# Acoustic Emission for Structural Monitoring of Historical Masonry: An In-Field Application
## 4 Case Study: Santa Maria Delle Grazie
### Experimental Setup  
The structure was assessed with both Acoustic Emission (AE) sensors (AE1, AE2, AE3, and AE4) and displacement transducers (D1, D2, D3, D4, and D5) in order to monitor the arch's response under a semi-static load test. A schematic representation of the setup is reported in Figure 4.  
Unlike applications involving concrete, for which the selection of AE sensing equipment is generally regulated by ASTM E1932, no specific standards or guidelines currently exist for masonry structures. In this study, due to the lack of a clear expectation regarding the frequency range of potential AE phenomena, broadband frequency sensors were  
Figure 4: Experimental setup. (**a**) plan scheme; (**b**) axonometric scheme. selected. The equipment used for the experimental campaign was provided by Vallen Systeme GmbH (Wolfrathausen, Germany). The sensors employed were of type VS150, with a sensitivity range between 100 and 450 kHz and a resonance peak at 150 kHz. The data acquisition system used was the AMSY-6 model. As a preliminary step, the masonry attenuation profile was characterised using a Hsu-Nielsen pencil lead break test [25] conducted in accordance with [41]. The use of high-frequency sensors effectively minimised background noise, which remained consistently low (approximately 20 dB) except for sporadic transient signals caused by the passage of vehicles with significant vibrational impact. The propagation range of the sensors was studied using an attenuation analysis (Figure 5). Based on amplitude readings recorded at increasing distances (e.g., 70.6 dB at 7 cm, 67.7 dB at 10 cm, and 39.2 dB at 40 cm), the attenuation profile demonstrated progressive signal decay with distance, crossing the evaluation threshold (41 dB) at around 36.3 cm, identified as the maximum effective sensor spacing. The analysis further quantified the near-field attenuation at 0.967 dB/cm and the far-field attenuation at 1.014 dB/cm, with a total signal loss of approximately 29.6 dB over the tested range. To ensure reliable detection within these constraints, the AE sensors were spaced at 30 cm intervals, remaining within the maximum operational range. In order to correlate the data obtained from the different instruments, the AE sensors were arranged linearly in proximity to the FRCM strips applied at the intrados. The FRCM reinforcement, due to its composite layering, might have influenced surface wave transmission. To mitigate this, AE sensors were positioned adjacent to but not over reinforcement bands. Preliminary attenuation analysis showed consistent signal profiles across both reinforced and unreinforced paths within the effective detection range. This calibration step was essential to optimise sensor placement and ensure effective signal acquisition. Although irregularities in masonry might affect propagation paths, sensor spacing was selected to ensure overlapping detection zones. While localised phenomena between sensors cannot be entirely ruled out, the use of the Arrival Time Method and observed redundancy in detection across multiple sensors provided confidence in signal capture adequacy. Figure 5: Attenuation analysis. To secure the sensors, magnetic clamps were used, mounted on backing plates directly fixed to the intrados of the arch. These magnetic clamps were equipped with a spring mechanism that allowed for adjustment of the sensor's pressure against the structure. Additionally, petroleum jelly was applied between the sensor and the surface as an acoustic couplant (Figure 6a,b). To prevent the sensors from being detached due to the weight of the cables, the acquisition unit was placed in the room on the ground floor directly above the tested arch. The cables were securely fastened to avoid exerting any downward force on the clamps (Figure 6c).