The long-term objective of the proposed research is to develop technology for the creation of directional microphones for hearing aids that will have essentially inaudible thermal and electronic noise. The dramatic reduction in the noise of these directional microphones will be accomplished by the integration of three novel technologies: 1) The development of a robust, biologically-inspired microphone diaphragm having low thermal noise. Microphones having ideal, noiseless electronic amplification will still produce noise in their output due to the random impacts of thermally excited air molecules on the diaphragm. The ability of the surrounding gas to impart energy to the diaphragm is directly related to the amount of vibration damping, or passive energy dissipation in the system. In the proposed study, optimized low-damping, and hence low-noise, diaphragm designs will be developed to create a directional microphone diaphragm having much lower thermal noise along with increased sensitivity to sound than can be achieve by currently available technology. 2) Optical sensing to convert the diaphragm motion into an electronic signal. In the proposed effort, a revolutionary low-noise optical method will be developed for converting the motion of the diaphragm into an electronic signal. This optical scheme provides a highly sensitive, low-noise method of obtaining an electronic output from the bio-inspired microphone diaphragms that adds negligible electronic noise. A miniaturized packaging scheme will be developed to integrate the optoelectronic components with the microphone diaphragm. 3) Electronic feedback for thermal noise reduction. As mentioned above, a key contribution of this research will be the development of directional microphone diaphragms having a minimum of passive damping. While low damping leads to low noise, it also leads to highly resonant, ringing response, which is certainly undesirable in a microphone. In the proposed effort, an electronic feedback system will be developed to incorporate electronic damping to achieve the desirable response benefits of damping without the associated thermal noise. This approach to thermal noise reduction and active response control has been adopted in other low-noise sensing applications but it has previously not been feasible in microphone applications. The combination of the low damping, directional microphone diaphragm and the optical sensing scheme to be developed here makes it possible to take advantage of this powerful technology in the design of low-noise miniature microphones.
