## 3 1/_f_ Noise in Damaging Structures: In-Situ AE Monitoring  
AE technique has been very usefully employed by the authors to investigate on a wide set of structural problems related to ancient masonry buildings [28; 29]. In particular, AE technique has been employed to assess the stability of the tallest structure in the city of Bologna (Italy), the Asinelli Tower, which, together with the nearby Garisenda tower, is the renowned symbol of the city (Figure 10) [30]. The Asinelli Tower dates back to the early XII Century, and it is the tallest leaning tower in Italy. The tower measures 97.30 m in height, the side is 8.00 m at the base, and 6.50 m at the top. Its leaning is of 2.38 m westward. The AE activity was detected in a masonry significant region for monitoring purposes and easy to reach. Six piezoelectric sensors (working in the range of 50-800 kHz) were attacked on the north-east corner of the tower, immediately above the terrace atop the arcade, at an average level of ~9.00 m above ground. In this area, the double-wall masonry has a thickness of ~2.45 m. AE monitoring began on 23 September 2010 at 5:40 p.m., and ended on 28 January 2011 at 1:00 p.m., covering, therefore, a 4-month period. The AE monitoring was carried on exploiting the above mentioned USAM system. To filter out the environmental background noise a detection threshold of 100 mV was settled (appropriate for masonry according to the authors' experience [28; 30]). From the monitoring process carried out on a significant part of this tower, it was possible to evaluate the incidence of seismic activity, vehicle traffic, and wind action on fracture evolution and damaging phenomena within the structure. In Figure 11, AE data obtained during the structural monitoring are represented in a log-log plot of power spectrum versus frequency, with a fluctuation of \(f^{-\gamma}\) (\(\gamma\) = 0.94). Therefore, the source noise revealed by AE signals sounds \"pink\", supporting the evidences of the cracking process found during the monitoring campaigns [28; 30]. We can say that, in the masonry region monitored by means of AE, pink noise fluctuations suggest that the material strength has been locally overcome, as in the case of the laboratory tests described above. Figure 10: The Garisenda Tower and the Asinelli Tower, in Bologna City center. AE technique was successfully applied by the authors, not only to assess historical monument stability, but also to evaluate the structural supports of mural paintings, and the state of conservation of the frescos. As a second object of analysis in this work, the results of the monitoring of Chapel XVII (also known as \"The Transfiguration of Christ on Mount Tabor\") of the Sacred Mountain of Varallo is chosen (Figure 12). The most ancient Sacred Mountain of northern Italy is the Sacred Mountain of Varallo, consisting in 45 monumental chapels dating back to the Renaissance period, and it is located at the top of a mount crest, among the green of the forests surrounding the City of Varallo. The chapels contain about 800 multicoloured life-size terracotta statues, representing Life, Passion and Death of [PERSON] [31]. Concerning Chapel XVII structural integrity, a vertical crack of about 3.00 m in length and a detachment of frescos are present, both on the north wall, which are the object of the AE monitoring campaign. AE sensors are employed to monitor the damage evolution of the structural support of the decorated surfaces of Chapel XVII: four are positioned around the vertical crack, while two are positioned near the frescos detachment. For the sensor pasting on valuable decorated surfaces, a suitable methodology is applied [32]. Figure 11: Log-log plot of AE power spectrum versus frequency for the Asinelli Tower, Bologna: \(1/f^{\gamma}\) noise, with \(\gamma=0.94\). Pink noise source (Correlated phenomena). Figure 12: External view of Chapel XVII of the Sacred Mountain of Varallo (Italy). The equipment used for signal acquisition and processing consists of the aforementioned six USAM units.