Designer’s guide: picking the right MEMS microphone for voice-control applications

Article By : Richard Quinnell, Editor-in-Chief

Interest is growing in voice control for electronic products, and a good design has to start with a good microphone

Interest is growing in voice control for electronic products, and a good design has to start with a good microphone

By Richard Quinnell, Editor-in-Chief

For decades, researchers have
been trying to give computers the ability to understand human speech, and it’s
finally ready for prime time. With the advent of personal voice assistants like
Apple’s Siri, the Amazon Echo, and Google Home, voice control of electronic systems
is now becoming a must-have feature. Starting with the right microphone is the
key to achieving optimal performance in such designs.

The technology of choice for
voice-controlled devices is the microelectromechanical systems (MEMS) microphone.
These have essentially replaced older electret condenser microphones for many
reasons. For one thing, MEMS microphones are small — as little as 2.5 x 1.6 x 0.9
mm. More importantly, however, they can offer stable performance that does not
drift over time and can have better phase matching to allow accurate
beamforming.

A MEMS microphone works by
capacitance. The micromachining of silicon produces an acoustic chamber with a
flexible plate as one wall, and pressure waves (sound) can move that plate. The
changing capacitance between the plate and the rest of the chamber creates an
electrical signal that represents the sound. Two types of MEMS microphones
exist. Analog microphones, like their larger counterparts, simply provide the
sensor signal — perhaps conditioned or filtered, but essentially unaltered. Analog
microphones thus require use of an external ADC if they are to be used in
machine voice recognition. Digital microphones include the ADC and other digital
elements onboard to convert the sensor signal into a digital data stream,
typically pulse-density-modulated.

MEMS-Fig-1-MEMS-structure

Fig.
1: A MEMS microphone is essentially a variable capacitor made by micromachining
a movable silicon plate in an acoustic chamber. (Source: STMicroelectronics)

MEMS microphone technology first
saw adoption in mobile devices and laptop computers, and these applications are
still driving the market. Most devices have multiple microphones positioned in
places such as near the bottom for phone voice pickup and adjacent to camera
lenses for video sound pickup. These microphones, however, are intended to
capture sound for human listening, which gives designers considerable freedom
in applying data compression and other filter algorithms. For machine
listening, however, audio processing needs are different.

Before evaluating a MEMS
microphone for voice control, developers first need to decide how the system is
to be used. A battery-powered, handheld device such as a remote control, for
instance, will probably always be held near the speaker’s mouth. The design may
thus require only one microphone, and acoustic parameters such as
signal-to-noise ratio (SNR) may not be as big a consideration as power
consumption. But in a device such as the Amazon Echo, which must reliably
understand voices coming from meters away in a noisy environment, key
parameters would be both the SNL (which determines the softest sound reliably
sensed) and the acoustic overload point (AOP), which is the loudest sound
pressure level (SPL) the sensor can handle without saturating.

A second upfront consideration
is where and how the microphone is to be mounted. MEMS microphones come in two
orientations. There is the top port, with the sound inlet aperture pointed away
from the mounting surface, and the bottom port. With the bottom port, the PCB to which
the microphone gets mounted must have a thru-hole that aligns to the aperture
in order for the sound to enter. While that may seem a more complicated
approach, bottom-port microphones currently dominate the market. Mobile device
designers have been mounting the microphone on flexible circuits that are then
attached to the device’s housing, and in these designs, the bottom port design
greatly simplifies assembly.

MEMS-Fig-2-IM69D130_LLGA-5-1_Combi

Fig. 2: A bottom-port microphone
has its acoustic aperture on the same side as its PCB mounting pads.

Also, in considering the mounting, vendors warn that the industrial design’s acoustic properties need attention.
Even the best microphone will deliver poor performance if the housing’s sound
aperture and the enclosure’s resonant chamber are not appropriate. Depending on
application, there may also need to be a gasket or other barrier to prevent
dust or water from entering the microphone’s sound aperture.

Fortunately, MEMS microphone
vendors can help. Vendors like Infineon, for instance, partner with acoustic
specialists to offer reference designs that customers can leverage. Others,
like STMicroelectronics, offer acoustic simulation services to help validate
customer designs based on the customer’s 3D drawings.

With these basic considerations
out of the way, designers can then evaluate the performance characteristics of
individual microphones to make a final selection. The key parameters for a
representative selection of available MEMS microphones are listed in a
spreadsheet, downloadable by registered EP readers. Some of these
specifications include:

  • Port sensitivity — the output signal magnitude
    given a reference 94-dB -SPL acoustic signal at the port.
    This helps indicate the softest sounds that the microphone will pick up.
  • SNR — the difference (in dB) between the
    microphone’s noise floor and the signal produced by a 94-dB, 1-kHz sound wave.
  • Dynamic range — the spread of sound intensity
    levels that the microphone can reliably capture without distortion. It is
    essentially the difference (in dB) between the microphone’s noise floor and its
    AOP.
  • Frequency range — the audio frequency range to
    which the microphone can respond without loss of sensitivity.
MEMS-Fig-3-Guide

Fig. 3: Click on the “Download Guides” button at the bottom of this article to download this
MEMS Microphone Selection Guide.

Some vendors have also indicated
special features of their microphone offerings that developers can consider.
One of the more intriguing is a low-power mode that allows a microphone to
operate in a less sensitive but always-on state. Operating in this state allows
the design to conserve power while remaining active in order to detect a “wake-up”
word, such as “Alexa.” Once the wake-up word is detected, the system can then
switch to full-sensitivity (and full-power) operation. This mode can be
especially valuable for battery-operated designs because it helps extend
battery life.

MEMS microphones are only the
first element of voice-activated system designs, however. They must often be
followed by signal processing that combines signals from multiple microphones
for noise reduction. Noise-reduction techniques include beamforming (multiple
microphones needed), which focuses the system’s acoustic response in the
direction of the speaker, and “barge-in” operation, which cancels out the sound that the device is itself generating so the speaker does not need to shout over it
to be heard.

This article only scratches the
surface of MEMS microphone operation and evaluation, of course. Fortunately,
there are many vendor tutorials and other guidelines online for further study.
These include:

You can also learn about some of
the voice signal processing options and their development kits by following the
article links below.

Related articles:
Audio
Pre-processing System Reference Design for Voice-based Applications Using C6747
How
to build your own Amazon Echo — or something like it
AcuEdge
dev kit simplifies Amazon Alexa Voice Service integration
XMOS
and Qualcomm launch voice-processing dev kits for cloud-based speech
recognition
Cirrus
joins chorus of Alexa pre-processing chip providers

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