# Edge-Computing Meshed Wireless Acoustic Sensor Network for Indoor Sound Monitoring
## 3 Sensor Design
### Microphone Selection and Test  
Three models of USB electret microphones were initially considered. An advantage of these types of microphones--compared to other microphone types such as Micro Electro Mechanical systems (MEMS)--is that they do not require an external Analogue-to-Digital Converter (ADC). Moreover, as they are plug-and-play devices, they do not require the installation of drivers. These two facts together make the set-up of sensing nodes in a WASN easier, contributing to the scalability of the system. The three specific models that were considered and evaluated are: Gyvazla (Gv) [34], Sandberg streamer model 126-19 (Sb) [35] and Saramonic SR-ULM10 [36]. Nevertheless, the SR-ULM10 was discarded because its gain cannot be adjusted, thus avoiding a proper calibration process, which is required in this prototype. These microphones were chosen based of technical specifications and cost per unit. While on the technical side the three models are comparable according to their manufacturers, the price per unit varies somewhat more. Specifically, the cost varies between EUR 10.99 and EUR 39. Furthermore, Gyvazla microphone is the cheapest and Saramonic SR-ULM10 the most expensive. In addition, the Sandberg streamer microphone model costs about EUR 29. Two microphones of each model were compared to assess whether there are substantial differences between microphones of the same model. First, they were calibrated using a calibrator (Bruel and Kjaer 4231) emitting a 1 kHz tone at 94 dB. In order to avoid clipping, the sound card input gain for the Gyvazla microphones was adjusted through the _alsmixer_, which is a graphical mixer program that allows one to adjust the volume and set-up sound settings in Linux. Specifically, it was set to 60% (15.0 dB) for the Gy1, and to 77% (22.5 dB) for the Gy2. Conversely, the default input gain was used for the Sandberg microphones (100%, \(-\)2.0 dB). Then, a calibration factor was computed for each of the four microphones, (i.e., the number that the recorded waveform of the calibration tone should be multiplied by to obtain 94 dB). The obtained factors are shown in Table 1.  
After the calibration process, different measurements were made to evaluate the linearity of the microphones. To this end, a loudspeaker Electro-Voice (ELX200-10) was used to emit 1 kHz tones with different reference levels, from 50 dB to 99.2 dB, in 10 dB steps. The tested microphones together with a free-field reference microphone (G.R.A.S. 40 BF) were placed at 1.5 m from the loudspeaker. This distance was chosen following the ISO 3382-2 [37]. Testing the linearity of microphones is important to see how they respond regarding to the level to which they are exposed. This gives an idea of the reliability of the measured level, regardless of whether the microphone sensitivity has been adjusted using the typical 94 dB calibration tone at 1 kHz. The results of the test are shown in Table 2 and pictures to show the set-up in the anechoic chamber are shown in Figure 1.  
If linearity performance shown in Table 2 is compared between the two models considering only the values above the background noise, the Sb microphone results in a better one. While the maximum deviation obtained with the Sb model is 1.4 dB (for 99.2 dB reference level), meaning that we measured 97.8 dB instead of 99.2 dB; for the Gv model, a deviation of 3 dB was measured (for 60 dB reference level), meaning that we measured 63 dB instead of 60 dB. If the results acquired between microphones of the same model are compared, levels obtained from Gv1 and Gv2 showed differences of about 2 dB for most of the reference levels. In contrast, the maximum difference between Sb1 and Sb2 is 0.6 dB only and just for one of the reference levels (99.2 dB). In addition, the background noise levels measured for the Sb model are noticeably lower than those measured for the Gv microphone. This background noise comes mainly from the electrical noise of the microphone together with the USB adapter.