The consumer electronics industry at-large has been able to digitize and shrink most of the device components and electronics. One of the last remaining barriers is the speaker — it remains comparatively heavy, bulky, and restrictive. This month, we explore new technology microspeakers and their challenges to transform the consumer electronics industry as microelectromechanical systems (MEMS) did to electret condenser microphones (ECMs). MEMS is the technology of very small devices, usually consisting of a micro-sensor/transducer and an application-specific integrated circuit (ASIC) that processes data or in the case of MEMS speakers amplification and codec functions.
Three examples of MEMS speaker chips. The Audiopixels speaker chip, USound MEMS speaker and the new Fraunhofer MEMS speaker chip.
Back in January 2015, I gave a talk on MEMS loudspeakers at the Association of Loudspeaker Manufacturing & Acoustics (ALMA) International’s Symposium and Expo (AISE) and followed that discussion with an article in the pages of Voice Coil. Five years later, and the time is now ripe for an update survey of MEMS audio technology — what is just now reaching the market and what is coming. This month’s focus centers on microspeakers and earphones, with MEMS microphones coming up next.
We will examine MEMS devices in general, provide a basic explanation of how the various types of MEMS speakers work, and what commercialization challenges are ahead. As the first MEMS speakers are now becoming viable commercial products, we also need to consider their practical applications, unit costs, and acoustical strengths and weaknesses.
So now it is reality check time — Could MEMS loudspeakers signal the end of speakers as we know them? As MEMS speakers are just starting to evolve into viable commercial products, what might be the impact on the speaker industry, practical applications, projected unit costs, and acoustical strengths and weaknesses? For decades, MEMS microphones were all show and no go. Yet progressively over the last few years, they have come to dominate the mobile audio device market. On the other hand, ATCO’s hypersonic array speaker was also supposed to take over the industry — but that was more of a stock market exercise — and today, it is only a boutique application.
NXT’s distributed mode loudspeakers (DML) also supposedly signaled the “end of the world for existing speaker technology,” but even 20 years later this flat-panel topology’s current proponents are still fighting for market share. The Tymphany linear array transducers (LATs) of a decade ago showed another different path to sound reproduction. Currently, Tymphany is successfully producing conventional but well-executed speaker designs and no longer offers LATs.
With these past “not-quite game changers” still fresh in our memories, how might MEMS speakers fit into the speaker industry? It is clear that speakers are going to be a tough application for MEMS technology as speakers need piston area and excursion to move air. The verdict is MEMS speakers might take some time to come to fruition.
This is a cross-section of an electrostatic IEM tweeter developed by Sonion, which designs and manufactures cutting-edge audio components and provides complete solutions to its customers who then manufacture hearing aids, in-ear earphones and hearables/wearables.
A Bit of MEMS History MEMS development over the last three decades has been slow and painful. The semiconductor industry’s favorite joke regarding MEMS development roadmaps are that they are calculated in dog years (seven times that of human years). However, MEMS devices became practical when they could be manufactured with high yields using integrated circuit (IC) fabrication and device packaging processes.
MEMS devices include microphones, accelerometers, vibration/shock sensors (e.g., burglar alarms and airbag sensors), gyros and now microspeakers and earphone transducers. The implementation of MEMS speakers is daunting compared to mics due to the far higher excursion requirements. Yet even the promise of MEMS microphones was slow to be achieved, with many development teams in the 1990s eventually giving up. Venture capital investments in MEMS mic startups rarely reached successful outcomes as the investors just did not have the staying power to keep pouring funds into research and process control.
There are quite a few steps in MEMS fabrication and getting high yields on every step always seemed to be another development phase away. It took more than 20 years for the first billion MEMS microphones, and two years for the second billion’s production, compared with monthly production now reaching about 1 billion monthly. Today, MEMS microphones totally dominate smartphones, tablets, laptops, portable media players, speech recognition systems, personal computers, surveillance cameras, 3D cameras, radars, anti-theft alarms, headphones, smart speakers, music recorders, and various smart home voice command appliances, including air conditioners, refrigerators, and service robots.
Back to the MEMS Microspeaker The microspeaker and earphone driver market is about $10 billion annually. Just considering the work needed to shift production lines, even automated speaker production line manufacturing, over to semiconductor foundries is mind boggling. The titans of microspeaker manufacturing typically have about 50,000 employees, while MEMS foundries producing similar quantities of devices have staffs of less than 500. Yes, the wafers from the foundry will still need to be “packaged” but a few zeros in workforce numbers are still lobed off…) With the rising cost of salaries in China, MEMS microspeakers will have a dramatic impact on staffing along with other far-reaching implications. But it is not just the fabrication of the transducers, but the promise of automated pick-and-place of MEMS speakers for surface-mount technology (SMT) board stuffing rather than hand soldering of billions of speakers. Let’s ponder practical applications, projected unit costs, and acoustical strengths and weaknesses.
