The MEMS Microphone Book - Table of Contents

 

1       Foreword. 1-17

2       Sound and Acoustics. 2-19

2.1        Sound. 2-19

   2.1.1         Audio Frequency Range. 2-21

   2.1.2         Ultrasound. 2-22

   2.1.3         Infrasound. 2-24

2.2        Importances of Different Frequencies. 2-25

2.3        Audio Frequencies. 2-26

   2.3.1         Sub Bass and Bass Frequencies. 2-26

   2.3.2         Low Midrange, Midrange and High Midrange. 2-29

   2.3.3         High Frequencies. 2-30

   2.3.4         Microphones vs. Audio Frequency Band. 2-31

2.4        Audio Frequency Bands. 2-32

2.5        A-Weighting. 2-33

2.6        Wavelength of Sound. 2-34

2.7        Sound Pressure Level 2-35

2.8        Sound Pressure Level Range (for a Microphone) 2-40

2.9        Sound Pressure Attenuation. 2-40

   2.10     Sound Propagation. 2-43

   2.10.1      Sound Propagation Factors. 2-43

   2.10.2      Absorption. 2-45

   2.10.3      Reflections and Reverberation. 2-46

   2.10.4      Diffraction. 2-50

   2.10.5      Other Propagation Factors. 2-51

2.11     Distance from the Source: Near Field vs. Far Field. 2-52

2.12     Helmholtz Resonance. 2-53

3       Microphone Basics. 3-56

3.1        Microphones. 3-56

   3.1.1         Sound Capturing. 3-56

   3.1.2         Microphone Types. 3-59

3.2        Structure of a Capacitive Microphone. 3-63

   3.2.1         Sound Port 3-65

   3.2.2         Front Volume. 3-66

   3.2.3         Membrane. 3-67

   3.2.4         Pressure Vent Hole. 3-69

   3.2.5         Membrane – Back-Plate Gap. 3-70

   3.2.6         Back-Plate. 3-71

   3.2.7         Back Volume. 3-72

3.3        Acoustic Resistances. 3-73

3.4        Directionality. 3-75

4       Key Value Indicator Introduction. 4-76

4.1        What Are Key Value Indicators? 4-76

4.2        Brief Key Value Indicator Descriptions. 4-78

5       Key Value Indicator Details. 5-85

5.1        Sensitivity. 5-85

   5.1.1         Definition of Sensitivity. 5-85

   5.1.2         Definition of dBFS. 5-88

   5.1.3         Sensitivity vs. Dynamic Range. 5-89

   5.1.4         Sensitivity Standardization. 5-90

   5.1.5         Sensitivity Matching. 5-91

   5.1.6         Sensitivity Tolerance. 5-92

   5.1.7         Sensitivity Variables. 5-94

   5.1.8         Sensitivity Measurement vs. Reflow. 5-95

5.2        Polarity. 5-95

5.3        Directivity / Directionality. 5-96

5.4        Frequency Response. 5-99

   5.4.1         Frequency Response Goals. 5-101

   5.4.2         Frequency Response Factors in a Microphone System. 5-104

   5.4.3         Frequency Response Matching. 5-107

   5.4.4         Ultrasonic Frequency Response. 5-108

   5.4.5         Transient Response. 5-109

5.5        Phase. 5-110

5.6        Phase Response. 5-112

5.7        Group Delay. 5-116

5.8        Interlude 1: Phase vs. System Performance. 5-117

   5.8.1         Phase Matching and Accuracy. 5-117

   5.8.2         Comb Filtering. 5-119

   5.8.3         Source Localization. 5-122

   5.8.4         Active Noise Cancellation. 5-123

   5.8.5         Echo Cancellation. 5-125

   5.8.6         Barge-in. 5-128

   5.8.7         Sound Frequency and Wavelength vs. Phase and Level Detection. 5-131

5.9        Interlude 2: Noise vs. Signal 5-134

   5.9.1         Acoustical Signal / Noise Contributors. 5-135

   5.9.2         Electrical Signal / Noise Contributors. 5-137

   5.9.3         Noise vs. Speech Intelligibility and Sound Quality. 5-142

5.10     Self-noise, Equivalent Input Noise and SNR. 5-147

   5.10.1      Self-noise. 5-147

   5.10.2      Equivalent Input Noise (EIN) 5-148

   5.10.3      Signal to Noise Ratio (SNR) 5-152

5.11     Interlude 3: Distortion Types. 5-153

   5.11.1      Distortion. 5-153

   5.11.2      Harmonic Distortion. 5-154

   5.11.3      Frequency Response Distortion. 5-155

   5.11.4      Phase Distortion. 5-155

   5.11.5      Amplitude Distortion. 5-155

