By Anne-Françoise Pelé
In the context of an aging population and increasing environmental noise exposure, hearing health is more important than ever. People are looking for smaller, more power-efficient, and higher sound-quality hearing aids, and MEMS microphones are well-positioned to meet those expectations.
Over 5% of the world’s population — or 466 million people — have disabling hearing loss, and more than 900 million people — one in every 10 people — are expected to have disabling hearing loss by 2050, according to the World Health Organization. Disabling hearing loss refers to hearing loss higher than 40 decibels (dB) in the better hearing ear in adults and higher than 30 dB in the better hearing ear in children.
Unsurprisingly, the global hearing aid market is estimated to reach $7.62 billion in 2023, reported ResearchAndMarkets.com, with a compound annual growth rate of 6.4% from 2019 to 2023. This market comprises hearing devices, cochlear implants, bone conduction systems, and diagnostic instruments.
A hearing aid detects sounds in order to amplify them for the user, and this requires a microphone. Traditional electret condenser microphones have long remained the most used microphones in hearing aids, but micro-electromechanical systems (MEMS) are increasingly displacing them.
“MEMS microphones have several advantages that make them the most suitable solutions for end applications such as hearing aids,” Manuel Tagliavini, principal analyst, MEMS & sensors, IHS Markit, told Electronic Products .
MEMS microphones offer a highly reliable, robust design with low susceptibility to particle ingress; an ultra-compact footprint, offering market-leading sensitivity-to-size ratio; technological advancements bringing optimal electro-acoustics performance, as well as highly versatile and tunable designs with programmable integrated circuits,” said Michael Knapp, vice president, communications & investor relations, Knowles Corp.
“With the first generation of hearing-health MEMS microphones entering the market only five years ago, we expect MEMS microphones to account for over half of the microphones used in the hearing health market in 2020,” he added.
Even if it is not comparable to mobile handsets, smart speakers, and earphones, the market for MEMS microphones used in hearing aids is rapidly growing. According to the latest IHS Markit projections, it will rise from $8 million in 2017 to $48 million in 2022, with a CAGR of 42% over the forecast period.
MEMS technology is now mature enough to meet medical-grade requirements. “After the billions of units produced and shipped mainly for the consumer electronics and wireless markets, they have reached the reliability required for medical applications,” said Tagliavini.
“Moreover, MEMS microphone suppliers have gained over the last few years good expertise in improving product size, power consumption, and price for applications such as smartphones and earbuds. These requirements, especially size and power consumption, are shared by hearing aids,” he added.
The most recent developments seen in MEMS microphones, Tagliavini continued, are linked to voice-recognition functionality and key features in smart speakers, but they are quickly growing in other segments, too. “Improvements are not only on the sensor side, with microphone arrays that enable beamforming and far-field sensing, but also on the software side [algorithms]. It is possible nowadays to classify different kinds of sounds [human voice, background noise, etc.] and amplify or attenuate them in the most appropriate way.”
These enhancements, which initially targeted consumer electronics applications, are particularly suited for hearing aid devices.
Knowles, which started developing MEMS microphones in the late 1990s, now offers its third and fourth generations of hearing-health MEMS microphones with the ambition to meet customers’ ever-rising electro-acoustic expectations, which means the highest sensitivity and lowest noise possible in a low-power, compact space, said Knapp. “As hearing aid aesthetics grow in importance, customers desire smaller component footprints that enable industrial design creativity.”
With a 70.5-dB signal-to-noise ratio (SNR) and 121-dB sound-pressure level, Knowles claims its third-generation MM20 platform is “the lowest-noise, highest-SNR-size ratio in the market.” It consists of an acoustic sensor, a low-noise input buffer, and an output amplifier. These devices are suitable for applications where excellent wideband audio performance and RF immunity are required.
The MM25 platform, Knowles’ fourth generation of hearing-health MEMS microphones, uses a programmable ASIC to achieve high versatility and optimization of its lowest-noise platform to allow high-performance beamforming and microphone pairing, said Knapp. The MM25 can be programmed for ultra-low-power currents between 18 µA and 31 µA.
Knowles said continued advancements in MEMS technology will bring new capabilities for hearing-health microphones that will allow for expanded voltage/current ranges and new applications while continuing to improve electro acoustics for hearing aid manufacturers.
