# Edge-Computing Meshed Wireless Acoustic Sensor Network for Indoor Sound Monitoring
## 3 Sensor Design
### Sensor Enclosure
Even though the box was functional, once printed, it was observed that the size of each sensing node would be too big to fit an indoor environment, which could lead to the perception of intrusion or obtrusion once deployed [3]. Figure 3: Raspberry Pi 4B without (**a**) and with (**b**) heat sink. (**a**) Raspberry Pi 4B without heat sink. (**b**) Raspberry Pi 4B with heat sink. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Box** & **Material** & **Dimensions (X \(\times\) Y \(\times\) Z)** & **Acronym** & **Figure** \\ \hline Large Non-holed & PLA & 146 \(\times\) 171 \(\times\) 74 mm & LNP & Figure 4a \\ Small Non-holed & PLA & 81 \(\times\) 129 \(\times\) 64 mm & SNP & Figure 4b \\ Small Slot-holed & PLA & 81 \(\times\) 129 \(\times\) 63 mm & SSP & Figure 4c \\ Small Honeycomb-holed & PLA & 86 \(\times\) 116 \(\times\) 140 mm & SHP & Figure 4d \\ Small Honeycomb-holed & TPU & 86 \(\times\) 116 \(\times\) 140 mm & SHT & Figure 4d \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Name of the created boxes, printing material, dimensions and the acronym used to identify them. Figure 4: Boxes’ designs. Perspective view of the assembled box, top view of the base and perspective view of the cover. (**a**) Large Non-holed PLA box (LNP). (**b**) Small Non-holed PLA box (SNP). (**c**) Small Slot-holed PLA box (SSP). (**d**) Small Honeycomb-holed PLA box (SHP) or Small Honeycomb-holed TPU box (SHT). Aiming to reduce the dimensions of the box, the SNP (see Figure 4b) box was created. In this second approach, the connector to power the RPi was placed outside the box. This way, a wire would be connected directly to the RPi port. Thanks to these, the box contains only the RPi device, which enables us to reduce its dimensions considerably (from 146 mm \(\times\) 171 mm \(\times\) 74 mm to 81 mm \(\times\) 129 mm \(\times\) 64 mm). However, as it can be observed in Figure 2, this box obtained worse thermal conditions for the RPi. For this reason, aiming to reduce the temperature of the RPi, the SSP box (see Figure 4c) was designed. It has the same design as the SNP box, but the cover is thinner and slot-holed. As the air flows upwards when it is warmed, the holes are placed in the highest part of the box. Also, there are holes at the base to enable the entrance of fresh air. After this third design, some temperature tests revealed that the RPi has better temperature conditions if it works vertically. Therefore, the SHP (see Figure 4d) was created containing the RPi placed vertically inside the box. This box is honeycomb-holed in order to avoid using supports while 3D printing and, therefore, avoid useless PLA and faster printing. Furthermore, in this design, the support for the microphone is changeable, as we noticed that printing it with Thermoplastic Polyurethane (TPU) material allows to better adapt to the microphone to the box, compared with the same support printed with PLA. This is thanks to the flexibility of the TPU. The last box model is SHT (see Figure 4d), printed using a TPU, which is a thermally conductive, electrically insulating plastic model Ice9 TM Flex (4 \(W\)/m\(\cdot\)\({}^{\circ}\)C) from TCPOLY [40]. Our original hypothesis was that it would help to dissipate the temperature from the RPi due to its thermally conductive properties. However, as shown in Figure 2, it has worse results than the one printed with PLA. Figure 2 illustrates the temperature of the RPi out and inside the boxes. As shown, the SNP box (brown line) obtains the worst results, followed by the LNP box (navy line). Actually, these two boxes reach higher temperatures than the naked RPi (grey line). This is due to the lack of holes in the box, causing a lack of proper ventilation. The SSP box (red line) is the first design that obtains lower temperatures than the naked RPi. Therefore, we appreciate that the effect of hiding the cover is important. Additionally, the purple line illustrates the effect of using the same box (i.e., SSP box), but placed vertically.