Qualitative Analysis of MEMS Microphones 16 th ANNUAL

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Qualitative Analysis of MEMS Microphones 16 th ANNUAL 2004 INTERNATIONAL MILITARY & AEROSPACE /

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

Introduction MEMS Microphones are desirable for NASA applications because they have: • Small Volume

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

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

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

Electret Microphone Overview • Small • Cheap • Lower Quality

Electret Microphone Overview • Small • Cheap • Lower Quality

MEMS Microphone Overview • Omnidirectional • 0. 5 m. A current draw • Free-plate

MEMS Microphone Overview • Omnidirectional • 0. 5 m. A current draw • Free-plate design • Higher Temperature Acoustical Wave Floating Diaphragm Insulated Spacers Backplate

General Comparison

General Comparison

MEMS Capacitive Microphone Design • Is an electrostatic transducer Acoustical Wave • Capacitance change

MEMS Capacitive Microphone Design • Is an electrostatic transducer Acoustical Wave • Capacitance change due to an external mechanical input (electrostatic transducer) • Clamped diaphragm introduces nonlinearities associated with in-built residual stress in the diaphragm • The Si. Sonic 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. Backplate Airgap Clamped Diaphragm Acoustical Wave Floating Diaphragm Insulated Spacers Blackplate

Si. Sonic MEMS Microphones SP 0101 NZ • 10 K Ohms Output impedance •

Si. Sonic MEMS Microphones SP 0101 NZ • 10 K Ohms Output impedance • 0. 5 m. A max. current drain SP 0102 NC • 100 Ohms Output impedance • 0. 25 m. A max. current drain SP 0103 NC • 100 Ohms Output impedance • 0. 35 m. A max. current drain • Integrated Amplifier SP 0101 NZ SP 0102 NC SP 0103 NC

Basic Structure of MEMS Microphone Diaphragm Spacers Base

Basic Structure of MEMS Microphone Diaphragm Spacers Base

SP 0101 General Outline Microphone Diaphragm Signal Power and Detection Circuit OUT Charge Pump

SP 0101 General Outline Microphone Diaphragm Signal Power and Detection Circuit OUT Charge Pump

SP 0102 General Outline Microphone Diaphragm Signal Power and Detection Circuit OUT Charge Pump

SP 0102 General Outline Microphone Diaphragm Signal Power and Detection Circuit OUT Charge Pump

SP 0103 General Outline Microphone Diaphragm Signal Power and Detection Circuit OUT Charge Pump

SP 0103 General Outline Microphone Diaphragm Signal Power and Detection Circuit OUT Charge Pump 20 d. B Amplifier

Basic Functional Analysis (Clamped and Free floating diaphragm) The model of clamped and free

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:

Frequency Response Analysis Overview • Measures output of microphones as frequency of sound source

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

SP 0101 NC 3 / SP 0102 NC 3 / SP 0103 NC 3

SP 0101 NC 3 / SP 0102 NC 3 / SP 0103 NC 3 Frequency Response Testing Hardware MEMS Microphone High-Pass Filter (10 Hz) nx Amplifier nx Buffer Software FFT Channel Select Hard Disk Voltmeter

Anechoic Chamber Test Setup

Anechoic Chamber Test Setup

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

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

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

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

MEMS Microphone Comparison Amplitude (% of 1 k. Hz Value) 100 Hz – 10

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

MEMS Microphone Comparison Amplitude (% of 1 k. Hz Value) 100 Hz – 25

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

MEMS Array Test Layout Anechoic Chamber MEMS Array Speaker Lab. VIEW Hardware Amplifier

MEMS Array Test Layout Anechoic Chamber MEMS Array Speaker Lab. VIEW Hardware Amplifier

MEMS Array Close-Up Numbering Convention MEMS Array Audio Source

MEMS Array Close-Up Numbering Convention MEMS Array Audio Source

MEMS Array Frequency Data 100 – 10000 Hz

MEMS Array Frequency Data 100 – 10000 Hz

MEMS Array Frequency Data 100 – 50000 Hz

MEMS Array Frequency Data 100 – 50000 Hz

MEMS Array Frequency Data 100 – 10000 Hz

MEMS Array Frequency Data 100 – 10000 Hz

MEMS Array Frequency Data 100 – 10000 Hz

MEMS Array Frequency Data 100 – 10000 Hz

MEMS Microphone Resonance Problem As can be seen from the last slide, testing showed

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

MEMS Microphone Resonance Data

MEMS Microphone Resonance Data

MEMS Microphone Resonance Data Normalized to 1000 Hz

MEMS Microphone Resonance Data Normalized to 1000 Hz

MEMS Microphone Resonance Reduction Filter

MEMS Microphone Resonance Reduction Filter

MEMS Microphone Resonance Reduction Filter

MEMS Microphone Resonance Reduction Filter

Directionality Testing Overview Linear Testing • Used to determine location of sound source ?

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

Linear Array Directionality Testing Linear Testing • Eight equidistant MEMS microphones • Lab. VIEW

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

Rotational Directionality Testing Anechoic Chamber Computer Lab. VIEW Hardware Speaker Microphone Stepper Motor 50

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

Rotational Directionality Testing Rotational Testing • MEMS microphones tested against electret • Rotated through

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

Background Noise Measurement of MEMS Microphones (MEMS Microphone isolated from ambient sounds and vibration)

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 d. B at 40 Hz) enable noise measurements at frequencies as low as 2 Hz. )

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

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

Environmental Testing Overview No Change Humidity Testing • Preliminary environmental tests • Lab. VIEW

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

Radiation Testing Overview Co-60 Cobalt-60 gamma source 50 x Amplifier V Radiation Testing •

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

Current MEMS Microphone Work

Current MEMS Microphone Work

Current MEMS Microphone Work

Current MEMS Microphone Work

Current MEMS Microphone Work

Current MEMS Microphone Work

Current MEMS Microphone Work

Current MEMS Microphone Work

Conclusions MEMS Microphones are adequate for many distributed or disposable systems External circuitry is

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.