## 3 Materials and Methods  
The increased necessity for continuous survey of infrastructures, ancient buildings, and monuments requires wireless transmission and processing of large amounts of data, thus allowing remote and real-time monitoring to point out the presence of evolving structural damage processes. To this purpose, the cooperation between the AE research unit of the Politecnico di Torino and the Italian company _Lunitek SRL_ (Sarzana-Italy) produced the _AEmission_(r) system for structural and seismic monitoring, based on AE data acquisition and transmission [23]. An array of eight piezoelectric (PZT) transducers is connected to this multi-channel system (each channel with a devoted memory of 64 Mb), \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mode n**} & \multicolumn{2}{c}{**Frequency**} & **Period** \\  & **[rad/s]** & **[cycle/s]** & **[s]** \\ \hline \(1\) & 2.5300 & 0.4027 & 2.4835 \\ \(2\) & 2.5687 & 0.4088 & 2.4460 \\ \(3\) & 10.208 & 1.6247 & 0.6155 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Eigenvalue analysis. Figure 6: Representation of the modal analysis: (**a,b**) The two modes of the tower in the y and the x directions, represented by flexural modalities, (**c**) The third mode showing a torsional modality. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Mode n**} & **Tran-X** & **Tran-Y** & **Tran-Z** & **Rotn-X** & **Rotn-Y** & **Rotn-Z** \\ \cline{2-6}  & **Mass (\%)** & **Mass (\%)** & **Mass (\%)** & **Mass (\%)** & **Mass (\%)** \\ \hline \(1\) & 31.452 & 25.482 & 0 & 18.006 & 22.289 & 0.001 \\ \(2\) & 25.427 & 31.456 & 0 & 22.288 & 18.054 & 0.001 \\ \(3\) & 15.108 & 0.220 & 0 & 0.187 & 12.861 & 19.762 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Participating modal masses. which automatically stores and processes significant parameters of each detected signal waveform (the cumulated number of events, duration, peak amplitude, and ring-down counts), allowing in situ damage localization and quantification from the recorded parameters. Processed data are periodically sent via GPRS/UMTS system to a remote server, making the continuous and simultaneous monitoring of individual structural elements or entire structures possible. In addition, taking into account the observed correlation between regional seismic activity and the AE activity detected during structural monitoring [8], this monitoring system would be helpful to preserve civil structures, infrastructures, and architectural heritage located in seismic areas [13]. Each channel consists of an analog-to-digital converter (ADC) module with a sampling rate of 10 mega-samples per second, which is adequate to measure frequency components up to 1 MHz, covering satisfactorily the typical frequency range of broadband PZT transducers. However, in the present application, the use of highly sensitive resonant transducers (with resonant frequency of 63 kHz) appeared more appropriate. During AE monitoring, the typical frequencies of signals emitted from micro-cracks and sources, generated in materials such as concrete and masonry, range between 50 kHz and 250 kHz. The sampling rate of the AE device (10 Ms/s) is between 200 and 50 times the AE signal frequency correlated to the damage evolution. In Figure 7, a typical AE signal is reported. The AE equipment used in the present paper allowed us to set the appropriate threshold value due to the monitored structure and the environmental conditions. As regards the amplitudes, the instrumentation used permitted the determination of two different modalities in the definition of the threshold values, beyond which the signal is effectively recognized and recorded (see Fig. 7a). The signal threshold value is normally defined in a range from 50 \(\mu\)V to 100 mV. The great variability on the thresholds that can be set by the _AEMission_ device allowed for the definition of the most appropriate one for each sensor set, based on the condition in which the monitoring is operating [6; 7; 8; 9; 10; 20].