# A Wearable Microphone Array Helmet for Automotive Applications
## 5 Conclusions  
A wearable helmet microphone array featuring 32 electret capsules has been developed, built, and characterized. It integrates a miniaturized 32-channel A/D converter, which avoids the bulky wiring that would occur with an analog solution and ensures a high S/N ratio thanks to the usage of the A\({}^{2}\)B digital bus. In addition, this solution allows only one Ethernet cable to come out of the helmet, maintaining good comfort and ease of use. The A\({}^{2}\)B signal is received by an external D/A box, which can deliver digital data to the PC via USB or convert the data back to the analog domain by means of two D/A converters, allowing us to record them with an external acquisition unit.  
The array helmet was compared with the Eigenmike32, a spherical microphone array that widely considered the reference equipment for spatial audio recording during the last decade. By analyzing two metrics for Ambisonics performance evaluation, namely Spatial Correlation and Level Difference, it was possible to assess that the proposed system shifted toward low frequencies, with the Ambisonics orders by one or even two octaves, making it accurate even at the lowest octave band, centered at 31.5 Hz, where the Eigenmike32 is not effective at all. These results have been proved with a laboratory test consisting of a noise source localization problem, making use of the sound color mapping technique. The array helmet was demonstrated to correctly localize the noise source at all frequencies, with a trend of increasing accuracy with frequency, as expected. The highest valid frequency for beamforming has been reduced to 1 kHz, due to the large size of the helmet compared to the number of capsules. However, such a frequency is still above the maximum frequency at which Active Noise Control systems for cars are effective. In conclusion, the proposed solution can be employed for the assessment and the development of road noise cancelling or engine order cancelling systems at the driver seat in driving condition.  
The presented array has two limitations. Most of the rolling noise from the wheels comes from below; therefore, the highest microphone density would be desired in the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Octave Bands Center** & \multicolumn{2}{c}{**Estimated DoA of Direct Sound Located at (0\({}^{\circ}\); 0\({}^{\circ}\))**} \\ \cline{2-3}
**Frequency [Hz]** & **Azimuth [\({}^{\circ}\)]** & **Elevation [\({}^{\circ}\)]** \\ \hline
31.5 & 0 & \(1\) \\
63 & \(-2\) & 3 \\
125 & 3 & 4 \\
250 & \(-1\) & \(1\) \\
500 & 1 & \(-1\) \\
1000 & 0 & \(-1\) \\ \hline \hline \end{tabular}
\end{table}
Table 7: Polar coordinates of the DoA of the direct sound at different octave bands for the array helmet.  
lower part of the helmet to increase the spatial resolution in the interested portion of space. However, this is not possible due to the presence of the neck. The second limitation consists in the movement of the driver's head, which is naturally unstable when driving a car, in particular while cornering. Therefore, when the recording is played back, a mismatch between the orientation of the listener's head and the orientation of the sound field may occur. However, this problem will be overcome in a future update of the helmet by installing a head-tracking system to record the quaternion of spatial rotation synchronously with the pressure signals. Such a solution will allow compensating the head's movements in post-processing, by counter-rotating the sound field after Ambisonics conversion.