# Edge-Computing Meshed Wireless Acoustic Sensor Network for Indoor Sound Monitoring
## 3 Sensor Design
### Sensor Enclosure  
For the enclosure of the components, a 3D box has been designed. The reason to design a box to cover the sensing node is twofold. On the one hand, it protects the computing unit in case of falls. On the other hand, it protects it from undesired manual disconnections of the different components (such as the USB microphone, the SD card or the power supply) and, moreover, protects the user from touching the computing unit when it is working at elevated temperatures. However, designing an optimal box that enables protection without interfering with the ventilation and heat dissipation of the computing unit is not trivial. In this section, different designs together with their associated overheating problems and solutions are introduced, assuming that no extra ventilation and heat dissipation was desired due to its high interference with audio gathering, and extra consumption and cost. To evaluate the temperature behavior of different set-ups, identical stress tests have been carried out under the same ambient temperature (21 \({}^{\circ}\)C) which has been verified using a DHT11 (manufactured by Sain Smart, Delaware, USA) [38] temperature sensor. The tests, which were conducted with an exhaustive software from the stressberry (pypi v0.3.3) [39], consisted of the following steps:  
1. Measuring the temperature for 1 h without stressing the RPi (i.e., relax period). 2. Measuring the temperature for 2 h stressing the RPi (i.e., stress period). 3. Measuring the temperature for 1 h without stressing the RPi (i.e., second relax period). Figure 2 shows the results of the stress tests conducted over the computing unit with different set-ups (i.e., with and without heat sinks and with different covering boxes). The temperature of a RPi 4B without any type of heat sink (i.e., naked device) has been taken as a reference (see Figure 3a). Figure 2: Temperature responses in time of the RPi using different setups (heat sink and enclosure) when performing a stress test. It can be appreciated that the RPi reaches 75 \({}^{\circ}\)C when it is under stress. It suffers an increment of 54 \({}^{\circ}\)C compared to the ambient temperature. Furthermore, during the first relax time previous to the stress, it works at 45 \({}^{\circ}\)C and, after the stress, it practically reaches the 45 \({}^{\circ}\)C in an hour. In order to help the RPi work under better conditions, an aluminium heat sink was placed on top of the RPi (see Figure 3b). This heat sink could not include a fan as it would affect the measurements of the microphones. As is known, the bigger the surface of the heat sink, the more dissipation it shows. For this, the selected heat sink covers all the Raspberry Pi. The results with this heat sink are shown in Figure 2. As it can be observed, it starts with 42 \({}^{\circ}\)C as baseline temperature when it is not under stress. Practically, it is the same initial temperature as when the RPi is naked. In the stress phase, it reaches 60 \({}^{\circ}\)C; which means an increment of 39 \({}^{\circ}\)C regarding the ambient temperature. After the stress, after an hour, it is capable of reaching back the 42 \({}^{\circ}\)C. Compared to the test where the RPi has no heat sink, the heat sink achieves a reduction of 15 \({}^{\circ}\)C when stressed. Using 3D modelling with SketchUp, multiple boxes (exposed in Table 3) have been created. These boxes are meant to contain all the hardware inside. They have been printed using Polylactic Acid (PLA), known for its low thermal conduction (0.13-0.16 W/m\({}^{\circ}\)C), which enables us to keep the enclosure cold and retain the temperature. All the boxes can be closed using a metric 4 (M4) screw. The first created model had the shape of a completely hermetic box, LNP (see Figure 4a). This box would contain the RPi device and a power connector (such as a 5V mobile charger). It has two holes; one for the microphone and another one for the wire to power the RPi.