Knowles’s latest generation of hearing-health MEMS microphones continue to shrink in size. (Image: Knowles Corp.)
On the other side of the Atlantic, Sonion a/s designs micro-acoustic and micromechanical technologies for hearing instruments, in-ear monitors, earphones, and hearables. “Hearing aid MEMS microphones are differentiated from electret microphones by the fact that MEMS are less sensitive to environmental conditions, such as humidity and temperature, and the designs allow for it to be reflowed directly on a PCB rather than using wires and hand soldering,” said a Sonion spokesman.
The Denmark-based company provides the O-Series MEMS microphones, co-developed with TDK-InvenSense, in two small form factors: 8 mm3 and 11 mm3 (volume). Key specs include an average current of 31 µA, sensitivity from -37 dB 1 V/Pa to -38 dB 1 V/Pa, and noise levels from 26 dB to 28.5 dB.
The company claims the newest P-Series has “improved SNR and offers higher sensitivity and a very low power consumption.” Sensitivity ranges between -35.5 and -38 dB 1 V/Pa, noise levels from 24.5 dB to 25 dB, and battery drain from 31 µA to 32 µA.
The P-series differentiates itself with its insensitivity to external light sources, said Sonion’s spokesman. “With semiconductors, some doping is needed to make it work. This means that if you have a certain type of n-doping or p-doping, it can react to light. That’s how LEDs work but in the opposite way. We make MEMS in such a way that they don’t respond to light.”
The package volume, 8.2 mm3 , is “relatively small, but it is not a priority at the moment. If we go smaller, we increase the noise, and if we increase the noise, the product is not usable anymore,” he added.
Sonion said its soon-to-be-released Q-Series comes with a flat response curve and without distortion (intermodulation distortion, or IMD, is below 10%). “If you look at the response curve of the P-series, you see a peak in the response around 10 Hz,” said Sonion’s spokesman. “This peak is not ideal, because any sound that comes in that frequency will be amplified compared to what you normally have.”
To prevent this, “you put some resistance in the circuit to lower the response to that frequency, especially with a low-pass filter,” he continued. For the Q-Series, Sonion managed to have a damped response curve “without adding the noise you would have with a low-pass filter.”
The Q-Series also provides sensitivity in the range of -35.5 dB 1 V/Pa to -37.0 dB 1 V/Pa, while the footprint and thickness are similar to the P-series and O-series (3.35 x 2.50 mm).
In labs
Most commercially available hearing aids use microphones to capture the external sound field. But because these microphones are usually located in an external element, they can cause discomfort, limit sports activities, or create a social stigma. There is a pressing need for implantable hearing devices that are easy to implant and have a high sensitivity. The University of Zürich and the Cochlear Technology Centre Belgium announced last year they were working on a MEMS condenser microphone-based acoustic receiver for totally implantable cochlear implants (TICI).
ams AG is working on ways to treat tinnitus, developing a new therapy using environmental noise. (Image: ams AG)
The implantable microphone, according to a paper published by the Acoustical Society of America, measures the pressure fluctuations in the cochlea (inner ear), which are induced by the outer and middle ear chain, a so-called intracochlear acoustic receiver (ICAR). An ICAR benefits from the amplification and directionality cues of the ear anatomy.
Also, on the research front, ams AG is working on new ways to treat tinnitus. Tinnitus is the perception of a persistent ringing, humming, or buzzing sound, often linked to hearing loss caused by damage to the inner ear, such as from normal aging or exposure to loud noise. It affects about 15% to 20% of the population.
The Austria-based company said it has developed a tinnitus therapy using environmental noise. Basically, it uses ams hardware for active noise cancelling and processes environmental noise into a personalized tinnitus training signal.
“This therapy has proven to be very efficient, and the more frequently tinnitus patients use it, the lower their tinnitus becomes. This therapy has a positive long-term effect,” said Verena Vescoli, senior vice president of R&D, ams AG, at the recent MEMS & Image Sensors Summit in Grenoble.
MEMS microphones are increasingly replacing traditional electret condenser microphones in hearing aids thanks to several advantages. These include reliable and robust designs with low susceptibility to particle ingress, small package size, and high performance, with advances continuing in the labs.
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