While MEMS microphones have taken the lion’s share of the microphone market, why are the microspeaker transducers almost 100% electro-dynamic (magnetic structure with a voice coil)? The 800 lb. gorilla blocking MEMS speakers is “pumping power.” While the micro-mechanism in MEMS mics only need to have enough movement to respond to the acoustic signal, MEMS speakers need to move air. But even MEMS mics have so little excursion capability that the acoustic overload point (AOP) is a serious consideration in spec’ing MEMS mics.
Some MEMS mics will latch up (the diaphragm will stick to the plates) if what they are mounted into is dropped or even if a car door is slammed. With conventional speakers, acoustic physics for sound output is the Xmax (excursion) times the piston area. The typical smartphone speaker diaphragm footprint is 10 mm × 15 mm and has about 0.5 mm Xmax peak excursion. The air moving power of MEMS speakers is significantly less than even the lowest performance microspeakers.
In every case of the unique transducers surveyed here, output is minuscule and outside of the application to in-ear monitors (IEMs) or hearing aids, they must be used in multiples. USound describes MEMS speakers as the “LED of the acoustics,” and the size and configuration of the array would be application-specific. Multiple speakers means multiple cost. Many of transducers here are made from wafers, which are sliced and diced and then packaged into complete speakers, much like MEMS mics. Wafer costs could be $500 to more than $1,000 depending on the process and diameter of the wafer. Using advanced math, if you need two (or four or half a dozen) MEMS devices for your application, the costs of both the the wafer and the packaging starts to add up quickly.
Speaker engineers following conventional wisdom means that for achieving the required sound levels and bass response, you need to have a large enough diaphragm moving far enough. Some of the new contenders point out their sound production technology does not follow the conventional physics of moving diaphragm transducers. Just a caveat here, perhaps the rules are different but they may come with a new set of problems.
The “Holy Grail” for these alternative MEMS speaker technologies is to become the next smartphone microspeaker. There are more than 1.6 billion mobile phones produced each year, each with at least two microspeakers — a receiver and speakerphone transducer. Less “pumping power”is required for headphones than speakerphone applications, still less for earphones (and hearing aids) and even less for earphone “tweeters.”
MEMS speakers promise to be ideal receivers for in-canal hearing aids and implantable hearing devices (i.e., cochlear and auditory brainstem implants). These applications have very small “air pumping volume” required for adequate acoustic output due to the enclosed duct and close proximity to the middle ear. A more ambitious step are in-canal IEM earphones, which require not much more acoustic output than implantable transducers. Between these two applications, IEM tweeter transducers are another application (many IEM earphones are two-way or more designs using balanced armature drivers).
The first MEMS speakers have already reached the market. There are a handful of MEMS solutions that replace conventional voice coil actuator with MEMS mechanisms while others are not precisely MEMS, but all are relevant for earphone and microspeaker applications. Each of these designs has significant development and manufacturing barriers to mass acceptance and productization.
Piezo Speakers One of the promising technologies is piezo speakers which already have a long history in the speaker industry. Motorola’s ceramic horn tweeters were used in "prosumer" speakers by the millions for decades. Piezo microspeakers have very low profiles, which are highly desirable for smartphones, and there has been a half dozen short-lived piezo microspeakers. The challenge has always been the limited excursion along with lack of adequate bottom-end or even lower midrange output. The redeeming aspect of piezo transducers is that while excursion of the ceramic element is limited, this can be somewhat addressed with larger and thinner ceramics, but also as the force of the ceramic element is high, enabling a cantilever to increase excursion. Now for our survey of these next-generation devices including an overview of their technology and development status.
In 2019, Austrian start-up USound brought their first MEMS microspeaker to market, enabling the company to target opportunities in wearables, headsets and embedded speakers.
USound USound is a fabless audio semiconductor company offering piezo silicon speakers based on MEMS technology. USound was able to overcome the limitation of traditional piezo transducers, and with its innovative MEMS concept have proven that they can generate relatively large displacements. USound has developed and shipped several hundred thousand of what it believes are the smallest and first MEMS loudspeakers in the world.
USound’s Co-Founder & CTO, Andrea Rusconi pointed out that their major selling points, confirmed by customers, are form factor and weight along with reflow solder compatibility. Reflow soldering was the one major motivation for the breakthrough of MEMS microphones in consumer electronics. USound’s solution with its MEMS processes (including microelectronics-grade packaging) works better for speaker manufacturing but also at product level because reflow soldering of the speaker enables audio modules manufactured in SMD lines with integrated electronics (i.e., connectivity, sensors etc.). Another major advantage of USound’s MEMS loudspeakers is their flexibility, with different versions for in-ear and also speaker applications.
USound microspeakers are currently offered for smartphones, earbuds, audio modules for augmented reality and virtual reality glasses, and numerous consumer wearables, as well as 3D surround sound headphones. Together with production partners STMicroelectronics, Flex and AT&S, USound has implemented a global semiconductor supply chain.
TDK TDK, best known for its sensors and electronics components, based its PiezoListen microspeakers on a haptic device. Twelve layers of piezoelectric material are stacked so that displacement and maximum sound level is increased with response down to 200 Hz. Intended for consumer products such as tablet computers and TVs. The speaker comes in two types: a "wide-range" type and "high-range" type. The wide-range type bandwidth is 400 Hz to 20 kHz. Though it does quite reach the low-end response of conventional speakers, it is adequate for tablets and laptops.
