Smartwatch Speakers Slim Down With Silicon

A year after introducing the first in-ear, silicon-based earbuds, xMEMS has unveiled a prototype of the latest version of its microspeakers—this time, for use as an open-air speaker, which is a more challenging task.

The Silicon Valley-based startup’s previous microspeakers brought microelectromechanical systems (MEMS) to wireless earbuds and boasted excellent sound quality. By modulating ultrasound signals, the speakers create high-fidelity sound in a light and compact device. The clarity of sound that the new silicon-and-piezoelectric chip, called Sycamore, produces is more like that of a smartphone speaker—decent, but far from the sound quality of in-ear alternatives. And like a smartphone speaker, Sycamore is intended for open-air audio produced by devices near or on the body. In particular, the speaker could be used in various wearable devices, like smart watches, XR glasses, or open earbuds, which clip around the ear instead of nestling within it.

For these applications, the advantage of using MEMS drivers instead of a conventional speaker is less about sound quality, and more about size. The microspeaker is about 1 millimeter thick, one-third the thickness of a coil driver, and removing the magnetic coils of conventional speakers brings its weight down by roughly 70 percent to 150 milligrams. Other speakers also require empty space behind the diaphragm, called back volume. The MEMS-based speakers significantly reduce the back volume needed.

For wearables, every millimeter and milligram matters; a heavy or bulky design could deter users, says Mike Housholder, xMEMS’ vice president of marketing and business development. That’s why the microspeakers are “perfect for smart watches.” Users seeking excellent audio quality would likely opt for in-ear buds or over-ear headphones. The thin, open-air microspeakers instead help deliver a sleek, “fashion-forward” product, Housholder says.

Sound From Ultrasound

Sycamore uses the same “sound from ultrasound” technology introduced in Cypress, xMEMS’ in-ear microspeaker. This tech produces ultrasound by vibrating robust silicon flaps coated in piezoelectric material. It then modulates the ultrasound to generate a full range of audible frequencies.

What’s new with Sycamore is a more efficient chip design. This enhanced efficiency means the speakers can deliver more decibels, making open air listening possible. The speaker also performs well in the bass frequency range, historically a weak spot for MEMS speakers. (In the first commercial headphones to use xMEMS technology, the silicon microspeaker was used only for the high-frequency “tweeter”; it was paired with a conventional dynamic driver “woofer” to produce mid-range and bass audio.)

In the company’s tests of its prototype speaker, Sycamore emitted similar or louder audio compared to the speaker on an Apple Watch Series 8 across most frequencies. Compared to Bose open earbuds, it lagged in mid-range but had stronger bass and treble frequencies.

The new speaker will be made with the same fabrication process as the earbud chip, Cypress. xMEMS will continue to partner with TSMC to manufacture the speakers, though they are now also using Bosch, a leading MEMS foundry.

Housholder says that by further improving the efficiency of the cell design, MEMS speakers may become loud enough for other applications, like phone or laptop speakers. But there are fundamental size limitations for the microspeakers, which are manufactured on a 300 millimeter wafer. Combining multiple chips can also bring up the volume, but it’s unlikely that your next loudspeakers will be made of MEMS.

xMEMS plans to begin sampling Sycamore in early 2025, with mass production expected in January 2026. In the meantime, the company’s full-range in-ear microspeakers will begin mass production in June 2025, followed by its all-silicon fan-on-a-chip in October of the same year.