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Expedition MEMS SpeakerDaniel Beer, Andreas Mannchen, Tobias Fritsch, Jan Kuller, Albert Zhykhar,

Georg Fischer, Frank Matthias Fiedler

To cite this version:Daniel Beer, Andreas Mannchen, Tobias Fritsch, Jan Kuller, Albert Zhykhar, et al.. ExpeditionMEMS Speaker. Forum Acusticum, Dec 2020, Lyon, France. pp.2921-2928, �10.48465/fa.2020.1122�.�hal-03231968�

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EXPEDITION ‘MEMS SPEAKER’

Daniel Beer Andreas Mannchen Tobias FritschJan Kuller Albert Zhykhar Georg Fischer Matthias Fiedler

Fraunhofer Institute for Digital Media Technology IDMT, [email protected]

ABSTRACT

There is a high market demand for portable audio de-vices. The increasing multifunctionality of the devicesconfronts system designers with the challenge of not be-ing able to demand larger system dimensions, higher en-ergy consumption or higher product prices. For this rea-son, all components must be miniaturized as far as pos-sible in terms of the required installation space and op-timized in terms of energy efficiency and manufacturingcosts. In the field of semiconductor manufacturing, oneapproach is known that has successfully solved such chal-lenges for accelerometers and microphones, namely theso-called MEMS technology (Micro-Electro-Mechanical-Systems). Therefore, the question arises whether MEMStechnology will enable a loudspeaker technology (MEMSloudspeaker) with which the requirements of future audiodevices can be better met than with the loudspeakers usedso far. The article gives an overview of MEMS loudspeak-ers, presenting their advantages but also challenges as wellas the general technological and economical conditions as-sociated with their use.

1. INTRODUCTION

Whether smartphones, headphones or hearables: Theworldwide demand for portable audio devices has in-creased significantly. According to a recent market study,headphones and headsets are the biggest growth drivers onthe global market for audio equipment [1]. In worldwidetotal sales revenue, an increase of almost 40% to around14 billion euros took place from 2017 to 2018 [2].

The functional diversity of today’s devices will continueto increase in the coming years. While headphones are al-ready offering added value in the integration of Bluetoothand active noise control (ANC), the development of fu-ture headphones will focus on additional functions suchas voice control, selective listening (‘smart ANC’) and au-tomatic sound equalization. Some smartphone manufac-turers and start-ups are introducing a new class of head-phones, so-called hearables, with far more functions. Ahearable can be described as a symbiosis of a headphone(usually of the in-ear variety, also called earphone) or hear-ing aid with a smartphone. Therefore, its extended fea-ture range will offer functions like voice control, a simul-taneous interpreter, navigation systems, and intelligent per-sonal assistants (e.g. Alexa, Siri, Cortana) on top of classicservices such as telephony, listening to music, and ANC.

In the second quarter of 2019, hearables were among thefastest-growing product segments in the global wearablesmarket with a share of 46.9%. In 2018 this share was only24.8% [3]. Although hearing aids do not at first glancebelong in the category of portable audio devices, manufac-turers are striving to expand their feature range, too. This isintended to enable hearing aid customers to connect to anduse additional services and to increase the attractiveness ofhearing aids and, consequently, the user acceptance.

The ever-growing functionality and advancing minia-turization of these devices pose new challenges for com-ponent and system developers in terms of the technical im-plementation. The individual components have to be re-duced in size or integrated with each other without impair-ing performance, sound quality, or battery lifetime. Con-sequently, the active components must work much moreefficiently. In addition, the market often demands devicesthat offer an extended feature range at, roughly, the estab-lished price of the predecessor models. Therefore, the sup-plier industry is faced with the following question: “Howcan my components support the system designer in dealingwith the challenges of size, efficiency, and price?” Withregard to the loudspeaker, it must be examined whether anoptimization or substitution of the existing technology willenable a technical improvement with unchanged or evenlower manufacturing costs.

An interesting approach for an alternative loudspeakertechnology is the MEMS technology, which has alreadybeen successfully utilized in the field of microphones [4].Thus far, however, the question remains: Can MEMS tech-nology also make a valuable contribution to loudspeakerapplications to help system manufacturers meet the chal-lenges of size, efficiency and price (cf. Tab. 1)?