Ultrasonic heterodyne sound generators has had their proponents and commercial audio designs from ATCO, Holosonics (Spot Light) and others, but these have been shoebox to ceiling tile size implementations. Work continues on MEMS implementations of ultrasonic heterodyne, ultrasonic shutter modulation, and digital sound reconstruction for microspeakers. Questions remain on achieving signal reproduction integrity, issues with the high levels of ultrasonics generated to achieve adequate audio levels, and attaining usable low-end frequency response.
Audio Pixels is one of the pioneers in the development and production of MEMS digital speaker chip technology. Its silicon chip can be used either as a stand-alone speaker or cascaded in any multiples of the same chip to achieve required performance specifications.
Audio Pixels Audio Pixels is one of the pioneers in the development and production of MEMS digital speaker chip technology. The company is directly generating sound from a digital audio stream. Audio Pixels holds innovative patents in the fields of electromechanical structures, pressure generation, acoustic wave generation, and control, signal processing, and packaging. Its silicon chip can be used either as a stand-alone speaker or cascaded in any multiples of the same chip to achieve required performance specifications.
This modular paradigm is comparable to “parametric speakers” such as phased arrays or using more transducers for increasing the dynamic range. Audio Pixels’ Digital Sound Reconstruction (DSR) technique is based on a theory introduced by Bell Labs in the 1930s. Originally a secure “digital” speech vocoder for military communications with a “digital speaker” to reconstruct the speech. The sound wave is generated from the summation of discrete pulses that are produced from an array of pressure generating micro-transducers. Within each transducer is an array of identical elements fine-tuned to a particular frequency. As with analog speakers, different frequencies are produced by varying the timing of the motion. Proof-of-concept continues to progress. Audio Pixels is in partnership with Sony as one of its MEMS foundry partners and ICsense for the ASIC design.
GraphAudio GraphAudio licensed the graphene audio work and patents from The Lawrence Berkeley National Labs in 2016 for development of commercialized audio products. GraphAudio has developed an electrostatic driver where the pure graphene diaphragm functions as part of the “motor.” Its initial products are earphones using a graphene diaphragm sandwiched between electrodes. When this field oscillates due to the audio signal, it causes the graphene to vibrate in a physical analogy to the audio electrical signal and this generates sound. It’s essentially an electrostatic speaker; but instead of a metalized polymer film diaphragm, graphene is used. Also in development is a studio microphone and super wideband measurement mic.
GraphAudio has developed an electrostatic driver where the pure graphene diaphragm functions as part of the “motor.” Here is an exploded view of an 8 mm speaker assembly.
Graphene diaphragms are very thin and light with a small spring constant so that the air itself damps its motion. The symmetrical push-pull electrostatic drive has been the core technology of the finest audiophile headphones and speakers and studio microphones. The ability to power graphene earphones and speakers using conventional mobile battery power expands their application from just the boutique end of the market. Batteries for the DC bias, work for graphene since they source only voltage and virtually no current. Since the power is tiny, there is no need for high current and small batteries suffice. Demonstration earphones have been produced and demonstrated with audiophile quality results.
Fraunhofer Hedging its bets, Fraunhofer, the German research institute is developing both piezo and capacitive (electrostatic) all-silicon MEMS-speakers. Its CMOS-compatible MEMS speaker is based on electrostatic bending actuators. Future work will focus on increased SPL and reduced distortion through optimized actuator design. Concurrently, development continues on a piezoelectric MEMS with concentrically cascaded lead zirconate titanate actuators making it the first integrated two-way MEMS speaker.
Fraunhofer, the German research institute is developing both piezo and capacitive (electrostatic) all-silicon MEMS-speakers. Its CMOS-compatible MEMS speaker is based on electrostatic bending actuators.
Designed to operate without a closed membrane to improve the acoustic performance, energy efficiency, and manufacturability. Extensive finite element analysis studies revealed an SPL of more than 79 dB in 10 cm distance at 500 Hz for a device 1 cm² in size operated at 30 V. At higher frequencies larger SPL values are calculated enabling a flat frequency response with 89 dB for frequencies above 800 Hz. Based on this concept, first speaker prototypes have been fabricated.
