Walter C. Babel III SAICQamar A. Shams NASA Langley Research CenterJames F. Bockman NASA Langley Research Center

Qualitative Analysis of MEMS MicrophonesQualitative Analysis of MEMS Microphones

16th ANNUAL 2004 INTERNATIONAL MILITARY & AEROSPACE /16th ANNUAL 2004 INTERNATIONAL MILITARY & AEROSPACE /AVIONICS COTS CONFERENCE, EXHIBITION &AVIONICS COTS CONFERENCE, EXHIBITION &

SEMINARSSEMINARS

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•High Voltage Required

Electret Microphone Overview

•Small•Cheap•Lower Quality

MEMS Microphone Overview

Backplate

Acoustical Wave

Floating Diaphragm

Insulated Spacers

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

General General Comparison

B&K 4134 Electret MEMS Volume >150 mm3 100 mm3 25 mm3 Support Electronics High Medium Low Power Low Low Low Voltage 200V 3V 1-3V Mass High Low Low Cost Very High Low Low Sensitivity High Medium Medium Range <100KHz <20 KHz <50 KHz Temperature Low Low Medium

MEMS Capacitive Microphone Design

Acoustical Wave

Clamped DiaphragmBackplateAirgap

Acoustical Wave

Insulated Spacers

Floating Diaphragm

Blackplate

• 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.

SiSonic MEMS Microphones

SP0101NZ SP0102NC SP0103NC

SP0101NZ•10K Ohms Output impedance

•0.5 mA max. current drain

SP0102NC•100 Ohms Output impedance•0.25 mA max. current drain

SP0103NC•100 Ohms Output impedance

•0.35 mA max. current drain

•Integrated Amplifier

Basic Structure of MEMS Microphone

Diaphragm

Spacers

Base

SP0101 General Outline

backV

Signal

CCV

OUT

Charge Pump

Detection Circuit

Microphone Diaphragm Power and Detection

SP0102 General Outline

backV

Signal

CCV

OUT

Charge Pump

Detection Circuit

Microphone Diaphragm Power and Detection

SP0103 General Outline

backV

Signal

CCV

OUT

Charge Pump

Detection Circuit

20 dBAmplifier

Microphone Diaphragm Power and Detection

extRextC

Basic Functional Analysis(Clamped and Free floating diaphragm)

The model of clamped and free floating movable plate capacitor is shown by:

FE can be calculated by differentiating the stored energy of the capacitor w.r.t. the position of the movable plate:

xKdt

xdmF

2

2

where F is the electrostatic attraction force caused by supply voltage V.

The mechanical elastic force FM can be expressed as:

xKF M

where K is a spring constant and is assumed to be linear.

)Vx

kAε

2

1(

dx

d - )CV

2

1(

dx

dF 202

E

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

SP0101NC3 / SP0102NC3 / SP0103NC3Frequency Response Testing

MEMSMicrophone High-Pass

Filter (10Hz)

Buffer

n x

nx Amplifier

Hardware

Software

FFT

Voltmeter

ChannelSelect

HardDisk

backV

Signal

CCV

OUT

Charge Pump

Detection Circuit

MEMS Microphone Power and Detection

Anechoic Chamber Test Setup

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

SP0101

SP0102

SP0103

Am

plit

ud

e (

V)

Frequency (Hz)

MEMS Microphone Comparison100 -10000 Hz

MEMS Microphone Comparison100Hz – 25kHz

Am

plit

ud

e (

V)

Frequency (Hz)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 5000 10000 15000 20000 25000

SP0101

SP0102

SP0103

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

SP0101

SP0102

SP0103

MEMS Microphone Comparison100Hz – 10kHz

Frequency (Hz)

Am

plit

ud

e (

% o

f 1kH

z V

alu

e)

MEMS Microphone Comparison100Hz – 25kHz

Am

plit

ud

e (

% o

f 1

kHz

Valu

e)

Frequency (Hz)

0%

100%

200%

300%

400%

500%

600%

0 5000 10000 15000 20000 25000

SP0101

SP0102

SP0103

MEMS MEMS Array Test Layout Test Layout

Anechoic Chamber

MEMSArray Speaker

Amplifier

LabVIEWHardware

MEMS Array Close-UpMEMS Array Close-Up

Audio Source

Numbering Convention

MEMS Array

MEMS Array Frequency ResponseLabVIEW Qualifying ProgramMEMS Channels 0 through 7

100-10000 Hz

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Frequency (Hz)

