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Thursday, October 20 • 10:00am - 10:20am
Novel highly miniaturized MEMS speaker technology for in-ear applications

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Driven by new wireless audio applications and the constant trend towards miniaturization, micro loudspeakers based on microelectromechanical systems (MEMS) are recently gaining a lot of interest. Like MEMS microphones, which have become the state of the art for mobile devices, semiconductor technology is used for manufacturing, enabling extraordinarily high miniaturization as well as cost-efficient production in high volumes. In recent years, various MEMS speaker technologies have been developed, primarily addressing in-ear applications, especially hearables. Despite great efforts, the miniaturization potential has not yet been fully exploited. Accordingly, the main challenge remains to increase the sound pressure level per device area.
To address this issue, in this paper a novel MEMS speaker technology is presented for the first time. The new concept builds on a previous development, where four triangular shaped piezoelectric bending actuators with a total area of 4 x 4 mm² were used to generate sound pressure levels of about 110 dB over the entire reproduction range in an IEC 60318-4 ear simulator. This concept is now extended by an additional acoustic shield and a modified actuator geometry, both increasing the sound pressure level and miniaturization at the same time.
The presented MEMS in-ear speaker features a silicon chip with a rectangular shaped actuator that consists of a 2 µm thin piezoelectric layer, two electrodes and a 20 µm thick silicon layer. Except from the clamped edge, the actuator is surrounded by a narrow 3 µm gap, providing good mechanical decoupling and thus enabling high actuator deflections. Along the gap a vertical acoustic shield is arranged, inhibiting significant gap widening when the actuator is deflected. Thus, the air flow through the gap is permanently suppressed by boundary layer effects, meaning that the actuator acoustically behaves like a closed membrane, while being mechanically decoupled at the same time. Compared to previous works, this yields a significantly higher average displacement and SPL at a given footprint.
The technology is demonstrated using a new in-ear demonstrator, featuring two MEMS loudspeakers with an emitting area of as small as 2.4 x 2.4 mm². Besides, measurements and simulations are presented which show that SPL values exceeding 90 dB/mm³ can be achieved for in-ear applications. Finally, an outlook on ongoing and future work is given.

avatar for Fabian Stoppel

Fabian Stoppel

Head Acoustic Systems and Microactuators, Fraunhofer Institute for Silicon Technology
Fabian Stoppel is an expert in the field of micro-electro-mechanical systems (MEMS) and holds a PhD degree in electrical engineering. Since 2010 he is with the Fraunhofer Institute for Silicon Technology (ISIT), Germany, where he is heading the group for acoustic micro systems. Dr... Read More →

Thursday October 20, 2022 10:00am - 10:20am EDT
  Transducers, Papers Oct 19 & 20