Sonion Sonion’s electrostatic IEM tweeter (electret) is designed for a smoother, more clean sound in the higher frequencies than traditional balanced armature IEM’s using standard tweeters. The Sonion electrostatic super-tweeter produces high frequencies from 7 kHz and upward. The driver comprises a specifically arranged dual electret cartridge that lowers symmetric distortion combined with a miniature transformer. This enables electrostatic performance in IEMs without the usual separate power supply for stepping up the voltage and supplying bias to the driver. The result is stunning audio quality with crystal clear undistorted sound that goes well beyond the limits of human hearing. The dimensions of their electrostatic tweeter are 3.55 mm × 3.55 mm × 2.54 mm (32 mm³), a single version is also available and measures 3.55 mm x 3.55 mm x 1.27 mm (16 mm³).
xMEMS Labs xMEMS Labs, a California MEMS startup has developed a MEMS speaker initially for earphone applications. Promising transducers of small size and low power consumption, with scalable design enabling the application’s SPL requirement defining the number and arrangement of “speaker cells.” Specifically a handful of cells may be sufficient for earbuds, but smartphones may require more. xMEMS claims ability to reach a range of frequencies as low as 20 Hz at least half the size of a conventional dynamic microspeaker. While xMEMs has not yet revealed specifics on its MEMS speaker technology, it is developing a complete MEMS process that reduce the manufacturing complexities which integrate the membrane and actuator making it uniquely capable for high-volume MEMS manufacturing. If you are curious, check out their patent on an “Air Pulse Generating Element and Sound Producing Device.”
xMEMS Labs, a California MEMS startup has developed a MEMS speaker initially for earphone applications.
This MEMS speaker survey is just the tip of the iceberg as I know of other initiatives that are still in the stealth mode. But as with MEMS microphone development, many of these efforts will dead-end, at least until the technology infrastructure catches up. VC
This article was originally published in Voice Coil, December 2019. Original Title: The Coming Impact of MEMS Audio in 2020. The article was edited from the original version.
ABOUT MIKE KLASCO
Mike Klasco is the president of Menlo Scientific, an audio consulting firm for the loudspeaker industry, located in Richmond, CA. He is a graduate from New York University, with post graduate work in signal processing, and he holds multiple patents licensed or assigned. For the past 35 years, since founding Menlo Scientific, he has worked on product development, design and technical support, liaison between suppliers and manufacturing facilities, sourcing guidance, and certification for a large number of companies. Mike specializes in materials and fabrication techniques to enhance audio performance, has over 500 articles published in audio magazines and technical journals, and to this day continues to be a regular contributor for audioXpress and Voice Coil magazines. He presented papers at the Acoustical Society of America, held positions as Session Organizer for the Audio Engineering Society, Chairman of the Committee on Acoustics, and is an AES Life Member.
Back in January 2015, I gave a talk about MEMS loudspeakers at the ALMA International (now named Audio and Loudspeaker Technologies International or ALTI) Expo, and I have regularly written about MEMS (microelectromechanical systems) speaker progress. A half decade later, it seems we are finally now moving beyond the industry's joke that MEMS’ development progress can be measured in dog years - it takes 7 human years for each year of MEMS’ progress!
MEMS’ microphone development over the first decade was painful. However, MEMS mics became practical when they could be manufactured with high yields using integrated circuit (IC) fabrication and device packaging processes. Today, MEMS devices include microphones, accelerometers, vibration/shock sensors (e.g., burglar alarms and airbag sensors), gyros and now µSpeakers and earphone transducers. And different types of MEMS microspeakers are now entering production.
It took more than 20 years for the first billion MEMS microphones, and two years for the second billion’s production, compared with monthly production now reaching about 1 billion units. Today, MEMS microphones totally dominate smartphones, tablets, laptops, portable media players, speech recognition systems, personal computers, surveillance cameras, 3D cameras, radars, anti-theft alarms, headphones, smart speakers, music recorders, and various smart home voice command appliances, including air conditioners, refrigerators, and service robots. Compared to microphones, the implementation of Piezo-MEMS in microspeakers is daunting, due to the far higher excursion requirements.
xMEMS has partnered with TSMC for mass production of its Montara monolithic piezo-MEMS microspeaker and has now confirmed the availability of Cowell, a second-generation ultra-small form factor full-range driver, measuring just 3.2mm x 1.15mm.
The big news is xMEMS Labs, a company founded in January 2018 and headquartered in Santa Clara, CA (click here for more insights), has announced it is now in mass production of its Montara, a monolithic MEMS “µspeaker” with a slim 1mm profile, already on its way to market. xMEMS now has 36 granted patents and more than 100 patents pending for its technology.
As audioXpress reported recently, xMEMS’ Taiwan fabrication partner, TSMC, is the world's largest semiconductor foundry (and partner for Apple's new SoC and SiP processors, found in AirPods, iPhones, iPads and MacBooks). Montara’s initial 1st tier design win is with Inventec Appliances Corp. (IAC), which selected it for the company’s flagship Chiline TR-X TWS Earbuds, powered by xMEMS Aptos, a Piezo-MEMS audio driver IC.
The µSpeaker and earphone driver market is valued at about $10 billion annually. Just considering the work needed to shift production lines, even automated speaker production line manufacturing, over to semiconductor foundries, the impact could be mind boggling. The titans of µSpeaker manufacturing typically have about 50,000 employees, while MEMS foundries producing similar quantities of devices have staff of less than 500. Yes, the wafers from the foundry will still need to be “packaged” but a few zeros in workforce numbers would be still lobed off. With the rising cost of salaries in China, MEMS microspeakers will have a dramatic impact on staffing along with other far-reaching implications.