Parameter Earphone/Hearable Hearing aidSound pressure level ≥ 100 dB ≥ 120 dBTotal harmonic distortion < 1% < 5%Bandwidth (-20 dB) 20 Hz–20 kHz 100 Hz–6 kHzSensitivity 105 dB/1 mW 105 dB/1 mWOperating voltage 3,6 V 3,6 VBattery lifetime 4 h–10 h 14 hSize [mm] (Ø, h) 7–14, 3–8 (LWH) 9×6×3Weight ≤ 2 g ≤ 1 gSignal processing varying noPrice < 3 USD < 5 USD

Table 1. Loudspeaker requirements for earphones andhearables as well as hearing aid devices [5].

10.48465/fa.2020.1122 2921 e-Forum Acusticum, December 7-11, 2020

In trying to at least get closer to answering that centralquestion, this article gives an overview of MEMS loud-speakers, examining technical as well as economical as-pects. To this end, the basics of MEMS technology (sec-tion 2) are presented before taking an extensive look at thehistory and current state of the art of MEMS loudspeak-ers as well as practical considerations for their use in audiodevices (section 3). This is followed by a thought experi-ment in section 4 that imagines a possible future in whichMEMS loudspeakers are ubiquitous, exploring the techno-logical and economical implications. Finally, the informa-tion given throughout the article is recapitulated and fac-tored into a conclusion in section 5.

2. MEMS TECHNOLOGY

The acronym MEMS stands for microelectromechanicalsystem and describes a form of miniaturized systemsthat include both electrical and mechanical functional el-ements. The following paragraphs aim to describe MEMSconsidering design aspects and the technologies used in thesemiconductor industry, based on the deliberations in [6].Further information on the basics of MEMS technologycan be found in [7].

MEMS technology enables the implementation ofstructures in the submicrometer range with high repeata-bility and the production of high quantities (parallel pro-cessing), permitting a high component quality at an attrac-tive cost. Depending on the selected fabrication process,MEMS are built up layer by layer by locally acting addi-tive and subtractive methods (e.g. vapor deposition andetching) in combination with lithography (Fig. 1).

initial point cantilever structure

photoresist etching coating

Figure 1. Illustration of a surface micromachining processby taking the example of a cantilever. The structure is builtup layer by layer through the recurring process steps: vapordeposition, exposure, and etching. Reprinted from [5].

MEMS chips are manufactured in clean rooms to pre-vent contamination of the chip surfaces with particulatematter. The starting material for the design of MEMS ismostly the so-called round wafer made of highly pure sili-con. A wafer can hold many chips, e.g. depending on theapplication and the required chip area, several thousand,which can be manufactured in parallel. The chips are thenseparated (dicing), their components are electrically con-nected (bonding), and a housing is provided (packaging),as illustrated in Fig. 2.

(1) (2)

(3)(4)

Figure 2. Illustration of the step-by-step construction ofa MEMS chip, starting with the pure wafer (1), manufac-turing of chips on the wafer (2), dicing into single chips(3), and finally the bonding and packaging (4). Reprintedfrom [6], based on [7, p. 34].

The processing of whole wafers at once can offer at-tractive production time and costs through parallel pro-cessing and distribution of fixed costs over a large numberof units [7, p. 533]. Therefore, the annual minimum pur-chase quantity of a typically-sized MEMS device (smallerthan 1 cm2) should be at least in the seven-digit range,which amounts to thousands of 200 mm wafers [8]. Sincethe number of processed wafers is fundamentally flexible,MEMS offer a high scalability of the production volume,enabling adaptability to dynamic markets. With the inte-gration of several functional units into one component orpackage, energy losses as well as installation space canbe reduced: e.g. electronic components, such as signalprocessors, can be integrated as application-specific inte-grated circuits (ASIC). Designed as surface-mounted de-vices (SMD), MEMS can be installed by fully automaticassembly lines with reflow ovens. This reduces assemblycosts and allows a dense board assembly.

In different application areas, MEMS components havealready been established, such as accelerometers forairbags. Regarding acoustic components, MEMS micro-phones have almost entirely substituted electret micro-phones in portable audio equipment since 2014 [4].

