Conventional microphones measure the motion of a compliant diaphragm using capacitive transduction. Criteria for high-fidelity motion detection conflict with criteria for low levels of thermal mechanical noise, or Brownian motion. This design conflict limits the SNR achievable by conventional MEMS microphones. Silicon Audio integrates optical interferometers with low-power, low-noise electronics to realize unprecedented performance in our omnidirectional microphones.
At Silicon Audio, our optical detection approach uses a semiconductor laser to illuminate a system with a surface that splits a portion of the beam, while allowing a portion of light to reflect from a pressure-sensitive diaphragm. Through the interference of these light waves, extremely high displacement detection resolution is achieved. This, in-turn, results in extreme sound pressure resolution and high SNR, up to 80 dB, for an omnidirectional microphone.
As shown in the prototype image to the left (photographed from the back through the KOH etch cavity), our omnidirectional microphone achieves a surface-mount form-factor consistent with the smallest MEMS microphones today. Realization of successful microphones calls upon our expertise at Silicon Audio in microfabrication of MEMS elements, optoelectronic integration of photodiodes and semiconductor light sources, and acoustical packaging and modeling.
D. Kim, C. T. Garcia, B. Avenson, and N. A. Hall, "Design and Experimental Evaluation of a Low-Noise Backplate for a Grating-Based Optical Interferometric Sensor," Journal of Microelectromechanical Systems, vol. PP (99), pp. 1-1, March 2014.
M. L. Kuntzman, C. T. Garcia, A. G. Onaran, B. Avenson, K. D. Kirk, and N. A. Hall, "Performance and Modeling of a Fully Packaged Micromachined Optical Microphone," Journal of Microelectromechanical Systems, vol. 20(4), pp. 828-833, August 2011.
N. A. Hall, M. Okandan, R. Littrell, B. Bicen, and F. L. Degertekin, "Simulation of Thin-Film Damping and Thermal Mechanical Noise Spectra for Advanced Micromachined Microphone Structures," Journal of Microelectromechanical Systems, vol. 17(3), pp. 688-697, June 2008.
N. A. Hall, M. Okandan, R. Littrell, B. Bicen, and F. L. Degertekin, "Micromachined Optical Microphone Structures with Low Thermal-Mechanical Noise Levels," The Journal of the Acoustical Society of America, vol. 122(4), pp. 2031-2037, October 2007.
N. A. Hall, B. Bicen, M. K. Jeelani, W. Lee, S. Qureshi, F. L. Degertekin, and M. Okandan, "Micromachined Microphones with Diffraction-Based Optical Displacement Detection," The Journal of the Acoustical Society of America, vol. 118(5), pp. 3000-3009, November 2005.
The omnidirectional microphone highlights Silicon Audio’s expertise in MEMS and our familiarity with the attributes of silicon microfabrication that, in combination with optical and electronic elements, result in superior sound and vibration sensing.
At Silicon Audio, our achievement of substantial improvements in fidelity and SNR, compared with other commercial MEMS microphones, is grounded in a deep understanding of advanced optics-based motion-detection principles.
Unique circuit design challenges are introduced in the realization of the first commercial optical microphone, including photocurrent to voltage converters and closed loop actuation electronics.
The optical microphone is free of many acoustical design constraints imposed on more conventional microphones, and advanced acoustical models are used to exploit this new space and achieve breakthrough performance.