% o

f M

EM

S A

rray

Ave

rag

e

MEMS0MEMS1MEMS2MEMS3MEMS4MEMS5MEMS6MEMS7

MEMS Array Frequency DataMEMS Array Frequency Data100 – 10000 Hz100 – 10000 Hz

MEMS Array Frequency Response (Raw Data)LabVIEW Qualifying ProgramMEMS Channels 0 through 7

100-50000 Hz

0

1

2

3

4

5

6

7

8

0 10000 20000 30000 40000 50000

Frequency (Hz)

ME

MS

Arr

ay R

aw D

ata

(V)

MEMS0MEMS1MEMS2MEMS3MEMS4MEMS5MEMS6MEMS7

MEMS Array Frequency DataMEMS Array Frequency Data100 – 50000 Hz100 – 50000 Hz

MEMS Array Frequency DataMEMS Array Frequency Data100 – 10000 Hz100 – 10000 Hz

MEMS Array Comparison

0.00%

100.00%

200.00%

300.00%

400.00%

500.00%

600.00%

700.00%

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Frequency (Hz)

No

rmal

ized

Ou

tpu

t (%

of

1000

Hz

Val

ue)

Average ArrayValuesB+K Microphone

MEMS Array Frequency DataMEMS Array Frequency Data100 – 10000 Hz100 – 10000 HzMEMS / Standard Comparison

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

120.00%

140.00%

160.00%

180.00%

200.00%

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Frequency (Hz)

No

rmal

ize

d O

utp

ut

(% o

f 10

00

Hz

Va

lue)

Percent Comparison

As can be seen from the last slide, testing showedevidence of sharp discrepancies between the B+K

standard and the MEMS microphones tested

Although many of the discrepancies can be attributed todifferences in holder types and not the microphonesthemselves the data seemed to indicate mechanical

resonances in the MEMS diaphragm

MEMS Microphone Resonance Problem

MEMS and Electret Microphone Comparison0.1V p-p from Function GeneratorPreAmp Gain to Speaker = 12dB, PostAmp Gain from Microphone= 50

Raw Data

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency (Hz)

Vo

ltag

e (V

olt

s)

MEMS Microphone

Electret Microphone

MEMS Microphone Resonance Data

MEMS and Electret Microphone Comparison0.1V p-p from Function Generator, PreAmp Gain to Speaker = 12dB, PostAmp Gain from Microphone= 50

Normalized to 1000 Hz Value

0%

200%

400%

600%

800%

1000%

1200%

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency (Hz)

% o

f 1

000

Hz

Val

ue

MEMS Microphone

Electret Microphone

MEMS Microphone Resonance DataNormalized to 1000Hz

+V

V35V

+

C41uF

+V

V29V

+

U2BLM318

+

U2ALM318

R6100k 40%

1kHz

V1-1/1V

C3.002uF

C2.001uF

C1.001uF

R51000k

R41k

R35000

R210k

R110k

+V

V35V

+

C41uF

+V

V29V

+

U2BLM318

+

U2ALM318

R6100k 40%

1kHz

V1-1/1V

C3.002uF

C2.001uF

C1.001uF

R51000k

R41k

R35000

R210k

R110k

MEMS Microphone ResonanceReduction Filter

Effect of Filter on MEMS MicrophoneData Normalized to 1000 Hz

100-20000 Hz

0%

50%

100%

150%

200%

250%

300%

350%

400%

450%

500%

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency (Hz)

% o

f 10

00 H

z V

alu

e

Filtered

Unfiltered

Overdriven Amplifer

MEMS Microphone ResonanceReduction Filter

Linear Testing•Used to determine location of sound source

Directionality Testing OverviewDirectionality Testing Overview

Rotational Testing•Used to determine “omnidirectionality” of microphone

MEMS Array ?? ??

Linear Array Directionality Testing

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

Anechoic Chamber

Stepper Motor

50x Amplifier

Speaker

Microphone

Stepper MotorControl Board

LabVIEWHardware

12V/1A Power Supply

Computer

Rotational Directionality Testing

Preliminary Directional Test - 1 kHz

Note: Circle has radius of 1.5 volts

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

Rotational Directionality TestingRotational Directionality Testing

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.)

Background Noise Measurement inAcoustic Isolation Vessel

PC MonitorScan Frequency

B&K ½” Mic (B&K 4134)

Environmental Testing Overview

Humidity Effect on MEMS Microphone

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

% (Humdiity)O

ffse

t (m

V)

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

No Change

Radiation Testing Overview

Radiation Testing•Preliminary radiation exposure tests (Co-60)•Capacitive elements = radiation detectors•No functional change for 4000 kpm (DC offset, noise)

50x Amplifiero’scope

Co-60Cobalt-60 gamma source

V

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 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.