3. MEMS LOUDSPEAKERS

Motivated in no small part by the success of MEMS mi-crophones, advancements in MEMS technology and de-sign have driven research and development in the fieldof MEMS loudspeakers. When evaluating MEMS loud-speaker approaches, an important distinction has to bemade between designs solely for the ultrasound regime andthose for audio applications. The latter usually present agreater challenge, because they require a relatively highmembrane deflection at low frequencies and a larger rel-ative bandwidth (20 Hz–20 kHz, i.e. three decades). Thisarticle only deals with MEMS loudspeakers that reproducehearable frequencies.


The following sections give an overview of the historyof MEMS speakers (section 3.1), current approaches (sec-tion 3.2), and special characteristics of MEMS speakersthat require consideration when using them in audio de-vices (section 3.3).

3.1 The early days

Early patent applications for MEMS loudspeakers ap-peared in the 90s, about 10 years after the first MEMS mi-crophone [9]. Despite the early patent applications, MEMSloudspeakers, are as of yet, more an element of researchprojects than a product on the market. Based on the his-tory of MEMS microphones, 30 years of development arealso expected for MEMS loudspeakers.

In 1994, Lee et al. filed a patent for a piezoelectricMEMS loudspeaker consisting of a single-side supportedmembrane of 2 mm × 2 mm [10]. The other membranesides are separated from the surrounding structure by a10 µm gap, as shown in Fig. 3. Following the applied au-

membrane

piezoelectric actuator

gap

Figure 3. Illustration of the piezoelectric MEMS loud-speaker of Lee et al. in 1994 [10]. Reprinted from [6].

dio signal, the piezoelectric actuator causes a cantilever-like membrane oscillation. The experimentally measuredsound was strongly colored by resonance effects and cov-ered the range of approximately 55 Hz–50 kHz (measuredin a 2 cm3 coupler). The sound pressure level (SPL)throughout this frequency range varied from roughly 50 dBup to 100 dB. At 8 Vpp (peak-to-peak), an averaged SPL ofabout 75 dB was reached. Due to its highly nonlinear fre-quency response function, with dips of up to 40 dB, thesound quality was moderate. Compared to the require-ments of loudspeakers for earphone applications (Tab. 1),it would not be satisfying.

In 1996/1997, Haradine et al. introduced a MEMS loud-speaker with an electrodynamic drive system [11]. In thisdesign, the permanent magnet is attached to a membraneand follows its movement. The voice coil is placed fixed inthe surrounding substrate (Fig. 4). The circular membranehas a diameter of 2 mm. In the acoustic measurement, thegenerated SPL reached only 45 dB on average, measured ina 2 cm3 coupler. The electrical power or voltage requiredfor this 45 dB is not mentioned in [11]. The acoustic per-formance was even lower than that of Lee’s demonstrators,not meeting the requirements given in Tab. 1 as a result.

membrane

magnet

voice coil

Figure 4. Illustration of the electrodynamic MEMS loud-speaker of Harradine et al. in 1996/1997 [11]. Reprintedfrom [6].

One of the first electrostatically driven MEMS loud-speakers for sound reproduction in the audio frequencyrange was introduced by Loeb et al. in 1999 [12]. Amembrane with a size of approximately 1.4 mm × 1.4 mmrepresents a movable electrode that interacts with a statorelectrode behind (Fig. 5). Measured using an IEC 60318-4

membrane

stator

Figure 5. Illustration of the electrostatic MEMS loud-speaker of Loeb et al. in 1999 [12]. Reprinted from [6].

ear simulator, this loudspeaker achieved an average SPL ofabout 75 dB at 27 Vpp with a bias voltage of 67 VDC. Thefrequency range extended from roughly 20 Hz to 7 kHz. Inspite of delivering the required frequency range for hearingaids (Tab. 1), this loudspeaker is unsuitable, e.g. due to itslow SPL.

3.2 Current MEMS loudspeaker approaches

Since the days of early MEMS loudspeaker experiments,many more scientific publications and patents in this fieldhave seen the light of day. While the same basic transducerprinciples have been employed for the most part (traceableeven to classic, non-MEMS loudspeakers, as a matter offact), advancements in MEMS technology and design haveeventually led to the implementation of truly promisingMEMS loudspeakers. This section presents a number ofcurrent MEMS loudspeaker designs, some of which havealready entered the market or are about to.