   5.11.6      Compression. 5-155

   5.11.7      Analog to Digital Conversion Distortion. 5-157

   5.11.8      Analog vs. Digital Clipping. 5-157

   5.11.9      Is Noise Distortion?. 5-158

   5.11.10         Distortion Sources. 5-158

5.12     Total Harmonic Distortion. 5-161

   5.12.1      Definition of THD. 5-161

   5.12.2      Distortion Requirements. 5-162

5.13     Total Harmonic Distortion + Noise. 5-165

5.14     Acoustic Overload Point 5-165

5.15     Dynamic Range. 5-168

   5.15.1      Definition of Dynamic Range. 5-168

   5.15.2      Improvement of Dynamic Range. 5-169

   5.15.3      Operating Voltage. 5-170

5.16     Current Consumption. 5-171

   5.16.1      Importance of Current Consumption. 5-171

   5.16.2      Operating Modes. 5-173

5.17     Power Supply Rejection (Ratio) 5-174

   5.17.1      Power Supply Rejection and Electrical Robustness. 5-175

   5.17.2      Power Supply Disturbance Rejection Performance Factors. 5-177

5.18     RF Immunity. 5-178

   5.18.1      Importance of RF Immunity. 5-178

   5.18.2      RF Immunity Factors. 5-180

5.19     ESD Immunity. 5-182

5.20     Light Sensitivity. 5-185

5.21     Microphone Stability, Matching and Reliability. 5-186

   5.21.1      Stability. 5-186

   5.21.2      Matching. 5-187

   5.21.3      Reliability and Robustness. 5-190

6       Electroacoustic Testing. 6-192

6.1        Sensitivity Measurements. 6-192

6.2        Polarity Measurements. 6-194

6.3        Directivity Measurements. 6-195

6.4        Frequency Response Measurements. 6-196

   6.4.1         Free Field Frequency Response. 6-196

   6.4.2         Diffuse Field Frequency Response and Sensitivity. 6-200

6.5        Phase Shift and Phase Response Measurements. 6-201

6.6        Group Delay Measurements. 6-204

6.7        Self-noise Measurements. 6-205

6.8        Equivalent Input Noise Measurements. 6-206

6.9        Signal to Noise Ratio Measurements. 6-206

6.10     Total Harmonic Distortion Measurements. 6-207

6.11     Total Harmonic Distortion + Noise Measurements. 6-208

6.12     Acoustic Overload Point Measurements. 6-209

6.13     Dynamic Range Measurements. 6-209

6.14     Current Consumption Measurements. 6-209

6.15     Power Supply Rejection (Ratio) Measurements. 6-210

6.16     Short Circuit Current and Output Load Capacitance. 6-211

6.17     RF Tests. 6-211

   6.17.1      Direct Injection Test 6-214

   6.17.2      Near-field Radiation Test 6-214

   6.17.3      TDMA Test (3GPP Idle Channel Noise Sending Test) 6-214

   6.17.4      Audio Breakthrough Test 6-215

6.18     Light Sensitivity Tests. 6-215

6.19     Mobile Phone Type Approval Tests. 6-216

6.20     Production Testing. 6-217

7       Electroacoustic Test Systems. 7-219

7.1        Electroacoustic Test Setup. 7-219

7.2        Electroacoustic Test Signals (Sensitivity, SNR, THD) 7-222

7.3        Requirements for Electroacoustic Test Systems. 7-223

   7.3.1         Mitigation of Test System Noise. 7-223

   7.3.2         Noise Measurement vs. Test System Noise. 7-226

7.4        Electroacoustic Test Environment 7-227

7.5        The Human Factor. 7-228

8       MEMS Microphones. 8-229

8.1        Package. 8-229

   8.1.1         Sound Port 8-231

   8.1.2         Building Blocks of the Package. 8-232

   8.1.3         Package Acoustics – Front Volume and Back Volume. 8-233

   8.1.4         Package Types: Bottom Port and Top Port 8-235

   8.1.5         Sealing Ring. 8-238

   8.1.6         Electrical Connections. 8-239

   8.1.7         Contact Pads. 8-242

   8.1.8         Other Package Specifications. 8-243

   8.1.9         Faraday Cage. 8-245

8.2        MEMS Sensor. 8-247

   8.2.1         Structure and Functionality. 8-247

   8.2.2         Properties and Performance of a Capacitive Sensor. 8-249

   8.2.3         Other Sensor Types. 8-259

8.3        ASIC. 8-260

8.4        Functionality of a MEMS Microphone. 8-263

8.5        MEMS Microphone Modeling and Simulations. 8-265

