## 3 In Situ Measurement Methods
In particular, the following room acoustical parameters were measured, as these are considered the most relevant parameters for the acoustical characterisation of open-air theatres [4]:  
* Reverberation time, RT, (s): duration required for the space-averaged sound energy density in an enclosure to decrease by 60 dB after the source emission has stopped. The integrated impulse response method was applied to obtain the RT from the IR [2]. RT can be evaluated on a smaller dynamic range than 60 dB and extrapolated to a decay time of 60 dB. It is then labelled accordingly. The RT in SR was derived from decay values of 5 dB to 25 dB below the initial level, and it was therefore labelled T\({}_{20}\). * Clarity, C\({}_{80}\), (dB): the balance between early- and late-arriving energy. This was calculated for an 80 ms early time limit, as the results were intended to relate to music conditions, using equation: \[\text{C}_{80}=10\,\log\frac{\int_{0}^{80}\text{p}^{2}(\text{t})\text{dt}}{\int_ {80}^{\infty}\text{p}^{2}(\text{t})\text{dt}}\] (1) where p(t) is the instantaneous sound pressure of the impulse response measured at the measurement point. * Sound Strength, G, (dB): the logarithmic ratio of the measured sound energy (i.e., the squared and integrated sound pressure) to the sound energy that would arise in a free field at a distance of 10 m from a calibrated omnidirectional sound source, as expressed in the following equations: \[\text{G}=10\,\log\frac{\int_{0}^{\infty}\text{p}^{2}(\text{t})\text{dt}}{\int_ {0}^{\infty}\text{p}_{10}^{2}(\text{t})\text{dt}}=\text{L}_{\text{PE}}-\text{ L}_{\text{PE},10}\] (2)in which  
\[\mathrm{L_{PE}}=10\ \log\ \left[\frac{1}{\mathrm{T_{0}}}\int\limits_{0}^{\infty} \frac{\mathrm{p^{2}(t)dt}}{\mathrm{P_{0}^{2}}}\right] \tag{3}\]  
and  
\[\mathrm{L_{PE,\ 10}}=10\ \log\ \left[\frac{1}{\mathrm{T_{0}}}\int\limits_{0}^{ \infty}\frac{\mathrm{P_{10}^{2}(t)dt}}{\mathrm{P_{0}^{2}}}\right] \tag{4}\]  
where:  
p(t) is the instantaneous sound pressure of the impulse response measured at the measurement point;  
p\({}_{10}\)(t) is the instantaneous sound pressure of the impulse response measured at a distance of 10 m in a free field;  
L\({}_{\mathrm{PE}}\) (dB) is the sound exposure level of p(t);  
L\({}_{\mathrm{PE,10}}\) (dB) is the sound exposure level of p\({}_{10}\)(t);  
p\({}_{0}\) is the reference sound pressure of 20 uPa;  
T\({}_{0}\) is the reference time interval of 1 s.  
In the above equations, t = 0 corresponds to the start of the direct sound, i.e., which corresponds to the arrival of the direct sound at the receiver, and \(\infty\) should correspond to a time that is greater than or equal to the point at which the decay curve has decreased by 30 dB [2]. G requires a calibration procedure for the sound power of the source. Different procedures have been described previously [2]. L\({}_{\mathrm{PE,10}}\) can be calculated from the sound pressure p\({}_{\mathrm{d}}\)(t) measured at a source-to-receiver distance d (\(\geq\)3 m) according to the following equation:  
\[\mathrm{L_{PE,10}}=\mathrm{L_{PE,d}}+20\log\left(\frac{d}{10}\right) \tag{5}\]  
where:  
L\({}_{\mathrm{PE,d}}\) (dB) is the sound exposure level of p\({}_{\mathrm{d}}\)(t), obtained from (3) (using p\({}_{\mathrm{d}}\) instead of p\({}_{10}\)). The Aurora plugin was used for the calculation of G with the firecrackers [31]. According to this procedure, the anechoic segment (direct sound) of each IR was used for calibration, providing the distance between the source and the receiver that allows for the estimation of L\({}_{\mathrm{PE,10}}\), and it is recommended to keep a length of the IR of at least 1 s and to silence the signal just after the end of the direct sound. In this way, the smearing out in time caused by the octave filtering does not push the energy outside the time window, even at a low frequency, and the correct value of the signal level can be computed.