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Walter C. Babel III SAIC Qamar A. Shams NASA Langley Research Center James F. Bockman NASA Langley Research Cen

Qualitative Analysis of MEMS Microphones 16th ANNUAL 2004 INTERNATIONAL MILITARY & AEROSPACE / AVIONICS COTS CONFERENCE, EXHIBITION & SEMINARS. Walter C. Babel III SAIC Qamar A. Shams NASA Langley Research Center James F. Bockman NASA Langley Research Center August 2004.

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Walter C. Babel III SAIC Qamar A. Shams NASA Langley Research Center James F. Bockman NASA Langley Research Cen

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  1. Qualitative Analysis of MEMS Microphones 16th ANNUAL 2004 INTERNATIONAL MILITARY & AEROSPACE / AVIONICS COTS CONFERENCE, EXHIBITION & SEMINARS Walter C. Babel III SAIC Qamar A. Shams NASA Langley Research Center James F. Bockman NASA Langley Research Center August 2004

  2. Introduction MEMS Microphones are desirable for NASA applications because they have: • Small Volume • Low Mass • Low Power • Low Voltage • Low Cost Before they can be used in mission-critical applications, they need to be thoroughly tested

  3. B&K 4134 Microphone Overview • Very High Quality • Industry Standard • PULSE System/Software • High Voltage Required

  4. Electret Microphone Overview • Small • Cheap • Lower Quality

  5. MEMS Microphone Overview • Omnidirectional • 0.5 mA current draw • Free-plate design • Higher Temperature Acoustical Wave Floating Diaphragm Insulated Spacers Backplate

  6. General Comparison

  7. MEMS Capacitive Microphone Design Acoustical Wave • Is an electrostatic transducer • Capacitance change due to an external • mechanical input (electrostatic transducer) • Clamped diaphragm introduces nonlinearities • associated with in-built residual stress in the • diaphragm • The SiSonic design uses a flat free-plate that • is held in proximity to the back plate by • electrostatic attraction. • As diaphragm is a free-plate (it has no edge • moments and has no tension), it has higher • fidelity than a clamped arrangement. Clamped Diaphragm Backplate Airgap Acoustical Wave Floating Diaphragm Insulated Spacers Blackplate

  8. SiSonic MEMS Microphones • SP0101NZ • 10K Ohms Output impedance • 0.5 mA max. current drain • SP0102NC • 100 Ohms Output impedance • 0.25 mA max. current drain SP0101NZ SP0102NC SP0103NC • SP0103NC • 100 Ohms Output impedance • 0.35 mA max. current drain • Integrated Amplifier

  9. Basic Structure of MEMS Microphone Diaphragm Spacers Base

  10. SP0101 General Outline Microphone Diaphragm Power and Detection Signal Detection Circuit OUT Charge Pump

  11. SP0102 General Outline Microphone Diaphragm Power and Detection Signal Detection Circuit OUT Charge Pump

  12. SP0103 General Outline Microphone Diaphragm Power and Detection Signal Detection Circuit OUT Charge Pump 20 dB Amplifier

  13. Basic Functional Analysis (Clamped and Free floating diaphragm) The model of clamped and free floating movable plate capacitor is shown by: where F is the electrostatic attraction force caused by supply voltage V. The mechanical elastic force FM can be expressed as: where K is a spring constant and is assumed to be linear. FE can be calculated by differentiating the stored energy of the capacitor w.r.t. the position of the movable plate:

  14. Frequency Response Analysis Overview • Measures output of microphones as frequency • of sound source is varied • Frequency changed from 100 Hz through 50,000 Hz • Non-linearities (power vs. Sound Intensity) • of speaker system factored out

  15. SP0101NC3 / SP0102NC3 / SP0103NC3 Frequency Response Testing Hardware MEMS Microphone nx Amplifier High-Pass Filter (10Hz) n x Buffer Software FFT Channel Select Hard Disk Voltmeter

  16. Anechoic Chamber Test Setup

  17. MEMS Microphone Comparison 100 -10000 Hz Amplitude (V) Frequency (Hz)

  18. MEMS Microphone Comparison 100Hz – 25kHz Amplitude (V) Frequency (Hz)

  19. MEMS Microphone Comparison 100Hz – 10kHz Amplitude (% of 1kHz Value) Frequency (Hz)

  20. MEMS Microphone Comparison 100Hz – 25kHz Amplitude (% of 1kHz Value) Frequency (Hz)

  21. MEMS Array Test Layout Anechoic Chamber MEMS Array LabVIEW Hardware Speaker Amplifier

  22. MEMS Array Close-Up Numbering Convention         MEMS Array Audio Source

  23. MEMS Array Frequency Data 100 – 10000 Hz

  24. MEMS Array Frequency Data 100 – 50000 Hz

  25. MEMS Array Frequency Data 100 – 10000 Hz

  26. MEMS Array Frequency Data 100 – 10000 Hz

  27. MEMS Microphone Resonance Problem As can be seen from the last slide, testing showed evidence of sharp discrepancies between the B+K standard and the MEMS microphones tested Although many of the discrepancies can be attributed to differences in holder types and not the microphones themselves the data seemed to indicate mechanical resonances in the MEMS diaphragm

  28. MEMS Microphone Resonance Data

  29. MEMS Microphone Resonance Data Normalized to 1000Hz

  30. MEMS Microphone Resonance Reduction Filter

  31. MEMS Microphone Resonance Reduction Filter

  32. Directionality Testing Overview • Linear Testing • Used to determine location of sound source ? ? • Rotational Testing • Used to determine “omnidirectionality” of microphone

  33. Linear Array Directionality Testing • Linear Testing • Eight equidistant MEMS microphones • LabVIEW acquires data • Weighted average determines sound location in x-axis

  34. Rotational Directionality Testing Anechoic Chamber Computer LabVIEW Hardware Speaker Microphone Stepper Motor 50x Amplifier Stepper Motor Control Board 12V/1A Power Supply

  35. Rotational Directionality Testing • Rotational Testing • MEMS microphones tested against electret • Rotated through 360 degrees in 3.6 degree steps • “Omnidirectionality” dependent on package style • For similar packages, electret and MEMS are similar Note: Circle has radius of 1.5 volts

  36. Background Noise Measurement of MEMS Microphones (MEMS Microphone isolated from ambient sounds and vibration) • Acoustic isolation is achieved by • means of high vacuum. • Microphone remains close to • room temperature and pressure • Attainable levels of isolation • (e.g., -155 dB at 40 Hz) enable • noise measurements at • frequencies as low as 2 Hz.)

  37. Background Noise Measurement in Acoustic Isolation Vessel B&K ½” Mic (B&K 4134) PC Monitor Scan Frequency

  38. Environmental Testing Overview No Change • Humidity Testing • Preliminary environmental tests • LabVIEW acquires data • No functional change for large humidity range

  39. Radiation Testing Overview Co-60 Cobalt-60 gamma source o’scope 50x Amplifier V • Radiation Testing • Preliminary radiation exposure tests (Co-60) • Capacitive elements = radiation detectors • No functional change for 4000 kpm (DC offset, noise)

  40. Current MEMS Microphone Work

  41. Current MEMS Microphone Work

  42. Current MEMS Microphone Work

  43. Current MEMS Microphone Work

  44. Conclusions MEMS Microphones are adequate for many distributed or disposable systems External circuitry is currently required to minimize effects of resonance of MEMS units Savings in space, weight, and cost make them useful for certain NASA applications, but cannot be considered a “replacement technology” at this time.

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