Measuring MEMS Microphones & Mic Arrays

The rapid adoption of “smart devices”-with smartphones and smart speakers leading the way—has made MEMS microphones ubiquitous to everyday life. The heart of any smart device is the Intelligent Virtual Assistant (IVA), enabling the use of voice commands to direct the device to do everything from playing audio content to providing a weather forecast and these voice commands are acquired via the integrated MEMS microphone or, more likely, an array of such microphones (mics).

MEMS (micro-electromechanical system) microphones are extremely small microphones that are used in a wide range of applications and, most commonly, use a PDM (pulse density modulation) stream as their digital output. MEMS mics, in addition to their role in smart devices, are integral components in a wide range of products and systems, including hearing aids, mobile computing, automotive systems, and IOT systems.

While the use of multiple mics in a smart device can provide distinct benefits to the end product—for example, using multiple mic inputs for improved SNR (signal-to-noise ratio)—it also complicates the testing process used to evaluate and characterize designs owing to the need for simultaneous, multichannel measurements. Additionally, engineers developing systems using beamforming mic arrays, and other DSP-based multichannel audio acquisition applications, depend on a true representation of phase alignment of the input channels to validate their algorithms and designs.

Within this section of AP.com are a variety of resources related to the measurement of MEMS microphones and mic arrays. Considering the intersection of these devices with smart speakers, and other smart devices, there is likewise a distinct amount of overlap the test complexities and measurement approaches recommended to address such challenges.

Another aspect of testing these devices, as indicated by brief mention of interchannel phase measurement above, will be a discussion of the importance of timing information, specifically the requirements for bit clock operation, decimation rates and master/slave clock functionality.

As with most other acoustic test scenarios, the conduct mic and/or mic array measurement requires several pieces of equipment, including a generator/analyzer for stimulus and acquisition; power supplies for the DUTs; and an amplifier for the speaker driver emitting the acoustic stimulus signal. An anechoic test chamber, reference measurement mic (or mics) and a sound level calibrator are also needed.

  • Making the Measurements
Understanding PDM Digital Audio

PDM stands for pulse density modulation. However, it is really better summarized as “oversampled 1-bit audio”, as it is nothing more than a high sampling rate, single-bit digital system. Learn about PDM digital audio in this article

Login to AP.com and download Understanding PDM Digital Audio.

PDM 16 Input Module

Acoustic test is challenging, so trying to measure the acoustic performance of a MEMS mic array can be especially daunting with the added requirement of validating phase alignment. The PDM 16 module, designed for the APx Series modular audio analyzers, provides the ability to incorporate all of the APx analyzer capability with your MEMS testing. This module provides 16 acquisition channels and can provide simultaneous analysis of all 16 channels on a single screen.

Download the datasheet.

Technote 140: Testing Microphone Arrays with the APx PDM 16 Module

In this technote, we discuss MEMS microphone array measurements and use a test board with 16 PDM mics configured in a line array to illustrate the typical measurements.

The technote specifically details the test procedure, and important considerations, for measuring key performance factors of the microphone array using a PDM 16-equipped APx525 audio analyzer.

Login to AP.com and download TechNote 140.



Understanding PDM Digital Audio

PDM stands for pulse density modulation. However, it is really better summarized as “oversampled 1-bit audio”, as it is nothing more than a high sampling rate, single-bit digital system. Learn about PDM digital audio in this article

Login to AP.com and download Understanding PDM Digital Audio.



PDM 16 Input Module

Acoustic test is challenging, so trying to measure the acoustic performance of a MEMS mic array can be especially daunting with the added requirement of validating phase alignment. The PDM 16 module, designed for the APx Series modular audio analyzers, provides the ability to incorporate all of the APx analyzer capability with your MEMS testing. This module provides 16 acquisition channels and can provide simultaneous analysis of all 16 channels on a single screen.

Download the datasheet.

Making the Measurements

Technote 140: Testing Microphone Arrays with the APx PDM 16 Module

In this technote, we discuss MEMS microphone array measurements and use a test board with 16 PDM mics configured in a line array to illustrate the typical measurements.

The technote specifically details the test procedure, and important considerations, for measuring key performance factors of the microphone array using a PDM 16-equipped APx525 audio analyzer.

Login to AP.com and download TechNote 140.