## 3 In Situ Measurement Methods  
Standard measurements have been performed in unoccupied conditions, with omnidirectional sound sources and receivers, as stated in the ISO 3381-1 [2]. Different considerations on ancient theatre measurements, defined during the European ERATO Project [3], were taken into account. The measurement results for SR have been used in Section 4 for the calibration of the simulation model and as references for the acoustic parameters predicted through computer simulations. The source and receiver positions for the theatre are shown in Figure 1b. Receivers were positioned on three radial axes of the cavea in the theatre, 1.2 m above the floor resembling the height of the ear of a sitting person. An omnidirectional microphone (Shoeps CMC 5-U, Durlach, Germany) was used to record the IRs. Ten receiver positions were considered. There was only a single microphone, meaning that all position measurements were carried out sequentially. Measurements were repeated two or three times for each source position for most of the receivers, in order to evaluate the repeatability of the results. Two source positions were investigated: S1 was  
Figure 1: Present conditions of the ancient theatre of Syracusae (**a**) and measurement set-up (**b**). S1 and S2 represent the source positions. R1 to R10 indicate the receiver positions. shifted horizontally by 1 m from the centre of the orchestra, in order to avoid any acoustical focus [29]; S2 was located behind S1, closer to the ancient scaenae froms position. The S1-S2 distance was equal to 7.6 m. Firecrackers were used as impulsive sources to measure the IRs (\"Raudo Manna New Ma1b\" Napoli, Italy and \"Perfetto C00015 Raudo New\", Napoli, Italy). The IRs were measured directly by recording the impulse produced by the firecracker blast. Firecrackers were used in S1 and S2, in order to overcome the problem of the low Signal-to-Noise Ratio (SNR): they maximise the SNR and this constitutes a significant advantage in outdoor measurements, but on the other hand, caution should be used as they are also more likely to be influenced by random effects (e.g., atmospheric conditions and random directivity). According to [PERSON] et al. [30], in the case of firecrackers the generated impulse is nearly omnidirectional. Its directivity index is, on average, around 1 dB for the octave bands between 125 Hz and 16 kHz. In addition, both its time curve and spectral power are highly repetitive, resulting in levels above 115 dB (reference 1 pW) within the aforementioned range. The Background Noise Level (BNL) was measured as an equivalent continuous A-weighted sound pressure level (L\({}_{\text{Aeq}}\)) over a period of 10 min, before the measurement sessions. The measured BNL was 45 dB (A), in unoccupied conditions. The sound source was positioned at a height of 1.5 m from the floor, and a custom-made tripod was used to hold the firecrackers in a fixed position. Aurora (version 4.4, Parma, Italy) was used as acquisition software. The air temperature and relative humidity were monitored during the whole measurement campaign, using a thermometer/hygrometer, Testo 608-H1 (Croydon South, VIC, Australia). The wind speed was measured by means of an anemometer, Testo 450-V1 (Croydon South, VIC, Australia). The environmental parameters acquired during the measurements campaign were t = 33 \({}^{\circ}\)C, RH = 65%, wind speed = 0.30 m/s. These did not change significantly during the measurement campaign. In order to characterise the acoustical conditions of a performance space, the ISO 3382-1 standard lists a series of parameters that can be obtained from the IRs measured at each receiver position. Although open-air theatres cannot be considered typical performance spaces, like closed theatres or concert-halls, the ISO 3382-1 standard was used as the reference for the acoustical characterisation.