay Time (**b**), and Optimal T\({}_{20}\) values at 500 Hz, based on room volume (**c**). Figure 11: Measurements results of the Definition (D\({}_{50}\)). Figure 12: Measurements results of the Lateral Fraction (LF). Figure 10: Measurements results of the acoustic parameters: Clarity indexes (C\({}_{80}\)) (**a**) and Clarity indexes (C\({}_{50}\)) (**b**). Figure 9a indicates that the averaged values of T\({}_{20}\) are around 0.5 s at mid frequencies, which are found to be very similar for the stalls and balcony. In conjunction with Figure 9c, the T\({}_{20}\) value at 500 Hz, it can be clearly seen in the results that the Monte Castello di Vibio theatre is below the ideal Reverberation Time for an opera house with this volume size [12]. It is more suitable as a radio studio and conference room. In terms of Early Decay Time (EDT), as shown in Figure 9b, it was found to be very similar between the stalls and balcony. Between 500 Hz and 4 kHz, the values fluctuate around 0.4 s, while at lower frequencies, the results rise to 0.6 s, and at higher octaves, the values drop to 0.2 s. The overall outcome is slightly lower than the optimum range set by the Opera House [13].