To start with, the Austrian company USound offersa hybrid MEMS loudspeaker technology [13], meaningit consists of a MEMS-based piezoelectric drive systemand an additionally applied membrane device. Driven bythe H-shaped piezo actuators, the membrane moves in apiston-like manner. USound has developed a dedicatedamplifier circuit as well as digital signal processing (DSP)with a feedback MEMS microphone and has recently pre-sented a hybrid MEMS loudspeaker module integrating allof these components [14]. USound hybrid MEMS loud-speakers have been available on the market since 2018.The acoustic performance of a USound Achelous speakeris considered further below.

Fraunhofer ISIT and Fraunhofer IDMT have presenteda narrow gap piezoelectric loudspeaker (NGPL) in [15].As opposed to the USound approach, it is not a hybrid de-sign, but instead only requires MEMS technology in itsproduction. It uses a piezoelectric drive system made upof an array of bending actuators that are separated by mi-croscopically narrow gaps and, therefore, functioning likea coherent membrane, acoustically speaking. This type ofMEMS loudspeaker has been successfully integrated in anearphone demonstrator with specifically designed externalamplifier and DSP [16]. As with USound above, the acous-tic performance of an NGPL is considered further below.

A third piezoelectric MEMS loudspeaker comes fromthe US-based company xMEMS. According to publiclyavailable information [17], the transducer resonance fre-quencies in the xMEMS design lie above 20 kHz, whilestill producing 115 dBSPL below 5 kHz in an IEC 60318-4 ear simulator. The loudspeaker module has a size of6.05 mm × 8.4 mm. Whether this interesting approach willbe able to achieve the impressive preliminary specifica-tions given on the company’s website in a final product,as of now remains to be seen.

Fig. 6 compares the frequency response functions ofthe Fraunhofer ISIT/IDMT NGPL earphone from [16](4 mm × 4 mm membrane) and the USound Achelous hy-brid MEMS speaker system (12 mm2 effective membranearea) to that of a standard consumer earphone with an elec-trodynamic driver (9 mm membrane diameter). Please notethat the NGPL demonstrator and the consumer earphonewere measured with a GRAS RA0401 ear simulator (spec-ified from 20 Hz to 20 kHz), while the USound Achelousearphone was measured with a standard ear simulator ac-cording to IEC 60318-4 (specified from 100 Hz to only10 kHz). Moreover, the three devices all follow distincttarget frequency responses with differently shaped sounds,e.g. various levels of bass boost. Finally, the drivingvoltages involved in these measurements differ immenselydue to the different actuator designs (the next section willdiscuss this aspect in more detail). All these differencesnotwithstanding, the comparison still clearly shows thatcurrent MEMS loudspeakers are capable of producing theSPL needed for earphone applications at an adequate size,as listed in Tab. 1. However, considering the requirementsfor hearing aids, MEMS loudspeakers still need increasedperformance.

0.1 1 10

80

90

100

110

120

frequency / kHz

SPL

/dB

NGPL (MEMS)Achelous (hybrid MEMS)consumer earphone (non-MEMS)

Figure 6. Comparison of the frequency response functionsof the Fraunhofer ISIT/IDMT NGPL earphone (drivenwith 20 Vpp, 10 VDC bias), a complete USound Acheloussystem (driven with 30 Vpp, 15 VDC bias), and a standardconsumer earphone (driven with 0.37 Vpp). Frequencyranges not specified by the respective ear simulator modeluse dashed lines.

3.3 Practical considerations for the usage of MEMSloudspeakers

While current MEMS loudspeakers meet, at the very least,some of the important specifications for in-ear audio appli-cations, significant differences between conventional loud-speakers and their MEMS counterparts remain. This sec-tion calls to attention some of the aspects that should beconsidered by system integrators and acoustic engineerswhen working with MEMS loudspeakers, and suggests so-lutions to some of those challenges.

3.3.1 Driving electronics

Conventional microspeakers almost exclusively employelectromagnetic driving mechanisms such as dynamic orbalanced armature. However, as shown in section 3.2, themajority of current MEMS loudspeaker approaches utilizethe reverse piezoelectric effect. Therefore, MEMS loud-speakers can not be integrated into an existing system bysimply interchanging a conventional speaker with a MEMSactuator: they require different excitation signals.

