Change in transfer function between air and bone conduction microphones due to mouth opening variation

Bone conduction microphones (BCM) offer an interesting solution for speech recording in noisy environments. They provide enhanced ergonomics and reduced sensitivity to ambient noise compared to conventional aerial microphones. However, BCM exhibit inherent limitations in intelligibility and sound quality, even in quiet conditions, restricting their widespread adoption. To address these limitations, filtering techniques employing transfer functions between a conventional Air Conduction Microphones (ACM) and BCM have been explored. However, this study demonstrates the inadequacy of this approach due to the non-constant nature of the transfer function. An experiment involving ten subjects revealed that the transfer function between an ACM and a BCM, derived with a direct oral excitation, varies with a mouth opening. Additionally, a numerical investigation using finite element methods confirmed that the mouth opening significantly impacts the transfer function between the oral cavity sound pressure and an air conduction microphone but has negligible effect on the transfer function between the oral cavity sound pressure and a BCM. This paper try to explain the amplitude variation of bone-conducted speech and air-conducted speech depending on the vowel pronounced and highlights the inapplicability of Perceptual Evaluation of Speech Quality (PESQ) metrics for BCM. It opens avenues for signal processing techniques aimed at improving the quality and intelligibility of BCM-recorded speech.