High Signal-to-Noise Ratio Miniature Microphones for Hearing Assistive Devices

High Signal-to-Noise Ratio Miniature Microphones for Hearing Assistive Devices Project Summary Hearing impairment is associated with a loss of fidelity to quiet sounds, while the threshold of pain remains the same. As such, hearing impairment causes a loss of dynamic range or “window” of detectable sound amplitudes. Directional sensing enables preferentially amplifying desired sounds without amplifying background noise. In modern hearing aids directional sensing is achieved with closely spaced microphone arrays. When the array aperture is small compared to the wavelength of sound, as is the case for a broad range of frequencies relevant to hearing aids, directivity is achieved by subtracting microphone signals with only a small relative phase difference. The resulting signals are small, and high SNR from the directional system demands high signal-to- noise ratio (SNR) from individual microphones. Capacitive microphones have undergone two decades of optimization and their SNR has plateaued. On the other hand, thin-film piezoelectric materials and related processing technologies are improving at a fast pace owing to their relevance to a wide range of Health and DOD applications. Single-crystal lithium niobate (LN) has recently been identified to have remarkably high sensing figures of merit when the electric field is measured in-plane and along the same axis as the induced stress, known as lateral field transduction. Recent advances in wafer processing have made in-plane polarization and lateral field transduction practical for micromachined bimorph LN diaphragms. Preliminary simulations of LN microphones accounting for dielectric loss and relevant packaging-induced noise predict that 7 dB improvement in SNR beyond state-of-the-art miniature hearing-aid microelectromechanical system (MEMS) microphones is possible. This improvement directly translates to 7 dB hearing-aid system SNR improvement when operating in directional mode. 7-dB hearing-aid system improvement is anticipated to have a significant positive impact on patient perception and intelligibility when hearing in noise. This STTR Phase 1 entails many challenges, including microfabricating the envisioned microphone die, piezoelectric material characterization of this new material in the audible frequency band, packaging into industry-standard size capsule, rigorous characterization of noise, SNR, and acoustic frequency response of the packaged microphone.