## 3 Setup and Results  
The experiments were performed on a solid wood modular box, in particular, a large cube (Figure 5) formed by two symmetrical parts. These parts are held together, on one side, by three hinges and, on the opposite side, by three large hooks, which allow them to be opened and closed. Thanks to this particular structure, we were able to test two scenarios: the first with the closed box, which represents an \"intact\" structure, and the second with the open structure, to simulate a damage or, in any case, a change in the state of the structure itself. Figure 6a,b shows the open structure. Figure 5: Photo of the intact structure. Figure 6: Node arrangement from an inside (**a**) and outside view (**b**). The disposition of the nodes with respect to the structure is also visible. Five nodes were used, four of which were placed on the outer sides of the box, the last one placed inside. The main reason for this choice is the non-overlapping of nodal points in order to detect a response as complete as possible. The aim of this setup is to verify the capability of the sensor network to coherently detect a modification in the evaluated structure. This is achieved by comparing the data acquired from each node. Three tests were carried out: in the first, the system was left to environmental stress alone; in the second, the system was directly disturbed by a randomic stimulus provided by a mechanical piston; in the third test, the structure was opened and subjected to a similar randomic stimulus as that of the previous test. For the first test, a 30 s time window was chosen, while for the last two tests, the parameter was increased to 60 s in order to test the system for a sufficient amount of time. For each repetition, we set a 500 Hz sampling frequency to ensure maximum data reliability and the full scale of both accelerometric sensors to \(\pm 2\) g because we expect that in an actual application of the system, it is subject to environmental stimuli, characterized by a low amplitude. The first test has the purpose of demonstrating the difference between main and backup accelerometers. The second and third tests are intended to show the spectral behavior of the box and how it changes as a result of a substantial change in the structure's layout. From the first test, we expect very good performance from the main sensor with respect to the second because of the different features; in particular, we expect several signals to be detected only by the first accelerometer because of the considerable difference in resolution of both devices. Results from the first test are reported in Figure 7a,b. It is noticeable how the main accelerometer could detect the minimal indirect structure perturbations, e.g., the passage of pedestrians and vehicles in an on-field application, differently from the backup accelerometer, where these stimuli are buried in noise. In the second test, the direct perturbation consisted of a periodic system stimulus. Figure 8 shows the comparison between the readings of the main and secondary sensors in node 5. As can also be seen in this case, the detection of the secondary sensor is much less clean than the main sensor. However, the peaks relating to the periodic stimulus also emerge clearly in the detection of the secondary sensor. From these analyses, it emerges that the secondary accelerometer can be either used in parallel with the main one, to sample the static trim of the structure, or, alternatively, if the main sensor fails, can be used as a backup solution even with a reduced resolution. Figure 7: Time response comparison of main (**a**) and backup (**b**) accelerometers in node 5. Finally, the third test began with the structure intact. After a few seconds, the structure was opened and subsequently subjected to periodic stimulation again. Figure 9a,b shows the difference between the primary and secondary node detections. As can be seen, the conclusions made with regard to Figure 9 are also valid in this case.