Conventional loudspeakers have a low electricalimpedance dominated by voice coil inductance, membranemass and suspension stiffness. MEMS loudspeakers, how-ever, provide a high electrical impedance of mainly capac-itive character to the power amplifier. Hence, specializedamplifiers are required. The design of such amplifiers doesnot pose a problem to experienced engineers, and inte-grated solutions are available. But highly miniaturized ver-sions providing the required voltages (up to 30 Vpp [14])over the entire audio frequency range are currently notavailable off the shelf.

Moreover, nearly all current MEMS loudspeaker ap-proaches require a DC bias voltage. The magnitude of thisDC bias varies from loudspeaker to loudspeaker, depend-ing on drive mechanism, materials, and loudspeaker size,reaching up to about 67 VDC in extreme cases [18]. The


DC bias has to be accurate. A wrong DC bias can reduceSPL output, have a negative impact on nonlinear distortion,or even damage the MEMS loudspeaker.

3.3.2 Controlling MEMS speakers with DSP

As is common with small devices required to deliver bigperformance, current MEMS loudspeakers operate in thelarge signal domain close to their physical limits—be itmembrane deflection or electrical overload. In this do-main, they need to be controlled in order to mitigate non-linear distortion by compensating for nonlinearities in thedriving mechanism. Fig. 7 gives an example of a reductionof the total harmonic distortion (THD) using DSP tech-niques, based on the results in [16]. The protection of

0.1 1 100.1

1

10

frequency / kHz

TH

D/%

without DSPwith DSP

Figure 7. THD (at 2 Vpp with a 1 VDC bias) of the NGPLearphone measured with and without DSP (measured withGRAS RA0401) [16].

the MEMS loudspeaker against overexcursion, overheat-ing, and electrical overload can also be of importance.These measures may or may not play a crucial role, de-pending on the design and underlying drive principle ofthe loudspeaker.

Furthermore, equalization of the linear transfer func-tion of the MEMS loudspeaker ensures good sound qual-ity. This is exemplified in Fig. 8 by matching the orig-inal, highly resonant NGPL MEMS earphone frequencyresponse with a target curve using equalization with a fi-nite impulse response filter (FIR-EQ) [16]. Because thequality factors of the resonances that need to be equalizedin the frequency response of a MEMS loudspeaker are of-ten quite high, even slight shifts of a resonance frequencydue to aging effects or a changing acoustic load may causethe designed filters to become useless or even counterpro-ductive. Adaptive control strategies, i.e. updating the fil-ters according to a changing system state, can solve thisproblem. To monitor the system state of the MEMS loud-speaker, the final system design needs to include sensors,which can be other MEMS devices like microphones, ac-celerometers, photodiodes, etc. Measuring the voltage andcurrent at the input of the loudspeaker offers another wayof observing the system state, which may be difficult de-pending on the MEMS design, however [6].

0.1 1 1070

80

90

100

110

120

130

140

frequency / kHz

SPL

/dB

without DSPwith DSP

Figure 8. Measured SPL (at 2 Vpp with a 1 VDC bias) of theNGPL earphone with and without processing (measuredwith GRAS RA0401). Here, DSP involves filtering theinput signal with a FIR-EQ filter to match the earphoneresponse with a target curve [16]. Reprinted from [6].

3.3.3 Acoustic Design Challenges

Because MEMS loudspeakers are small devices, physicalphenomena have to be taken into account during the designprocess that are usually negligible for large loudspeakers.This refers, specifically, to thermoviscous losses, which af-fect the propagation of sound waves in narrow air-filledstructures. Tangential forces occur at the transition be-tween the air and the wall surface of a solid. Such forces re-sult in friction, which removes energy from the wave fieldand converts it into heat [19, p. 298]. A more detailed dis-cussion of this topic, which the following paragraphs arebased on, is offered in [20].

The importance of thermoviscous losses and the result-ing acoustic impedances is demonstrated by a simulationmodel of the NGPL MEMS earphone from [16] in Fig. 9.In this case, thermoviscous acoustics were applied to sev-eral regions. The MEMS loudspeaker is housed in an ear-phone enclosure that plugs into an artificial ear and in-cludes a back volume with a sound duct radiating into thefree field.

ear simulator

MEMS chip

Figure 9. Cross-section of the simulation model of theNGPL MEMS earphone mounted on an ear simulator.Reprinted from [6].


