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Vibration Sensitivity of One-Port and Two-Port MEMS Microphones

Published in : IEEE Sensors Journal (Volume: 24, Issue: 18, September 2024)
Authors : Francis Doyon-D’amour, Carly Stalder, Timothy Hodges, Michel Stephan, Lixue Wu, Triantafillos Koukoulas, Stephane Leahy, Raphael St-Gelais
DOI : https://doi.org/10.1109/JSEN.2024.3423658
Summary Contributed by:  Raphael St-Gelais (Author)

Two-port micro-electro-mechanical system (MEMS) microphones (mics) are gaining attention for their potential to achieve higher directional sensitivity compared to traditional one-port MEMS mics. This ability gives them many potential applications requiring robust acoustic performance in vibration-prone environments like automotive and industrial settings, especially in fields where noise reduction and clearer audio recording are critical (e.g., augmented/virtual reality, everyday electronics, online conferencing systems, etc.).

However, measuring acoustic pressure in two-port mics typically requires a softer sensing element (i.e., lower natural frequency) than in one-port mics, which could presumably make them more prone to interference from external vibrations. Two-port mics measure the small pressure difference that exists between two points (i.e., two ports) in the sound wave. This approach provides valuable information not only on the sound amplitude but also on the direction of propagation of sound. Unfortunately, the pressure difference between two points in the sound wave is fundamentally smaller than the absolute acoustic pressure measured by traditional one-port mics. Two-port mics, therefore, typically rely on more compliant sensing elements, which raises questions about their possible increased sensitivity to external vibration.

In this work, the researchers derive a universal expression for sensitivity to the vibration of two-port mics, which are then confirmed experimentally by characterizing several emerging two-port mic technologies. Vibration sensitivity measurements are also performed on one-port mics, thus providing a direct comparison between the two sensing approaches. Experiments are carried out using a custom shaker apparatus consisting of an electrodynamic exciter with a custom mounting plate to accommodate up to two mics and one reference accelerometer. Two-port mics with various acoustic packages are tested, demonstrating the broad applicability of the model.

The experimental findings show that the acoustically-referred vibration sensitivity of two-port MEMS mics, in units of measured acoustic signal per external acceleration (i.e., Pa per g), does not depend on the stiffness of the sensing element nor its natural frequency. Mics with softer sensing elements are expected to be more sensitive to vibration. However, they are also found to be equally more sensitive to sound, making the acoustically referred vibration sensitivity constant. Consequently, the derived expression for the acoustically referred vibration sensitivity only depends on widely known quantities, namely air density, speed of sound, and the external vibration frequency. The researchers also find that the sensitivity to vibration in two-port mics is inversely proportional to the frequency of the vibration (S_(Pa⁄g)∝1/ƒ), as opposed to the frequency-independent behavior observed in one-port mics.

To conclude, the acoustically-referred vibration sensitivity of two-port mics is found to be easily predictable by known physical quantities while varying very weakly between different mic models. Acoustic designers employing two-port mics should, therefore, expect predictable spurious acoustic signals from external vibration, independently of the mic model, packaging, or manufacturer. This vibration sensitivity is independent of the mic’s natural frequency (i.e., its stiffness) and inversely proportional to the frequency of the vibration (S_(Pa⁄g)∝1/f), regardless of the acoustic package.

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