But it is not just the fabrication of the transducers, but the promise of automated pick-and-place of Piezo-MEMS µSpeaker for surface-mount technology (SMT) board stuffing - rather than hand soldering of billions of speakers - that is bound to create the most immediate impact.
xMEMS introduced Cowell, the world’s smallest monolithic MEMS µspeaker, targeting the replacement of balanced armature drivers in true wireless earbuds and hearing aid designs.
There are quite a few steps in MEMS fabrication and getting high yields at every step always seemed to be another development phase away. Earlier-generation MEMS earphones and speakers were based on a two-step fabrication process where the “motor” was fabricated in a MEMS process, then sent for final assembly of the diaphragm/suspension into the package. The most recent generation of Piezo-MEMS earphone transducers are completely fabricated in the MEMS process resulting in a high yield, very tight uniformity, and the environmental robustness ideal for true wireless stereo (TWS) applications.
In full-range occluded (in-ear) applications, these transducers achieve smoother and more extended full-range response compared to balanced armatures. In two-way or higher designs with a dynamic “woofer” driver higher sensitivity is also achieved. Future smartphones will have ultra-wide band receivers and eventually MEMS for the hands-free speakerphone.
xMEMS’ Montara is the world’s first monolithic Piezo-MEMS µSpeaker. Monolithic, meaning the implementation of both actuation and diaphragm in silicon, resulting in superior part-to-part consistency in frequency response and phase. Montara’s fast mechanical response results in the industry’s lowest group delay and phase shift and can be seen in the smooth ultra-wideband response to beyond 70kHz. For manufacturers, these characteristics reduce calibration and speaker matching. The 1mm profile, SMT-ready package and IP58 rating for dust/water simplify system design, integration, and assembly.
All this makes piezo-MEMS microspeakers ideal receivers for over-the-counter (OTC) in-canal hearing aids and implantable hearing devices (cochlear and auditory brainstem implants). These applications have small “air pumping volume” required for adequate acoustic output due to the enclosed duct and close middle ear proximity.
The side-firing Montara xMEMS microspeaker and the smaller side-firing Cowell in side by side comparisons. In the second "dime comparison" we can see Cowell side by side with an equivalent balanced armature receiver.
And There Is More... This week, xMEMS announced Cowell, the world’s smallest monolithic MEMS µSpeaker. At just 1.15mm high and 3.2mm wide, the Cowell side-firing package only weighs 56 milligrams and delivers an impressive 110dB SPL at 1kHz. Cowell provides up to 15dB of gain above 1kHz for improved speech-in-noise performance and greater vocal and instrumental clarity versus electrodynamic and balanced armature µSpeakers. This is also the first Piezo-MEMS to use xMEMS’ second generation M2 speaker cell architecture, offering improved SPL/mm2, enabling increased loudness in smaller form factors. And Cowell engineering samples are available now, with mass production in early Q2 2022.
Cowell’s attributes will enable designers of TWS hearables unique benefits in high-res audio and immersive sound, as well as OTC hearing aids. For TWS applications, Cowell can be implemented as a full-range driver in occluded earbud architectures or as a small, high-performance tweeter paired with an electrodynamic woofer driver in non-occluded or leaky two-way solutions.
For hearing aid applications, Cowell is a full-range driver that is 45% smaller than an equivalent balanced armature receiver, making receiver-in-canal applications possible. The driver's superior high-frequency response, lack of in-band resonance peaks, and 15dB of gain above 1kHz make it ideal for addressing high-frequency hearing loss and improving speech intelligibility in noise. Lastly, Cowell’s speaker diaphragm is vented (front-to-back), enabling the relief of air pressure that may build up over time inside the ear canal, resulting in reduced fatigue with increased long-term wear and listening comfort.
The side-firing package and 1mm-thin profile provides for placement flexibility in the earbud making space for larger batteries and additional sensor components. And the SMT-ready package boasts an IP58 rating for dust/water, simplifying system design, integration, and assembly.
xMEMS has samples and evaluation kits ready for companies working on new designs. For more information, visit www.xmems.com
This article was originally published in The Audio Voice newsletter (#351), November 2021.
ABOUT MIKE KLASCO
Mike Klasco is the president of Menlo Scientific, an audio consulting firm for the loudspeaker industry, located in Richmond, CA. He is a graduate from New York University, with post graduate work in signal processing, and he holds multiple patents licensed or assigned. For the past 35 years, since founding Menlo Scientific, he has worked on product development, design and technical support, liaison between suppliers and manufacturing facilities, sourcing guidance, and certification for a large number of companies. Mike specializes in materials and fabrication techniques to enhance audio performance, has over 500 articles published in audio magazines and technical journals, and to this day continues to be a regular contributor for audioXpress and Voice Coil magazines. He presented papers at the Acoustical Society of America, held positions as Session Organizer for the Audio Engineering Society, Chairman of the Committee on Acoustics, and is an AES Life Member.
SonicEdge MEMS Ultrasound Speaker-In-Chip: Next Gen MEMS for TWS Earbuds and Hearing Aids
February 24 2023, 00:00
As Monty Python once said, “And Now for Something Completely Different.” Ultrasonic MEMS Speakers!
