REDCO AUDIO Measuring MEMS Microspeakers Instructions
- June 13, 2024
- REDCO AUDIO
Table of Contents
Practical Test & Measurement
Measuring MEMS
Microspeakers
Measuring MEMS Microspeakers
MEMS microspeakers are now commercially available and seeing fast evolution
and adoption in true wireless stereo (TWS) earbuds and in-ear monitors (IEM).
With the emergence of these small form-factor transducers comes the need to
address how to measure its performance and ensure product quality. By Steve
Temme (Listen Inc.) and Michael Ricci (xMEMS)
MEMS microspeakers are driving one of the fastest technological transitions in
the miniature speaker market. MEMS transducers first gained widespread
attention in the late 2000s with the first commercially available MEMS
microphone, and over the course of a decade, they replaced electret
microphones in the majority of mobile devices. The MEMS speaker revolution is
shaping up for an even faster transition. Early speakers incorporating MEMS
technology were launched in 2014, but it was not until 2021 that the first
fully piezoelectric MEMS microspeakers became commercially available.
Manufacturers are already rapidlyadopting these into TWS and pro-audio IEMs,
earning praise from consumers and product reviewers alike, and propelling this
technology into the mainstream. MEMS Speaker Overview
MEMS microspeakers are constructed in an entirely different way from
conventional miniature speakers. Rather than using inductive coils and
magnets, they rely on a voltage-driven capacitive actuator, and they are
entirely manufactured using a monolithic solid-state fabrication process in a
semiconductor wafer foundry. This fabrication process delivers much tighter
speaker-to-speaker consistency than is possible with a conventional driver
made up of an assembly of many moving parts. They offer full-range
performance, eliminating crossover networks that introduce complexity and
sometimes even phase differences. This is particularly important considering
this year’s other big audio trend—spatial audio, where the phase response is
critical to accurate reproduction. Piezo-MEMS speakers are also compact in
size and low power, making them economical on both space and power
consumption.
Finally, they are surface-mount technology (SMT) reflowable, offering yield
and quality advantages in device manufacturing. While these advantages are
leading to rapid adoption in devices such as hearing aids, earbuds,
professional in-ear monitors, smart glasses, and more, they are not without
their challenges. The maximum sound pressure level (SPL) in the low
frequencies is typically less than is required for a leak-tolerant active
noise cancelling (ANC) system compared with conventional headphone drivers,
limiting their use for ANC earbuds to a mid/high frequency tweeter in a two-
way configuration.
However, in recent months, manufacturers have made rapid progress in
overcoming these limitations, thus broadening their applications. For example,
xMEMS’ new Cypress Piezo-MEMS speaker, showcased at the recent Consumer
Electronics Show (CES), delivers up to 140dB SPL in the low-frequency
band—more than sufficient for ANC applications. The cost of MEMS-driven
products is also decreasing as this technology is more widely adopted. While
early MEMS earbuds were priced well over $1000, recently launched models such
as the Creative Labs Aurvana bring this technology’s high-quality sound to TWS
products below a $200 price point. As this technology moves toward becoming
the de facto standard, the outlook for this market is positive.
The Importance of Measurements
Any time a new technology is introduced, measurements are critical. Designers
rely heavily on models to predict performance before building devices. While
such models are well established and based on decades of data for conventional
drivers, models for MEMS speakers are still in their infancy. It is therefore
critical to validate and iterate models with actual measurements for them to
be valuable design tools. Measurement is also critical for both MEMS device
manufacturers and those companies using their products to thoroughly evaluate
performance and ensure product quality.
Measuring Finished MEMS Headphones
Finished products such as earbuds are simple to measure. A MEMS headphone can
be tested in exactly the same way as any other headphone using an audio test
system with a head-and-torso simulator (HATS) or ear couplers, and a Bluetooth
interface if needed. Figure 1 shows a typical headphone test configuration
that could be used for testing a pair of wired MEMS earbuds. The test method
is simple.
The test signal is played through the headphones and captured using a HATS or
ear couplers, where it is sent to the computer for detailed analysis of
frequency response, phase, distortion, and more. Measuring MEMS Drivers
Raw MEMS drivers are more complicated to measure due to their small form-
factor and power requirements. That said, the basic measurement principles are
the same as any other driver; the complexity is in the fixturing. In addition
to an audio test system (interface plus software), a typical configuration
requires an ear simulator coupler to capture the signal, custom fixturing to
hold the MEMS driver in place, and a special driver circuit for the MEMS
device. Piezo-MEMS devices have a very high input impedance and draw very low
current. Typically, MEMS speaker manufacturers provide their own driver
circuit to deliver voltage bias and boost converter to step up the voltage,
and this should be used for making measurements.
Figure 2 shows a schematic diagram of the test configuration used to measure a
MEMS speaker, and Photo 1 shows the test jig, coupler, and charge amplifier in
more detail.
To measure the Piezo-MEMS speaker, an xMEMS Montara Plus (Photo 2), the xMEMS
“Aptos” Class H charge amplifier supplies the required bias voltage and up to
24dB of gain to power the device. A 3D printed device test jig is connected to
a 711 ear simulator coupler to simulate an in-ear measurement. An AudioConnect
2 audio interface powers the coupler and is connected to SoundCheck for
analysis. Aside from the charge amplifier and custom fixture, the test setup
is very similar to that used for a conventional speaker.
