Measuring Digital MEMS Microphones

MEMS Microphone Substitution

This sequence demonstrates a method for testing a digital MEMS microphone frequency response with a source speaker that is not or cannot be equalized. The method involves playing a sine sweep through the source and simultaneously measuring it with a reference microphone and the microphone under test. Because the measurement microphone is assumed to have a relatively flat response, it is used to acquire the frequency response of the source speaker. By subtracting the response of the reference microphone from the DUT microphone the response and sensitivity of the device under test is found. Limits are set around both the frequency response and the sensitivity, the default values are for a typical MEMS microphone and should be adapted to your device.

The final display shows four graphs. The top left X-Y graph displays the data at its absolute level in dBFS. The lower left graph shows the same response curve but normalized to 0 dB at 1 kHz. This is a common way of displaying microphone frequency response and applying limits. The two graphs on the right display the raw response curves of the reference and the DUT microphones.

Required Software

• SoundCheck 18 Basic or higher

Required Hardware

• Portland Tool & Die DCC-1448(Listen p/n 5810) or PQC-3048(p/n 5811)
• Listen AmpConnect(Listen p/n 4042) or another audio interface
• If not using an AmpConnect, then an external power amplifier is needed
• Source Speaker
• Measurement microphone e.g Listen SCM3 (p/n 4002)

Setup Diagram

Hardware Setup and Calibration

1. Calibrate the measurement microphone as per the instructions in the SoundCheck manual.
2. Connect the MEMS microphone and digital MEMS interface per Digital Microphone Test Configuration.

Get Sequence

Download Sequence (zip file) (contains all 3 sequences)
Download Sequence documentation (PDF)

Measuring Digital Microphone PSR (Power Supply Rejection)

This sequence demonstrates a method for measuring a digital MEMS microphone’s power supply rejection performance (PSR). The PSR of a microphone reflects the ability of the device to reject or suppress external electrical interference. Such sources have historically been dominated by 50 and 60 Hz hum from electrical mains, but the 217 Hz GSM TDM pulse is now often of greater concern. This sequence measures PSR at 217 Hz but is easy to modify to test at any frequency.

The test is based on using a Portland Tool and Die DCC-1448 to provide a digital MEMS microphone with a DC supply with a calibrated AC signal, simulating electrical interference. SoundCheck then records the audio from the DUT, analyzes it with a spectrum analyzer and extracts the RMS energy at the specific frequency of the simulated electrical interference and returns the PSR value. The setting of frequency, waveform type and amplitude of the simulated electrical interference is controlled entirely from within SoundCheck.

The final display shows a graph of the spectrum of the device under test and the measured PSR value. A typical limit is applied to the PSR value and should be adapted to your DUT.

Required Software

• SoundCheck 18 Basic or higher

Required Hardware

• Portland Tool & Die DCC-4149 (Listen p/n 2018) note: PQC-3048 cannot be used for this application

Setup Diagram

Hardware Setup and Calibration

1. Connect the MEMS microphone and digital MEMS interface per Digital Microphone Test Configuration Notes.
2. Make sure that you have run the Windows Setup.exe for your device, instructions can be found in the manual for the
3. This test is entirely electrical. You may need to mechanically mute the microphone if PSR performance is exceptionally low or ambient acoustical noise is very high.

Get Sequence

Download Sequence (zip file) (contains all 3 sequences)
Download Sequence documentation (PDF)