Let’s start with the general speaker population. We characterize speakers by how they move air, more particularly their various types of motors and diaphragms.
The moving force can be electro-dynamic - the usual voice coil in a magnetic structure driving a cone or dome - or a planar film diaphragm with laminated conductor pattern in a magnetic field. Oskar Heil’s air motion transformer (AMT) features an accordion-pleated diaphragm using the squeezing of the pleats to move air, but otherwise similar to tensioned diaphragm magnetic planars. One recent reincarnation of the AMT, (sort of, on a microscale) could be the Arioso MEMS speaker, which uses electrostatic actuators integrated inside 20µm wide bending strips, which are energized by vibrations from the audio signal voltage. Arioso Systems was recently acquired by Bosch.
Another more esoteric approach are electrostatic planars, which use a metalized diaphragm between grids. Microelectromechanical systems (MEMS) speakers are a new crop, mostly in that their acoustic production can be capacitive or piezo-electric as the driving force for the diaphragm. Admittedly I have given all these the short shift and each has enough variants to fill a book.
Now we move on to another universe of transducers that do not quite follow the usual rules of acoustic production. These are the modulated and parametric speakers that have been proposed and are now moving through development.
Perhaps you might remember back to 2009 to Woody Norris and his Hypersonic Sound Speaker (HSS). These were a class of ultrasound transducers that generated audible sound from modulated ultrasound. The modulated ultrasound passes through a nonlinear medium that acts as a demodulator. Projecting a narrow beam of modulated ultrasound substantially changes the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly and extracts the modulation signal from the ultrasound, resulting in sound that can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes.
Click to see the video with Woody Norris’ presentation of the Hypersonic Sound Speaker
Basically, the heterodyne speaker uses ultrasonic modulators of two beams with the difference creating the audio. This technology allows a beam of sound to be heard only in a small well-defined area; for a listener outside the beam the sound pressure decreases substantially.
And Now... A Unique Ultrasonic Shutter MEMS-Based Speaker System Percolating in stealth mode for a decade is the work of Moti Margalit, CEO, and Ari Mizrachi (COO) of SonicEdge, today leading a growing team, including some in their Danish subsidiary. The core of the design is one or more apertures driven by the ultrasonic carrier signal and one or more movable and over-sized obstruction elements that are configured to modulate the ultrasonic carrier signal (think of a shutter) thereby generating audio.
The ultrasound propagates through a time varying acoustic channel, which is an acoustic modulator. A large channel enables the ultrasound to pass, and a small channel attenuates the ultrasound. The modulator shifts shifting the ultrasound frequency creates a constant volume velocity source. A standard speaker in earphone applications has a constant sound level for lower frequencies and drops short at higher frequencies (black line). A volume velocity source provides a constant volume velocity of air regardless of frequency and in earphone applications has increasing SPL for lower frequencies and extended SPL at high frequencies (green line).
As can be seen from the graphs, this technique overcomes the challenge of large excursion and piston size in MEMS speakers by using a radically new approach based on active modulation of ultrasound. The SonicEdge speaker is composed of membranes generating modulated ultrasound and acousto-mechanical demodulators that frequency shift the ultrasound to audio.
The MEMS speaker is based on existing process modules for ultrasound transducers, such as Capacitive Micromachined Ultrasound Transducers (CMUT) or Piezoelectric Micromachined Ultrasonic Transducers (PMUTs).
The resulting game-changing speaker is a constant volume velocity source, generating-resonance free audio and ideally suited for non-occluded earphones and headsets with ANC functionality.
The company's SE1000 solution includes the MEMS speaker chip that generates sound combined with an ASIC that drives the transducer.
Back in 2018 I first came across this technology in my consulting work and some of the many patentsthat belong to SonicEdge. Since then, work has continued and SonicEdge was formed to complete development of the concept. Around this time last year SonicEdge emerged from “stealth mode” and first appeared in the MEMS microspeaker directory published in Voice Coil May 2022. Currently, SonicEdge is conducting POC projects with an evaluation board for strategic customers, with commercial availability of its first MEMS speaker (SE1000), anticipated for later in 2023 (specifications below).
This ultrasound nano-speaker and frequency transformers are realized as a solid-state MEMS device manufactured monolithically leveraging the semiconductor eco system for scalability and price trajectory. Coming from the semiconductor fab as a final device in minimal size, which includes a MEMS chip that generates sound and an ASIC amplifier chip that drives the MEMS chip, SonicEdge has developed a unique technology that generates ultrasound from multiple acoustic pixels, each as wide as a strand of hair. The back cavity is 10x reduced in size compared to state-of-the-art 11mm drivers and therefore supports new design paradigms for earphones. The lack of a magnetic structure also relieves the TWS pain-point of providing enough separation between the antenna and the transducer.
There are advantages in size and sound quality, no mechanical vibrations, potentially low power consumption, and more SPL than what some of the earlier MEMS speaker efforts. The minuscule vibration produced with this sound product technique enables a lot more acoustic echo cancelation (AEC) and feedback margin in applications such as TWS and hearing aids, along with full-duplex speaker-as-mic functionality, together with some other sensing capabilities.
