The Woodruff School

SUMMARY

Measuring acoustic pressure gradient is critical in many applications such as directional microphones for hearing aids and sound intensity probes. This measurement is especially challenging with decreasing microphone size that reduces the sensitivity due to small pressure port spacing. Novel, micromachined biomimetic microphone diaphragms are shown to provide high sensitivity to pressure gradients on one side of the diaphragm with low thermal mechanical noise. These structures have a dominant mode shape with see-saw like motion in the audio band responding to pressure gradients as well as spurious higher order modes sensitive to pressure. In this dissertation, integration of a diffraction based optical detection method with these novel diaphragm structures to implement a low noise optical pressure gradient microphone is described and experimental characterization results are presented. This optical detection scheme also provides electrostatic actuation capability from both sides of the diaphragm separately which can be used for active force feedback. A 4-port electromechanical equivalent circuit model of this microphone with optical readout is developed to predict the overall response of the device to different acoustic and electrostatic excitations. The model includes the damping due to complex motion of air around the microphone diaphragm and it calculates the detected optical signal on each side of the diaphragm as combinations of separate individual vibration modes. This equivalent circuit model is verified by experiments and used to predict the microphone response with different force feedback schemes. Single sided force feedback is used for active damping to improve the linearity and the frequency response of the microphone. Furthermore, it is shown that using two sided force feedback one can significantly suppress or enhance the desired vibration modes of the diaphragm. This approach provides an electronic means to tailor the directional response of the microphones with significant implications in device performance for various applications. As an example, the use of this device as particle velocity sensor for sound intensity and sound power measurements is investigated. Without force feedback, the gradient microphone provides accurate particle velocity measurement until 2 kHz after which the pressure response of the second order mode becomes significant. With two-sided force feedback, the calculations show that this upper frequency limit may be increased to 15 kHz. This improves the pressure residual intensity index by more than 15 dB in the 100 Hz-15 kHz range, matching the Class I requirements of IEC1043 standards for intensity probes without a need for multiple spacers.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Baris Bicen

TIME:	Wednesday, April 14, 2010, 3:00 p.m.

PLACE:	Love Building, 210

TITLE:	Micromachined Diffraction Based Optical Microphones and Intensity Probes with Electrostatic Force Feedback

COMMITTEE:	Dr. F. Levent Degertekin, Chair (ME) Dr. Kenneth Cunefare (ME) Dr. Jerry H. Ginsberg (ME) Dr. Oliver Brand (ECE) Dr. Ronald Miles (ME-Binghamton University)

SUMMARY Measuring acoustic pressure gradient is critical in many applications such as directional microphones for hearing aids and sound intensity probes. This measurement is especially challenging with decreasing microphone size that reduces the sensitivity due to small pressure port spacing. Novel, micromachined biomimetic microphone diaphragms are shown to provide high sensitivity to pressure gradients on one side of the diaphragm with low thermal mechanical noise. These structures have a dominant mode shape with see-saw like motion in the audio band responding to pressure gradients as well as spurious higher order modes sensitive to pressure. In this dissertation, integration of a diffraction based optical detection method with these novel diaphragm structures to implement a low noise optical pressure gradient microphone is described and experimental characterization results are presented. This optical detection scheme also provides electrostatic actuation capability from both sides of the diaphragm separately which can be used for active force feedback. A 4-port electromechanical equivalent circuit model of this microphone with optical readout is developed to predict the overall response of the device to different acoustic and electrostatic excitations. The model includes the damping due to complex motion of air around the microphone diaphragm and it calculates the detected optical signal on each side of the diaphragm as combinations of separate individual vibration modes. This equivalent circuit model is verified by experiments and used to predict the microphone response with different force feedback schemes. Single sided force feedback is used for active damping to improve the linearity and the frequency response of the microphone. Furthermore, it is shown that using two sided force feedback one can significantly suppress or enhance the desired vibration modes of the diaphragm. This approach provides an electronic means to tailor the directional response of the microphones with significant implications in device performance for various applications. As an example, the use of this device as particle velocity sensor for sound intensity and sound power measurements is investigated. Without force feedback, the gradient microphone provides accurate particle velocity measurement until 2 kHz after which the pressure response of the second order mode becomes significant. With two-sided force feedback, the calculations show that this upper frequency limit may be increased to 15 kHz. This improves the pressure residual intensity index by more than 15 dB in the 100 Hz-15 kHz range, matching the Class I requirements of IEC1043 standards for intensity probes without a need for multiple spacers.