8.6        MEMS Microphone Manufacturing. 8-268

8.7        Cost and Price of a MEMS Microphone. 8-271

8.8        Size of a MEMS Microphone. 8-274

8.9        Materials and Environmental Regulations. 8-276

9       MEMS vs. ECMs and Normal-Scale Microphones. 9-277

9.1        MEMS vs. ECM. 9-277

9.2        MEMS Microphones vs. Normal-Scale Microphones. 9-281

   9.2.1         Membrane Properties vs. Capturing Accuracy. 9-281

   9.2.2         Membrane Size vs. Sensitivity. 9-284

   9.2.3         Membrane Size vs. Off-axis Capturing. 9-284

   9.2.4         Performance, Size, Cost and Versatility. 9-287

10    Microphone Needs of a Device Manufacturer 10-291

10.1     What Is Important for a Device Manufacturer?. 10-291

10.2     Risk Mitigation. 10-293

10.3     Sources of Microphone Requirements. 10-294

11    Implementation Goals. 11-297

11.1     Implementation Goals: Capturing Performance. 11-298

11.2     Implementation Goals: Reliability. 11-299

11.3     Implementation Goals: Low Cost 11-300

11.4     Implementation Goals: Small Size. 11-302

11.5     Implementation Goals: Stability and Accuracy. 11-303

11.6     Choosing the Right Microphone for an Application. 11-304

   11.6.1      Microphone Package Type. 11-305

   11.6.2      Microphone Size. 11-305

   11.6.3      Electrical Interface. 11-306

   11.6.4      Performance. 11-307

   11.6.5      Robustness. 11-311

   11.6.6      Current Consumption. 11-311

   11.6.7      Price. 11-312

12    Acoustical Implementation. 12-313

12.1     Acoustic Channel 12-313

12.2     Goals for Acoustical Implementation. 12-315

   12.2.1      Acoustic Design. 12-315

   12.2.2      Acoustic Stability. 12-316

12.3     Acoustical Implementation Guidelines. 12-317

   12.3.1      Sound Channel and Microphone Acoustics. 12-317

   12.3.2      Acoustical Factors in the Sound Channel 12-320

   12.3.3      External Acoustical Factors. 12-321

   12.3.4      Implementation for Ultrasound. 12-322

   12.3.5      Acoustic Simulations. 12-324

12.4     Acoustical Implementation Examples. 12-325

13    Mechanical Implementation. 13-327

13.1     Mechanical Implementation of MEMS Microphones. 13-327

13.2     Acoustic Sealing. 13-328

13.3     Acoustic Sealing Guidelines. 13-330

   13.3.1      Tolerance Chain Example. 13-335

   13.3.2      Challenging Sealing Cases. 13-337

   13.3.3      Testing of Sealing and Leaks in Device Production. 13-339

13.4     Mechanical Implementation to Prevent Echo. 13-340

13.5     Handling Noise. 13-342

13.6     Implementation Size. 13-345

   13.6.1      Implementation Size of a Bottom Port Microphone. 13-346

   13.6.2      Implementation Size of a Top Port Microphone. 13-347

13.7     Microphone Placement in a Device. 13-349

   13.7.1      Locations in Relation to the Surface of the Device. 13-349

   13.7.2      Locations in Relation to Corners, Edges and Flat Surfaces. 13-350

   13.7.3      Locations in a Microphone Array. 13-350

   13.7.4      Locations in Relation to Noise Sources within the Device. 13-353

   13.7.5      Locations in Relation to Heat Sources. 13-354

   13.7.6      Locations in Relation to Wanted Sound Sources. 13-355

   13.7.7      Locations in Relation to Ambient Noise Sources. 13-355

   13.7.8      Locations in Relation to Sources of Mechanical Stresses. 13-359

   13.7.9      Industrial Design / Device Appearance. 13-360

13.8     Protection of MEMS Microphones in a Device. 13-360

   13.8.1      Protection from Contamination. 13-360

   13.8.2      Protection from Abuse. 13-364

14    Electrical Implementation: Analog Microphones. 14-365

14.1     MEMS Microphone Interfaces. 14-365

14.2     Implementation Goals for Analog Microphones. 14-366

14.3     Analog Microphone Interfaces. 14-369

   14.3.1      Differential Interface. 14-369

   14.3.2      Single-ended Interface. 14-373

   14.3.3      Analog Interface Voltage vs. Sound Pressure Level 14-374