Having taken a good listen at CES this year, the device sounded excellent, as did some of the other recent MEMS earphones. SonicEdge’s product roadmap also includes bass reflex acoustic design supporting full-range free-field speakers for AR/VR and mobile device applications.
SonicEdge developed a unique technology that generates ultrasound from multiple microtransducers, and then transforms the ultrasound to human-audible sound using a patented acousto-mechanical frequency transformer or acoustic modulator, contained on a solid-state MEMS device.
This article was originally published in The Audio Voice newsletter, (#411), February 23, 2023.
ABOUT MIKE KLASCO
Mike Klasco is the president of Menlo Scientific, an audio consulting firm for the loudspeaker industry, located in Richmond, CA. He is a graduate from New York University, with post graduate work in signal processing, and he holds multiple patents licensed or assigned. For the past 35 years, since founding Menlo Scientific, he has worked on product development, design and technical support, liaison between suppliers and manufacturing facilities, sourcing guidance, and certification for a large number of companies. Mike specializes in materials and fabrication techniques to enhance audio performance, has over 500 articles published in audio magazines and technical journals, and to this day continues to be a regular contributor for audioXpress and Voice Coil magazines. He presented papers at the Acoustical Society of America, held positions as Session Organizer for the Audio Engineering Society, Chairman of the Committee on Acoustics, and is an AES Life Member.
At The Association of Loudspeaker Manufacturing & Acoustics (ALMA) International in January 2015, I gave a talk on microelectromechanical systems (MEMS) loudspeakers, and quite a few inquiries followed. As a follow up to last month’s focus on microspeakers, we thought it might be time for an article on MEMS microspeakers. This is really new technology, so new that only recently were functional lab prototypes operational.
Photo 1: This diagram illustrates how a MEMS microphone chip operates. (Image courtesy of STMicroelectronics)
MEMS is the technology of very small devices, usually consisting of a micro-sensor/transducer and an application-specific integrated circuit (ASIC) that processes data (e.g., codec functions and amplification). We will examine MEMS devices in general, provide a basic explanation of how a MEMS speaker works, and what commercialization challenges are ahead. When MEMS speakers are viable commercial products, we will also need to consider their practical applications, projected unit costs, and acoustical strengths and weaknesses.
A Bit of MEMS History MEMS development over the last three decades has been slow and painful. The semiconductor industry’s favorite joke regarding MEMS development schedules are that they are calculated in dog years (seven times that of human years). However, MEMS devices became practical when they could be manufactured using integrated circuit (IC) fabrication and device packaging processes (see Photo 1). MEMS mainstream consumer electronics devices include accelerometers, vibration sensors (e.g., burglar alarms and air bag sensors), and microphones.
For decades, the promise of MEMS microphones was slow to be achieved, with many development teams eventually giving up. Venture capital investments in MEMS companies rarely reached successful outcomes as the companies just don’t have the patience or staying power to keep pouring funds into research. There are quite a few steps in MEMS fabrication and getting high yields on every step always seemed to be another development phase away (see Photo 2). It took more than 20 years for the first billion MEMS microphones, and two years for the second billion’s production. Today, MEMS microphones are in most smartphones, tablets, and laptops.
MEMS accelerometer prototypes were successfully developed at Analog Devices, Inc. (ADI) in 1989, which has processes similar to MEMS microphones. By the mid-1990s, ADI successfully commercialized air-bag sensor accelerometers. Knowles also began development of its MEMS microphone program around the same time. In 2003, Knowles commercially launched its SiSonic MEMS microphones. Other early MEMS microphone developers included Sonion and Akustica (now part of Bosch).
MEMS first-generation microphones were noisy and could just reach 50 dBa signal/noise (S/N). But with continual improvement, the sensitivity increased and the noise floor dropped. By 2010, a few vendors reached 60 S/N, and now the best reach 70 S/N! While microelectrical engineers measure MEMS microphone success by performance specifications, perhaps a more real-world criteria is that MEMS microphones represent more than half of all microphones produced last year. Invensense, Knowles, STMicroelectronics, Akustica, AAC, Goertek, and many others have MEMS microphones in mass production. Many commodity MEMS microphone vendors are really MEMS assemblers buying the MEMS “microphone” element and the ASIC (preamp and sometimes codec), and putting this together in an IC package.
Photo 2: Akustica’s single-chip MEMS microphone establishes an industry first in small size, low cost, and high performance. It is show compared to the point of a pen (a) and in actual view (b).
Audio MEMS Beyond Microphones Expanding MEMS development from microphones to sound generators is an epic challenge as both the radiating area and the excursion are limited. Yet, MEMS devices have strong next-step development as an ideal receiver for in-canal hearing aids and implantable hearing devices (e.g., cochlear and auditory brainstem implants). These applications have very small “air pumping volume” needed for adequate acoustic output due to their close proximity to the middle ear. An even more ambitious step are in-canal earphones (in-ear monitors), which require not much more acoustic output than implantable transducers. While the industry at large has been able to digitize and shrink all other device electronics, the last remaining barrier is the speaker, which remains large, heavy, bulky, and very analog.