For comparison, simulations with and without thermo-viscous losses were performed. The MEMS loudspeakerdriving signal was simulated at 2 Vpp with a 1 VDC bias.A measurement under the same conditions validates thesimulation results in Fig. 10. When simulating with ther-

0.1 1 10

60

70

80

90

100

110

120

130

frequency / kHz

SPL

/dB

MeasurementSim with losses

Sim lossless

Figure 10. Simulated SPL with and without thermovis-cous losses, and measured SPL [16] of the NGPL MEMSearphone (measured with GRAS RA0401). Reprintedfrom [20].

moviscous losses, the resulting acoustic impedances arecalculated correctly. The resonances and cancellations inthe frequency response are attenuated appropriately and,therefore, the simulation results approximate the measure-ment well. In the lossless simulation model, the acousticimpedances inside narrow geometries are no longer mod-eled correctly. This results in a drop of lower frequen-cies, additional resonances and cancellations. Because thesound duct at the back of the enclosure now has a smallacoustic impedance compared to the ear simulator, thesound energy is radiated out of the back of the earphonethrough the duct. This example shows that thermoviscouslosses are not negligible in the design process of minia-turized acoustic actuators and sound guides with small di-mensions [20].

3.3.4 Harnessing the potential: the all-in-one MEMSloudspeaker package

While it may seem at first glance like the technical chal-lenges described above could prevent a widespread use ofMEMS loudspeakers, solutions are indeed available. Andthese solutions even open up new possibilities by playingto the advantages of the MEMS technology.

The core concept that simplifies the usage of MEMSspeakers in audio devices for engineers and system in-tegrators is the MEMS loudspeaker package, sometimesalso called module, as shown in Fig. 11. Apart from thespeaker itself, it ideally includes: a power supply, a DSP, adigital-to-analog converter, an amplifier, a DC bias genera-tor, and an optimized acoustic packaging, either in one sin-gular package called ‘System in Package’ (SiP) or, minia-turized even further, on the same silicon substrate, called‘System on Chip’ (SoC). This is where one of the greatstrengths of MEMS lies. By using standardized microfab-

enclosure

ASIC electrical contact

MEMS chip

sound outlet

Figure 11. Schematic representation of a MEMS loud-speaker package comprising the MEMS component, theASIC, and the acoustic packaging. Adapted from [5].

rication technologies throughout the fully automated man-ufacturing process, a MEMS loudspeaker system can eas-ily include electronic components implemented as ASICand even its acoustic packaging, while maintaining a com-pact form factor.

The MEMS loudspeaker as one complete package es-pecially suits so-called True Wireless Systems, since thoseare stand-alone devices that need their own wireless re-ceivers, digital-to-analog converters, power supplies, andamplifiers, anyway. However, MEMS loudspeaker pack-ages can also provide standardized interfaces to designersof more traditional systems, regardless of the underlyingsound reproduction principle. Such interfaces offer thepossibility of freely interchanging loudspeakers of differ-ent types without worrying about impedance or compati-bility. They might be implemented as 3.3 Vcc for powersupply and I2S terminals for digital audio connectivity. It iseven possible to include further MEMS components suchas microphones or other sensors in a SiP or SoC fashion,enabling adaptive DSP techniques.

On top of improving the sound quality of the MEMSspeaker and protecting it, DSP opens up the possibility ofadapting the acoustic output of the MEMS loudspeakerpackage to varying requirements—bounded by physicallimitations, of course. In this vein, adaptive DSP is cru-cial to the success of MEMS loudspeakers: It allows oneand the same design of a MEMS speaker package to beused in a possibly large number of different products byaltering the input-output characteristics of the system. Inthe field of MEMS, this is a great advantage, as it enablesthe mass-production of a single device at a low unit price.

4. A FUTURE WITH MEMS LOUDSPEAKERS?

Compared to the current state, future earphones, hearables,and hearing aid devices will be equipped with a lot morefeatures. Based on its characteristic properties describedabove, it is possible to specify the potential of the MEMStechnology or the MEMS loudspeaker for future portableaudio devices. Furthermore, the technological and eco-nomic influence on market can be estimated, in this caseexemplified by the earphone and hearable market.


