LAUSR

This paper introduces a directional MEMS microphone designed for hearing aid applications appropriate to low-frequency hearing impairment, inspired by the hearing mechanism of a fly, the female Ormia ochracea. It uses both piezoelectric and capacitive sensing schemes. In order to obtain a high sensitivity at low frequency bands, the presented microphone is designed to have two resonance frequencies below the threshold of low-frequency hearing loss at approximately 2 kHz. One is around 500 Hz and the other is slightly above 2 kHz. The novel dual sensing mechanism allows for optimization of the microphone sensitivity at both frequencies, with a maximum open-circuit (excluding pre-amplification) acoustic response captured via differential piezoelectric sensing at approximately −46 dB (V) ref. 94 dB (SPL) at the resonance frequencies. The corresponding minimum detectable sound pressure level is just below −12 dB. The comb finger capacitive sensing was employed due to a lower electrical response generated from a ground referenced single-ended output by the piezoelectric sensing at the first resonance frequency compared with the second resonance frequency. The capacitive sensing mechanism, connected to a charge amplifier, generates a −28.4 dB (V) ref. 94 dB (SPL) acoustic response when the device is excited at either of the two resonance frequencies. Due to the asymmetric geometry and the $400~\mu \text{m}$ thick substrate, the device is predicted to perform as a bi-directional microphone below 3 kHz, which is shown by the measured directional polar patterns. [2018-0036]