## 2 IoT System Description
### Hardware: A Multisensor IoT-WSN  
The IoT-WSN proposed in this paper consists of wireless sensing nodes, which are fed using a suitable tuned photovoltaic energy harvester [62]. Figure 1 shows the main components of the first prototype of one wireless sensing node of the WSN. In particular, the figure cited above shows (1) the power supply unit of the sensing node (which consists of a 5 W-18 V polycrystalline silicon photovoltaic panel, a 12 V-20 A recharge circuit, and a 12 V-12 Ah battery), and (2) an ultralow-power IoT board [63], which is the core of the sensing node.  
In more detail, the IoT board includes several MEMS sensors (i.e., a 3D accelerometer, a microphone, a temperature/humidity sensor, magnetometer, barometer, etc.) and a microcontroller. The IoT board can wirelessly transmit the data gathered by the sensors, using different standards and protocols, i.e., Wi-Fi, Bluetooth, and NFC tags. It is important to underline that the NFC protocol allows for further applications, e.g., the interaction/exchange of information with the theme park visitors during normal and emergency conditions, also in the absence of a remote connection, thus allowing a backup communication channel with the users. Note that, to obtain a more detailed environmental and structural monitoring, the IoT board was also equipped with a smoke sensor (to detect carbon monoxide, liquid petroleum gas, and smoke), a flame sensor, and an additional microphone. The latter was added to receive the vibro-acoustic response [64; 65; 66] of the structure on which the units were installed. In more detail, the additional microphone was isolated from the airborne noise through a cover (inside the box) and isolating material (between the box and the structure), and was able to receive the acoustic signals (a.k.a., vibro-acoustic signature of the structure) that travelled into the structure. Proper analyses of these acoustic signals (i.e., feature extraction, multidomain analysis for cracks identification and monitoring, structural health status classification; [64; 65; 66]) allow one to carry out the Structural Health Monitoring (SHM) of the structures where the sensing node is attached, which is also applicable in the case of road monitoring, where the use of self-powered and smart sensors is envisioned [67].  
Figure 1: First prototype of the wireless sensing node of the proposed WSN.  
The study reported in this paper refers to two case studies that were carried out using, as a reference place, an Italian theme park. Figure 2a shows the map of the park, where lines point out boundaries and main routes (solid and dashed, respectively), numbers (from 1 to 6) point out the positions in which the sensing nodes of the WSN were installed, and the positions of the two exits (called Exit 1 and Exit 2) and the two assembly points (called AP 1 and AP 2) are indicated. Figure 2 also shows three prototypes of wireless sensing nodes installed at different points of the theme park, i.e., a road pavement near the Exit 1 (see Figure 2b, which refers to point 1 in Figure 2a), a masonry structure (historical military fort; see Figure 2c, which refers to point 5 in Figure 2a), a light pole (see Figure 2d, which refers to point 6 in Figure 2a). Importantly, the prototype cited above have been subsequently equipped with Wi-Fi cameras that allow computer vision analyses aimed at defining the spatiotemporal distribution of the visitors of the theme park per given area.