14.4     Analog Microphone Implementation Guidelines. 14-377

   14.4.1      General Guidelines. 14-377

   14.4.2      Output Signal Path. 14-379

15    Electrical Implementation: Digital Microphones. 15-382

15.1     Electrical Implementation of Digital Microphones. 15-382

15.2     Pros and Cons of Digital Microphones. 15-383

15.3     PDM Interface. 15-386

   15.3.1      Disadvantages of PDM 15-389

   15.3.2      Conversion from PDM to an Analog Signal 15-391

   15.3.3      Conversion from PDM to PCM (and Back) 15-391

   15.3.4      Other PDM MEMS Microphone Specifications. 15-392

15.4     Other Digital Interfaces. 15-394

   15.4.1      I2S / PCM. 15-394

   15.4.2      Other Digital Interfaces. 15-395

15.5     Digital Microphone Implementation Guidelines. 15-398

   15.5.1      General Guidelines. 15-398

   15.5.2      Signal Connections in a Digital Microphone System. 15-400

15.6     PDM Microphone Clock & Timing. 15-402

   15.6.1      Audio Bandwidth, Clock Frequency and Oversampling Ratio. 15-402

   15.6.2      Microphone Timing. 15-403

   15.6.3      Requirements for Host System Clock. 15-404

   15.6.4      Requirements for Timings of Microphones. 15-406

15.7     IO Voltage Levels. 15-408

15.8     Requirements for Digital Signal Paths. 15-410

15.9     Left/Right Selection. 15-412

15.10       Sleep Mode. 15-413

15.11       Microphone Powered Off 15-414

16    Electrical Implementation: EMC, RF, ESD. 16-415

16.1     Electromagnetic Compatibility (EMC) 16-415

   16.1.1      EMC. 16-415

   16.1.2      Consequences of EMC Problems. 16-417

16.2     Conducted and Radiated Disturbances. 16-419

   16.2.1      Conducted Disturbances. 16-419

   16.2.2      Radiated Disturbances. 16-420

16.3     Disturbance Mitigation Guidelines. 16-421

   16.3.1      Microphone Component 16-421

   16.3.2      Circuit Design (Schematic) 16-421

   16.3.3      Grounding. 16-422

   16.3.4      Layout and Component Placement 16-424

   16.3.5      Shielding and Faraday Cage. 16-427

   16.3.6      Disturbance Filtering. 16-429

   16.3.7      Power Supply Disturbance Mitigation. 16-433

16.4     Electrostatic Discharges (ESD) 16-433

   16.4.1      ESD. 16-433

   16.4.2      Improvement of ESD Immunity. 16-434

17    Directional Microphone Systems. 17-436

17.1     Directional Microphones. 17-436

   17.1.1      Benefits of Directional Microphones. 17-436

   17.1.2      Microphone Directionality (Directivity) 17-438

   17.1.3      Drawbacks of Directional MEMS Microphones. 17-440

17.2     Directional Microphone Arrays. 17-442

17.3     Endfire Arrays. 17-444

   17.3.1      Endfire Arrays – Pressure Gradient Microphones. 17-446

   17.3.2      Endfire Arrays – Combination Microphones. 17-452

   17.3.3      Delays for Cardioid Arrays. 17-457

   17.3.4      Proximity Effect 17-458

   17.3.5      Inverse Square Law. 17-462

17.4     Broadside Arrays. 17-464

17.5     Microphone Array Design Considerations. 17-467

   17.5.1      Distance between Microphones (+ Acoustic Variables) 17-467

   17.5.2      Array Variables. 17-468

17.6     Omnidirectional Microphones vs. Directional Capsules vs. Directional Arrays. 17-471

18    Wind vs. MEMS Microphones. 18-474

18.1     Wind Noise. 18-474

   18.1.1      Wind Noise Types. 18-476

   18.1.2      Wind Noise Testing. 18-477

   18.1.3      Wind Noise in Directional Microphones. 18-477

18.2     Wind Noise Reduction Techniques. 18-479

   18.2.1      Acoustical Wind Noise Reduction. 18-481

   18.2.2      Electrical Wind Noise Reduction. 18-485

   18.2.3      Other Wind Noise Reduction Possibilities. 18-488

19    Reliability Fundamentals. 19-490

19.1     Microphone Reliability. 19-490

19.2     Reliability Factors. 19-491

   19.2.1      The Microphone. 19-491

   19.2.2      The Device. 19-493

19.3     Microphone Reliability Problems in Devices. 19-495

   19.3.1      Importance of Well-working Microphones. 19-495