An actual commercial MEMS speaker is not yet ready, but I wrote this article while I was in Israel in mid-February. At the time, Audio Pixels had succeeded in reaching the third phase of a four-part commercialization program for the MEMS loudspeaker, including functional prototypes. Audio Pixels can now focus on advancing aspects related to mass production and commercialization of MEMS loudspeakers.
Figure 1: This diagram shows Audio Pixels’ digital sound reconstruction (DSR) methodology.
Reality Check Could MEMS loudspeakers signal the end of speakers as we know them? When MEMS speakers are viable commercial products, what might be the impact on the speaker industry, practical applications, projected unit costs, and acoustical strengths and weaknesses?
For decades, MEMS microphones were all show and no go. Yet in the last few years, they have come to dominate the mobile audio device market. On the other hand, ATCO’s hypersonic array speaker was also going to take over—but that was more of a stock market exercise—and today, it is only a boutique application. NXT was also supposed to bring the “end of the world for existing speaker technology,” but now this flat-panel topology has earned only a tiny market share. The Tymphany linear array transducers (LATs) of a few years ago is another example of a different path to sound reproduction. Currently, Tymphany is successfully producing conventional but well-executed speaker designs and no longer offers LATs. With these past “not quite game changers” still fresh in our memories, how might MEMS speakers fit into the speaker industry? It is clear that speakers are going to be a tough application for MEMS technology as speakers need piston area and excursion to move air. One of the first attempts of a MEMS “speaker on a chip” was in 2002, which featured individual addressing of the micro-diaphragms, a mechanism similar to Texas Instruments’ digital light processing (DLP) used for video projectors. Ten years ago at the height of the DLP era, Texas Instruments was the one of the largest MEMS manufacturers.
Another early effort took place at the MEMS Laboratory, Department of Electrical and Computer Engineering, Carnegie Mellon University in Pittsburg, PA. This microspeaker array used 256 elements of 200 × 200 μm each. Marginally operational, it could reach only low SPL due to the small amplitude excursion of 10 μm. Other limitations were the low 8-kHz sampling rate, a very low SNR, and an asymmetric impulse response. So, it seems the most promising MEMS speaker technology will come from Audio Pixels. The company has pursued an entirely new technique to directly generate sound waves from a digital audio stream.
Figure 2: Here is a cutaway diagram of a proposed Audio Pixels digital MEMS loudspeaker.
Audio Pixels holds innovative patents in the fields of electromechanical structures, pressure generation, acoustic wave generation, and control, signal processing, and packaging. Its silicon chip can be used either as a standalone speaker or cascaded in any multiples of the same chip to achieve required performance specifications. This modular paradigm is comparable to “parametric speakers” such as phased arrays or using more transducers for increasing the dynamic range.
Audio Pixels’ technique is called Digital Sound Reconstruction (DSR) (see Figure 1). DSR is based on a theory introduced by Homer Dudley at Bell Labs in the late 1930s. The original project at Bell Labs was a secure “digital” speech vocoder for military communications and the “digital speaker” reconstructed the speech on the receiving end. The desired sound wave is generated from the summation of discrete pulses that are produced from an array of pressure generating elements or microtransducers (see Figure 2).
Unlike analog speakers, mini-speakers do not require a large dynamic range. In Audio Pixels’ design, an array of identical elements is used, all fine-tuned to a particular frequency. As with analog speakers, different frequencies are produced by varying the timing of the motion. There is no need for external DACs, and these transducers could have higher energy efficiency than existing speaker topologies, potentially lower harmonic distortion, and improved flatness (with simple EQ).
Yuval Cohen, Audio Pixels’ CTO, stated that “the MEMS-based Digital Sound Reconstruction platform enables the market for audio speakers to follow the evolution of the video display market from large, heavy analog tube-based monitors to the digital flat-panel displays of today. Driving the rationale for change in audio speakers is the ever-increasing demand for smaller, thinner, clearer sounding, more power-efficient speakers.” While actual commercial MEMS speakers might not be available too soon, the technology is on the path to commercialization. Functional prototypes have been demonstrated and Audio Pixels is in partnership with Sony as one of its MEMS foundry partners and ICsense for the ASIC.
Article originally published in Voice Coil, May 2015.
ABOUT MIKE KLASCO
Mike Klasco is the president of Menlo Scientific, an audio consulting firm for the loudspeaker industry, located in Richmond, CA. He is a graduate from New York University, with post graduate work in signal processing, and he holds multiple patents licensed or assigned. For the past 35 years, since founding Menlo Scientific, he has worked on product development, design and technical support, liaison between suppliers and manufacturing facilities, sourcing guidance, and certification for a large number of companies. Mike specializes in materials and fabrication techniques to enhance audio performance, has over 500 articles published in audio magazines and technical journals, and to this day continues to be a regular contributor for audioXpress and Voice Coil magazines. He presented papers at the Acoustical Society of America, held positions as Session Organizer for the Audio Engineering Society, Chairman of the Committee on Acoustics, and is an AES Life Member.
