Walter C. Babel III SAIC Qamar A. ShamsNASA Langley Research Center James F. BockmanNASA Langley Research Center Qualitative Analysis of MEMS Microphones 16th ANNUAL 2004 INTERNATIONAL MILITARY & AEROSPACE / AVIONICS COTS CONFERENCE, EXHIBITION & SEMINARS
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
MEMS Capacitive Microphone Design Acoustical Wave Clamped Diaphragm Backplate Airgap 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 Signal OUT Charge Pump Detection Circuit Microphone DiaphragmPower and Detection
SP0102 General Outline Signal OUT Charge Pump Detection Circuit Microphone DiaphragmPower and Detection
SP0103 General Outline Signal OUT Charge Pump Detection Circuit 20 dB Amplifier Microphone DiaphragmPower and Detection
Basic Functional Analysis (Clamped and Free floating diaphragm) The model of clamped and free floating movable plate capacitor is shown by: F E can be calculated by differentiating the stored energy of the capacitor w.r.t. the position of the movable plate: where F is the electrostatic attraction force caused by supply voltage V. The mechanical elastic force F M can be expressed as: where K is a spring constant and is assumed to be linear.
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 / SP0103NC3 Frequency Response Testing MEMS Microphone High-Pass Filter (10Hz) Buffer n x Amplifier Hardware Software FFT Voltmeter Channel Select Hard Disk
Anechoic Chamber Test Setup
Amplitude (V) Frequency (Hz) MEMS Microphone Comparison Hz
MEMS Microphone Comparison 100Hz – 25kHz Amplitude (V) Frequency (Hz)
MEMS Microphone Comparison 100Hz – 10kHz Frequency (Hz) Amplitude (% of 1kHz Value)
MEMS Microphone Comparison 100Hz – 25kHz Amplitude (% of 1kHz Value) Frequency (Hz)
MEMS Test Layout MEMS Array Test Layout Anechoic Chamber MEMS Array Speaker Amplifier LabVIEW Hardware
MEMS Array Close-Up Audio Source Numbering Convention MEMS Array
MEMS Array Frequency Data 100 – Hz
MEMS Array Frequency Data 100 – Hz
MEMS Array Frequency Data 100 – Hz
MEMS Array Frequency Data 100 – Hz
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 Problem
MEMS Microphone Resonance Data
Normalized to 1000Hz
MEMS Microphone Resonance Reduction Filter
MEMS Microphone Resonance Reduction Filter
Linear Testing Used to determine location of sound source Directionality Testing Overview Rotational Testing Used to determine “omnidirectionality” of microphone ? ?
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 Motor Control Board LabVIEW Hardware 12V/1A Power Supply Computer Rotational Directionality Testing
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 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 in Acoustic Isolation Vessel PC Monitor Scan Frequency B&K ½” Mic (B&K 4134)
Environmental Testing Overview 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 Amplifier o’scope Co-60 Cobalt-60 gamma source VV
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.