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


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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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slide1

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

slide2

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

slide3

B&K 4134 Microphone Overview

  • Very High Quality
  • Industry Standard
  • PULSE System/Software
  • High Voltage Required
slide4

Electret Microphone Overview

  • Small
  • Cheap
  • Lower Quality
slide5

MEMS Microphone Overview

  • Omnidirectional
  • 0.5 mA current draw
  • Free-plate design
  • Higher Temperature

Acoustical Wave

Floating Diaphragm

Insulated Spacers

Backplate

slide7

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

slide8

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
slide10

SP0101 General Outline

Microphone Diaphragm

Power and Detection

Signal

Detection Circuit

OUT

Charge Pump

slide11

SP0102 General Outline

Microphone Diaphragm

Power and Detection

Signal

Detection Circuit

OUT

Charge Pump

slide12

SP0103 General Outline

Microphone Diaphragm

Power and Detection

Signal

Detection Circuit

OUT

Charge Pump

20 dB

Amplifier

slide13

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:

slide14

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
slide15

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

slide17

MEMS Microphone Comparison

100 -10000 Hz

Amplitude (V)

Frequency (Hz)

slide18

MEMS Microphone Comparison

100Hz – 25kHz

Amplitude (V)

Frequency (Hz)

slide19

MEMS Microphone Comparison

100Hz – 10kHz

Amplitude (% of 1kHz Value)

Frequency (Hz)

slide20

MEMS Microphone Comparison

100Hz – 25kHz

Amplitude (% of 1kHz Value)

Frequency (Hz)

slide21

MEMS Array Test Layout

Anechoic Chamber

MEMS

Array

LabVIEW

Hardware

Speaker

Amplifier

slide22

MEMS Array Close-Up

Numbering Convention

      

MEMS Array

Audio Source

slide27

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

slide29

MEMS Microphone Resonance Data

Normalized to 1000Hz

slide32

Directionality Testing Overview

  • Linear Testing
    • Used to determine location of sound source

?

?

  • Rotational Testing
    • Used to determine “omnidirectionality” of microphone
slide33

Linear Array Directionality Testing

  • Linear Testing
    • Eight equidistant MEMS microphones
    • LabVIEW acquires data
    • Weighted average determines sound location in x-axis
slide34

Rotational Directionality Testing

Anechoic Chamber

Computer

LabVIEW

Hardware

Speaker

Microphone

Stepper Motor

50x Amplifier

Stepper Motor

Control Board

12V/1A Power Supply

slide35

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

slide36

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.)
slide37

Background Noise Measurement in

Acoustic Isolation Vessel

B&K ½” Mic (B&K 4134)

PC Monitor

Scan Frequency

slide38

Environmental Testing Overview

No Change

  • Humidity Testing
    • Preliminary environmental tests
    • LabVIEW acquires data
    • No functional change for large humidity range
slide39

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)
slide44

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.