Broadband MEMS Microphone Arrays with Reduced Aperture Through 3D-Printed Waveguides

TL;DR

The paper tackles grating-lobe limits in in-air ultrasound MEMS microphone arrays by reducing the acoustic aperture with a 3D-printed baffle, enabling closer inter-element spacing without PCB constraints. The approach leverages spatial Nyquist theory to push the beamforming bandwidth, increasing $f_{ ext{max}}$ from $v/(2d)$ for a given spacing to a higher value as $d$ decreases, demonstrated here from 45.13 kHz to 95.25 kHz when spacing is reduced from 3.8 mm to 1.8 mm. Experimental results show up to ~15 dB signal attenuation due to the baffle, but grating lobes are effectively suppressed in the baffled configuration, with calibration and simulation data agreeing on reduced spatial aliases. The method offers a cost-effective path to more robust ultrasonic sensing in robotics and enables better emulation of bat HRTFs for high-frequency perception tasks.

Abstract

In this paper we present a passive and cost-effective method for increasing the frequency range of ultrasound MEMS microphone arrays when using beamforming techniques. By applying a 3D-printed construction that reduces the acoustic aperture of the MEMS microphones we can create a regularly spaced microphone array layout with much smaller inter-element spacing than could be accomplished on a printed circuit board due to the physical size of the MEMS elements. This method allows the use of ultrasound sensors incorporating microphone arrays in combination with beamforming techniques without aliases due to grating lobes in applications such as sound source localization or the emulation of bat HRTFs.

Figures (3)

• Figure 1: a) Wireframe representation of the 30 waveguides inset, incorporated into the 3D-printed baffle structure of which the render is also shown in the top left corner, that reduce the inter-element spacing of the MEMS microphones from 3.8mm to 1.8mm. This reduced inter-element microphone array layout effectively increases the $f_{\mathit{max}}$ at which grating lobes would be introduced in the directivity pattern from 45.13kHz to 95.25kHz. b) Shows the fabricated 3D-baffle structure with the SLA-printed inset and the outer FDM-printed assembled with the eRTIS ultrasound sensor.
• Figure 2: a) The Welch Power Spectral Density is shown comparing both the recorded non-baffled recordings with the baffled recordings, in which the blue curve shows the non-baffled PSD and the red curve shows the baffled PSD. This shows that by using the 3D-printed baffle add-on in front of the microphone the signal gets attenuated by a maximum of approximately 15dB. b) Shows the HRTF of the left ear of a Micronycteris Microtis bat and the synthetic HRTF approximations of a microphone array layout with an inter-element spacing of 3.8mm and 1.8mm
• Figure 3: The effect of using baffles on the directivity pattern is shown. The columns represent the raw measured data, the calibrated measured data and simulated data with the uneven rows being the non-baffled array setup and the even rows the baffled array setup. In a) the Senscomp 7000 transducer has a pan angle of $0°$ in relation to the microphone array, in b) the pan angle is $-20°$ and in c) the pan angle is $40°$.