The test itself is a standard speaker test, with levels and frequency range
set appropriately for the device. It plays the stimulus (e.g., a stepped sine
wave) and analyzes the response, displaying frequency response, impedance, and
distortion. MEMS Measurement Results
The frequency response, shown in Figure 3, is very flat up to 1kHz. In the
higher frequencies, there are a couple of resonances—one due to the
Practical Test & Measurement
piezoelectric material and the other due to the resonance of the coupler,
which simulates the human ear. MEMS speakers exhibit fewer resonances than
conventional speakers due to the simplicity of their design. They have fewer
moving parts than a conventional speaker and consequently fewer break-up
modes. Once the device is measured and the resonances known, they can be
accounted for in the product design to minimize their effect. The higher
frequency resonances in the MEMS speakers are actually easier to design around
as they are out of the most sensitive range of the human ear. Perhaps future
development might result in these resonances being moved out of the audible
range altogether. Naturally this information also provides useful data points
for refining MEMS speaker models, which will, in turn, help design better
speakers. Figure 4 shows the impedance of the MEMS speaker. You can see here
that it’s a very different shape from a conventional loudspeaker impedance
(Figure 5). Although the values are much higher, it’s very linear, which makes
it easy to compensate for when designing products. It’s advantageous to use
frequency-normalized distortion measurements rather than conventional
distortion measurements when measuring the total harmonic distortion (THD) of
MEMS speakers or any other loudspeaker, as the difference can be quite
significant. Normalized distortion measurements compare the harmonic levels to
the fundamental level at their measured frequency before their ratio is
plotted, rather than the fundamental level at the excitation frequency. This
provides more accurate results as it removes the effect of the non-flat
frequency response from the THD.
This makes it easier to see the peaks in the THD response independent of the
peaks and dips in the fundamental response, and easier to set limits.
Figure 6 shows harmonics plotted at their measured frequency (not at their
excitation frequency) along with the fundamental for the Montara Plus piezo-
MEMS speaker. You can see that the fundamental bumps at the high frequencies
are superimposed on the harmonics.
In Figure 7, the orange line shows the THD calculated at the conventional
stimulus frequencies (THD) and the blue line shows the normalized THD,
calculated at the measured stimulus frequencies. Removing the effect of the
non-flat frequency response from the distortion makes it easier to see the
peaks in the distortion response independently of the peaks and dips in the
fundamental response. It shows that the THD is very low up to 10kHz. As with
the frequency response, an understanding of the high-frequency distortion
enables it to be minimized with good product design. It is clear from the
normalized measurement that there is a high Q at resonance between 8kHz and
9kHz, but apart from that there is very little distortion, so design efforts
can be focused on planning around that one peak. If a design was based on the
conventional distortion measurements shown by the orange curve, time would be
wasted trying to solve the distortion between 4kHz and 5kHz on the graph that
does not really exist.
Conclusions
MEMS speaker technology is rapidly evolving to meet today’s mobile audio
requirements. The simplicity of construction results in features such as flat
frequency response, linear impedance, and predictable distortion that
facilitates their design into consumerelectronics products.
Accurate audio measurements of both raw drivers and finished goods are
essential to develop and refine models, which are still in their infancy
compared to more established driver technologies. While finished goods
containing MEMS devices can be measured
in the same way as their conventional counterparts, raw MEMS drivers require
custom fixturing and an appropriate amplifier.
With these test components, they can be measured in the same way as a
conventional driver.
The distortion in a MEMS speaker is typically highly focused around one
frequency, and normalized THD, often called frequency normalized distortion,
is highly recommended over conventional or non-normalized distortion
measurements to accurately evaluate this distortion.
About the Authors Steve Temme is is founder and President of Listen,
Inc., manufacturer of the SoundCheck audio test system. Steve founded the
company in 1995, and over the past 29 years the company has remained on the
cutting edge of research in audio measurement, introducing many new
measurement algorithms and techniques. In recent years, Listen’s product
development focus has been on creating the algorithms, interfaces and test
methods to measure today’s newest devices including MEMS speakers and
microphones, as well as automotive audio and various wireless and
cloudcommunicating devices. Prior to founding Listen, Steve worked for many
years as an acoustic test and measurement applications engineer at Brüel &
Kjær, and also as a loudspeaker design engineer at Apogee Acoustics.
He holds a BSME from Tufts University, has authored numerous papers on
acoustic testing, and has lectured extensively throughout the world.
Michael Ricci is the Senior Director of Electroacoustic Engineering at xMEMS
Labs, in Santa Clara, CA, where he built and maintains multiple test labs to
support Piezo-MEMS µSpeaker integrations for consumer hearables and
professional audio products. His technical background includes 25 years of
semiconductor device testing with a focuson MEMs transducers. Michael has held
Senior FAE and Principal Electroacoustic Engineering positions with Bosch
Sensortec (Akustica MEMs Mics) and Knowles Electronics supporting strategic
accounts with MEMs microphone and balanced armature integrations. Industry
affiliations include Acoustical Society TAG voting member, the Audio
Engineering Society (AES), the National Association of Music Merchants (NAMM),
and the American Society of Composers, Authors, and Publishers (ASCAP).
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