   19.3.2      Consequences and Cost of Reliability Problems. 19-496

   19.3.3      Device Reliability Risk Mitigation. 19-499

19.4     Causes for Problems: Troubleshooting. 19-500

   19.4.1      Causes for Quality and Reliability Problems in a Device. 19-501

   19.4.2      Causes for Quality and Reliability Problems in a Microphone. 19-503

20    Reliability Hazards. 20-507

20.1     Environmental Factors. 20-507

   20.1.1      General Environmental Factors. 20-507

   20.1.2      Manufacturing: Reflow and Reworking. 20-508

20.2     Contamination. 20-510

   20.2.1      Effects of Liquid Contamination. 20-512

   20.2.2      Effects of Solid Contamination. 20-513

   20.2.3      Consequences of Contamination. 20-514

20.3     Mechanical Abuse. 20-515

20.4     Pressure Shocks. 20-519

21    Improvement of Microphone Reliability. 21-521

21.1     Immunity Against Environmental Factors. 21-521

21.2     Contamination Immunity. 21-524

   21.2.1      Solid Contamination – Internal Particles. 21-525

   21.2.2      Solid Contamination – External Particles. 21-527

   21.2.3      Liquid Contamination and Fumes. 21-529

   21.2.4      Summary: Contamination Immunity Improvement 21-530

   21.2.5      Prevention of Contamination Ingress. 21-531

21.3     Mechanical Reliability. 21-532

21.4     Pressure Shocks. 21-535

22    Reliability in Device Production. 22-536

22.1     Reliability in Production. 22-536

   22.1.1      Mechanical Threats in Device Assembly. 22-537

   22.1.2      Pick & Place Process (and Tape & Reel Packaging) 22-538

   22.1.3      Vacuum. 22-540

   22.1.4      Contamination. 22-540

   22.1.5      Cleaning. 22-541

   22.1.6      Ultrasonic Operations. 22-542

   22.1.7      Excessive Temperatures. 22-542

   22.1.8      ESD. 22-542

22.2     Reflow Soldering. 22-543

   22.2.1      Reflow. 22-543

   22.2.2      Moisture Sensitivity Level 22-546

22.3     Sealing Ring and Solder Paste. 22-547

   22.3.1      Sealing Ring. 22-547

   22.3.2      Application of Solder Paste. 22-547

22.4     Reworking. 22-549

23    Reliability Testing and Verification. 23-552

23.1     Reliability Testing of MEMS Microphones. 23-552

23.2     Drop and Impact Tests. 23-553

   23.2.1      Guided Drop Test 23-555

   23.2.2      Tumble Test (Unguided Drop Test) 23-556

23.3     Pressure Shock Tests. 23-557

   23.3.1      Compressed Air and Vacuum Tests. 23-557

   23.3.2      Acoustic Sound Pressure Tests. 23-557

23.4     Vibration Tests. 23-558

23.5     Environmental Tests. 23-559

23.6     Contamination Tests. 23-562

23.7     ESD Tests. 23-563

23.8     Reflow Tests. 23-565

23.9     Latch-up Test & DC Safety Check. 23-566

   23.9.1      Latch-up Test 23-566

   23.9.2      DC Safety Check. 23-567

23.10       Maximum Ratings Tests. 23-567

23.11       Use Case Tests. 23-568

   23.11.1         Pressure and Sound Sensor Displacement Tests. 23-570

   23.11.2         Temperature and Humidity Tests. 23-570

   23.11.3         Deformation Tests. 23-572

23.12       Failure Analysis. 23-572

23.13       Reliability Simulation. 23-574

23.14       Transportation Packaging Testing. 23-575

23.15       Quality. 23-576

24    Appendix – Calculations: Decibel, Sensitivity, Input SPL, Output Voltage, Noise. 24-577

Definition of Decibel 24-577

Electrical Signal Level Conversions: Volt – dBV – Volt 24-577

Sound Pressure Conversions: Pascal – dBSPL – Pascal 24-578

Combination of Decibels. 24-578

Peak Value, Root Mean Square Value and Crest Factor. 24-578

Sensitivity of a Microphone. 24-579

Output Voltage at 94dBSPL. 24-580

Output Voltage at a Given Sound Pressure Level 24-581

Microphone Input SPL at a Given Output Voltage. 24-582

EIN at a Given Output Noise Level 24-583

Decibel Ratios. 24-584

EXTRA: Sound Pressure Levels vs. Output Voltages. 24-585

25    Updates and Notes. 25-587

26    References. 26-588

27    Index. 27-591