As mentioned above, MEMS technology can enablelow per-unit production costs, but merely if minimum an-nual purchase quantities of several millions are guaran-teed. Only earphone and hearable manufacturers with asignificant market share can reach these numbers and usea MEMS foundry to capacity, i.e. several thousands ofwafers each year. Even then, they will probably use thesame design of a MEMS loudspeaker package in all ear-phone or hearable products in order to avoid the high costinvolved in developing several different designs. Smallerplayers on the earphone and hearable market will likelyeither have to share a MEMS loudspeaker technology be-tween them, or stick to traditional loudspeakers and, as aresult, traditional audio products. A similar situation ex-ists today in the field of hearing aids: Sometimes, differenthearing aid manufacturers use the same balanced armaturetransducer in their products.

This raises the question: If they always use the sameMEMS loudspeaker package, how can earphone or hear-able manufacturers set themselves apart from their com-petition or create individual product characteristics, e.g.sound branding? Of course, the frequency response can beadjusted by means of the acoustic design of the earphoneenclosure. The costs resulting from this additional devel-opment step may, however, be reduced or entirely avoidedbecause MEMS loudspeaker packages offer a technical ad-vantage over conventional loudspeakers: they combine theloudspeaker structure and a DSP unit. The DSP unit allowsfor a digital adaptation of the loudspeaker characteristics tovarying operating conditions (e.g. different acoustic loads)and an individualization (e.g. sound branding). Further-more, the possibility of MEMS loudspeaker designs withintegrated sensors offers more flexibility for an advancedadaptive control, e.g. for self-initialization or compensa-tion of manufacturing tolerances, aging effects, and operat-ing errors. Although the conventional loudspeaker can alsobe combined with a DSP unit, its integration in the MEMSloudspeaker package results in comparably low space andenergy requirements. Moreover, the combination of com-ponents in the MEMS loudspeaker package results in a re-duction of functional units and interfaces for the systemmanufacturer.

The use of MEMS loudspeakers also changes the prod-uct development and production flow of the audio de-vices equipped with them. According to the modern con-cepts of semiconductor manufacturing, this means a sub-divided value-added chain in which specialized manufac-turing partners implement the production of MEMS loud-speakers by cost-efficient use of their existing infrastruc-ture (Fig. 12). The basic loudspeaker idea or technicalspecifications regarding its electroacoustic properties aswell as dimensions are developed by the system manufac-turer. Typically, a so-called design house then performsthe acoustic, mechanical, and electrical construction ofthe MEMS loudspeaker element according to the speci-fications. Aside from specialized design and simulationsoftware, this task requires special skills and experiencein MEMS design and fabrication. The resulting design

specifications MEMS and ASIC designs

headphone industry design house MEMS foundry

MEMS loudspeaker package

Figure 12. Schematic representation of the expected prod-uct development and production flow resulting from thewidespread use of MEMS loudspeakers. Adapted from [5].

is given to the MEMS manufacturing partner, a MEMSfoundry.

Before the actual production of the MEMS loudspeaker,however, the foundry first needs to develop a reliableand reproducible production technology. This is done inclose cooperation with the design house to ensure that theMEMS speaker meets the specification initially devised bythe system manufacturer.

For classical MEMS products, such as microphones orinertial sensors, the following assembly and packaging in-volves yet another partner, the so-called assembly house.However, since the small size of MEMS loudspeakers forin-ear applications is an important application criterion, itis expected that in the future, the integration of the elec-tronics, the implementation of the acoustic volume, as wellas the contacting and mounting of interfaces for the finalproduct will already take place in the wafer process (wafer-level assembly and packaging). This will render the as-sembly house unnecessary, resulting in a faster and morecost-effective production.

The conditions under which MEMS loudspeakers canachieve attractive manufacturing costs as well as advancedtechnical properties make it clear that MEMS loudspeakersare used either completely or not at all. A niche existenceis hardly conceivable—in the MEMS context, it is clearlydisadvantageous compared to a complete substitution ofthe traditional technology. However, if MEMS loudspeak-ers substitute the main speaker technologies in of today inearphones, hearables, and even hearing aids, it will affectthose markets significantly.

