Review of MEMS Speakers for Audio Applications

Microelectromechanical systems (MEMS) speakers are compact, scalable alternatives to traditional voice coil speakers, promising improved sound quality through precise semiconductor manufacturing. This review provides an overview of the research landscape, including ultrasound pulse-based and thermoacoustic sound generation, classifying MEMS speakers by actuation principle: electrodynamic, piezoelectric, and electrostatic. A comparative analysis of performance indicators from 1990-2025 highlights the dominance of piezoelectric MEMS with direct air displacement, focusing on miniaturization and efficiency. The review outlines upcoming research challenges and identifies potential candidates for achieving full-spectrum audio performance. A focus on innovative approaches could lead to wideband adoption of MEMS-only speakers.

💡 Research Summary

This paper presents a comprehensive review of micro‑electromechanical systems (MEMS) speakers for audio applications, covering the period from 1990 to 2025. It begins by highlighting the contrast between the widespread adoption of MEMS technology in sensors, microphones, and other devices and the continued reliance on century‑old voice‑coil speakers in consumer audio. The authors map the growth of publications, showing a sharp increase in MEMS speaker research after the early 2000s, especially in the piezoelectric domain.

The review first defines the essential acoustic metrics—sound pressure level (SPL) and total harmonic distortion (THD)—and explains two standard measurement approaches: ear‑coupler (closed‑volume) measurements suitable for low‑frequency (<2–3 kHz) analysis, and free‑field measurements that model the speaker as a point source radiating into half‑space. Equations linking volume displacement to SPL (Eq. 5) and free‑field pressure to displacement, frequency, and distance (Eq. 6) are provided to enable cross‑study comparisons.

Three sound‑generation concepts are examined. Direct displacement, the classic mechanism used by conventional loudspeakers, is realized in MEMS by moving a diaphragm (or cantilever) to compress and rarefy air. Ultrasound‑pulse‑based designs operate the actuator at ultrasonic carrier frequencies (>20 kHz) and encode audio via pulse‑density modulation (digital sound reconstruction) or amplitude‑modulated pumping. These approaches can boost low‑frequency SPL through double‑sideband suppressed‑carrier modulation but suffer a 1/f SPL roll‑off at higher audio frequencies. Thermoacoustic (thermophone) devices generate sound by rapid heating of the surrounding gas; while theoretically broadband, practical implementations are limited by high power consumption and low efficiency.

The core of the paper classifies MEMS speakers by actuation principle. Electrodynamic actuation uses the Lorentz force (F = B·l·i) generated by a current‑carrying coil in a magnetic field. Although this method can deliver high SPL, scaling is constrained by the size of permanent magnets and the need for relatively large currents. Piezoelectric actuation exploits the inverse piezo effect (S = d·E) in ferroelectric thin films, most commonly in d₃₁ mode with unimorph or bimorph cantilevers. By applying a DC bias to reduce hysteresis, piezo MEMS achieve SPLs up to 140 dB (especially at low frequencies) and THD below 1 %, while maintaining power efficiencies above 30 %. Electrostatic actuation relies on voltage‑induced forces between movable and fixed electrodes (F = ½(V_AC+V_DC)²·∂C/∂x). The inherent non‑linearity leads to distortion at high drive voltages, but push‑pull electrode configurations and large DC bias mitigate this effect, enabling low‑voltage operation.

A quantitative comparative analysis of reported devices shows that piezoelectric MEMS dominate the performance landscape, offering the best combination of SPL, THD, and efficiency. Electrodynamic MEMS provide high output but are limited by magnetic integration challenges, while electrostatic MEMS excel in low‑power scenarios but struggle with linearity. Ultrasound‑pulse and thermoacoustic concepts remain niche, primarily of interest for specialized applications such as directional sound projection or ultra‑compact actuation where conventional diaphragms are impractical.

The authors identify several research challenges: achieving sufficient low‑frequency SPL in sub‑millimeter footprints, reducing drive‑voltage‑induced distortion in electrostatic designs, improving the thermal management of thermoacoustic devices, and developing CMOS‑compatible high‑voltage drivers for piezoelectric actuation. They also point to emerging opportunities, including hybrid architectures that combine piezoelectric displacement with electrostatic linearization, 3‑D stacked diaphragm structures to increase effective radiating area, and novel ferroelectric materials with higher d₃₁/d₃₃ coefficients.

Finally, the paper outlines future directions. For true wireless stereo (TWS) earbuds and AR/VR headsets, MEMS speakers must meet stringent SPL, THD, power‑budget, and form‑factor requirements. Advances in high‑density packaging, on‑chip bias generation, and adaptive control algorithms are expected to bridge the gap between laboratory prototypes and commercial products. The authors conclude that while MEMS speakers have not yet supplanted traditional voice‑coil technology, the convergence of semiconductor scaling, material innovation, and system‑level integration positions them to become a mainstream solution for full‑spectrum audio in the coming decade.