## 3 Materials and Methods
Figure 8: _Cont._  
The second part of the Bell Tower subjected to the AE monitoring is the timber structure sustaining the bells, positioned inside the bell cell at the top of the tower (see Figure 9). In this case, the wooden structure supporting the bells was monitored with an eight-channel acoustic emission data acquisition system to evaluate the evolution of the cracking pattern evolution over the monitoring time (see Table 7 for the sensor positioning). The sensors were positioned in the wood frame sustaining the heaviest bells, as shown in Figure 8. The structure was monitored for a period of time equal to one month; in particular, the monitoring was carried out from 8 August to 8 September 2019. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensor n** & **Coordinate x [cm]** & **Coordinate y [cm]** & **Coordinate z [cm]** \\ \hline
1 & 21.51 & 123.5 & 15.00 \\
2 & 59.15 & 111.05 & 0.00 \\
3 & 10.208 & 83.63 & 0.00 \\
4 & 105.53 & 81.50 & 0.00 \\
5 & 133.15 & 7.50 & 0.00 \\
6 & 51.50 & 22.01 & 0.00 \\
7 & 26.00 & 15.82 & 15.00 \\
8 & 72.20 & 71.48 & 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Sensor positions on the south tower facade. Figure 8: AE localization results. In (**a**), the portion of the masonry structure where the AE sensors were positioned. The localized points have been indicated with black points, for a total of 62 AE sources (**b**). Three circles are reported in correspondence to the center of AE source clusters. Two levels of cluster are reported with circle radius proportional to the number of AE source inside the areas. (**c**) The results of the numerical analysis reported the portion of the structure with higher compressive stress. (**d**) The same area is reported considering the crack position the erosion areas and the damaged surfaces. The analysis of the data extracted from the AE device followed similar steps previously observed for the monitoring of the masonry structure of the Bell Tower (see Figure 10). Figure 10a shows the AE cumulative number recorded during the monitoring period and the number of EA events for each day for the timber frame. The \(b\)-value trend was obtained working on time windows containing at least 100 AE. As observed in the case of the masonry structure, the \(b\)-value assumed numbers greater than 1.5, in correspondence with the first monitoring phase (first 10 monitoring days). After 11 days, the analysis results entered the critical band \(1.5<b\)-value \(>1.0\) (Figure 10b). In addition, as seen in Figure 10c, the \(\beta_{t}\) exponent showed an evident growth after 17 monitoring days. This increment is not perfectly correlated with the b-value decrease, but the two phases can be considered just in correspondence. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensor n** & **Coordinate x [cm]** & **Coordinate y [cm]** & **Coordinate z [cm]** \\ \hline
1 & 48.15 & 22.50 & 0.00 \\
2 & 18.51 & 7.50 & 0.00 \\
3 & 73.26 & 11.00 & 0.00 \\
4 & 31.29 & 0.00 & \(-\)7.00 \\
5 & 80.14 & 29.89 & \(-\)7.00 \\
6 & 26.50 & 19.90 & \(-\)15.00 \\
7 & 43.69 & 04.00 & \(-\)15.00 \\
8 & 76.70 & 18.90 & \(-\)15.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Sensor positions on the wood frame sustaining the bells. Figure 9: Timber structure positioned inside the bell cell at the top of the tower: (**a**) Plan of the cell where the timber frame layout is reported. The monitored portion is localized. (**b**) AE sensors disposition on the wood frame. Finally, the sources of AE events were localized. The position of the sensors and the localized sources are shown in Figure 11a,b. The localized points have been indicated with black points, for a total of 24 AE sources. In this Figure, two circles are reported in correspondence to the barycenter of AE source clusters. Two levels of cluster are reported with circle radii proportional to the number of AE source inside the areas.