5. CONCLUSION

MEMS loudspeaker approaches for audio reproductionhave been known for more than 25 years. Step by step theirtechnical performance has improved so that some currentdesigns can keep up with some conventional microspeak-ers or balanced armature transducers. Through the inte-gration of the MEMS transducer, the driving electronics,the DSP, sensors, and the acoustic packaging in one unit,MEMS loudspeaker packages enable a relatively straight-forward adaptation of this new technology by system man-ufacturers, while demanding little space or energy. Byutilizing adaptive DSP techniques, the same design of aMEMS loudspeaker package can be used in a multitude


of products, resulting in mass-production at an attractivelylow per-unit cost.

Whether MEMS loudspeakers will be able to developa clear unique value proposition in the future that will al-low them to replace the currently utilized transducers re-mains to be seen. The relevance of the loudspeaker com-ponent in fulfilling the system requirements for future ear-phones, hearables, and hearing aids (inexpensive, small inspite of multifunctionality, and energy-efficient) will playa big role in that regard. An eventual substitution of clas-sic loudspeakers with MEMS would result in a significantchange in the product development and production flowand possibly in the composition of market participants.

6. REFERENCES

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[3] “Hearables Are the New Wearables,Says IDC.” https://www.idc.com/getdoc.jsp?containerId=prUS45488019.Accessed: 2020-10-28.

[4] “Acoustic MEMS and Audio Solutions – From Tech-nologies to Markets,” Yole Developpement, 2017.

[5] D. Beer, A. R. Clerici, F. Stoppel, and L. Ehrig,“Mems-lautsprecher - ein paradigmenwechsel,” DEGAAkustik J., vol. 3, pp. 24–39, 2020.

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[7] C. Liu, Foundations of MEMS. Prentice Hall, UpperSaddle River, 2 ed., 2012.

[8] R. A. Lawes, “Manufacturing costs for microsys-tems/mems using high aspect ratio microfabricationtechniques,” Microsyst. Technol., vol. 13, no. 1, pp. 85–95, 2007.

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[10] S. S. Lee, R. P. Ried, and R. M. White, “PiezoelectricCantilever Microphone and Microspeaker,” Solid-StateSensor and Actuator Workshop Hilton Head, SouthCarolina, 1994.

[11] M. Harradine, T. Birch, J. Stevens, and C. Shear-wood, “A micro-machined loudspeaker for the hear-ing impaired,” in Proc. of International Solid StateSensors and Actuators Conference. (Transducers’ 97),Chicago, vol. 1, pp. 429–432, IEEE, 1997.

[12] W. A. Loeb, J. J. Neumann, and K. J. Gabriel, “MEMSDigital-to-Acoustic Transducer With Error Cancella-tion,” Patent: US20050013455 A1, Field: 09.13.1999,Priority: 09.13.1999.

[13] “Specifications of Achelous hybrid MEMS loud-speaker.” https://www.usound.com/product/mems-speaker-achelous/. Accessed: 2020-10-28.

[14] M. Hansler, G. Muraro, C. Novotny, R. Murphy,J. Spotl, and A. R. Clerici, “Mems based audio speakermodule,” in J. Audio Eng. Soc. Convention 148, 2020.

[15] A. Mannchen, D. Beer, F. Niekiel, J. Nowak, F. Stop-pel, and B. Wagner, “Analysis of the mechanical vi-bration and acoustic behavior of a piezoelectric memsmicrospeaker,” in J. Audio Eng. Soc. Convention 143,2017.

[16] A. Mannchen, F. Stoppel, T. Brocks, F. Niekiel,D. Beer, and B. Wagner, “Design and electroacousticanalysis of a piezoelectric MEMS in-ear headphone,”in Audio Eng. Soc. Conference on Headphone Technol-ogy, 2019.

[17] “Preliminary specifications of XSM-2300 Mon-tara MEMS loudspeaker.” http://xmems.com/products/. Accessed: 2020-10-28.

[18] J. J. Neumann and K. J. Gabriel, “CMOS-MEMSmembrane for audio-frequency acoustic actuation,”Sens. Actuators, A, vol. 95, no. 2, pp. 175 – 182, 2002.

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[20] D. Beer, J. Kuller, A. Zhykhar, T. Fritsch, andA. Mannchen, “Mems based speakers – soundsgood?!,” in GMM-Fb. 97: Mikro-Nano-Integration,GMM-Fachberichte, VDE VERLAG, Berlin, 2020.


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