Advantages of MEMS Speakers in True Wireless Stereo (TWS)

MEMS speakers provide unique features and compelling advantages compared to electrodynamic and balanced armature speakers. The application-specific benefits of MEMS speakers must be separated into two main kinds of applications: Free field and occluded ear applications. In this article, we will focus on the latter.

When we talk about occluded ear applications, we refer to audio systems in which the speaker, mounted in an earbud, sits inside or at the entrance of the ear canal. The air volume between the speaker and eardrum is isolated from the outside by using a sealing ear tip, typically made from silicone. In this kind of application, the ear can exclusively capture the sound emitted by the loudspeaker; the earbud’s sealing attenuates external sounds.

True wireless earbuds have been dominating the market since the launch of the Apple AirPods in 2016. As several brands already jumped on the TWS trend, differentiation is critical to stand out. TWS are the predominant choice among consumers because of their portability, form factor, and freedom of movement they provide.

Manufacturers are looking for ways of enhancing the user experience of TWS — either by improving the sound quality, the noise-cancelling capabilities, the battery lifetime, adding sensor functions, adding connection via NFC, or improving the design and ergonomics.

The main advantage that MEMS speakers offer in this setting is the overall small form factor. This includes the dimension of the loudspeaker and the back and front volume. Both, thanks to the MEMS speaker, can be kept to a minimum without degrading the acoustical performance.

The image below shows how little space a MEMS loudspeaker (right side) takes up compared to a traditional 6 mm dynamic loudspeaker (left side) commonly used in today’s TWS earbuds.

Comparison between VS voice coil speakers and USound MEMS speakers

A further advantage is the large bandwidth that a single MEMS driver can achieve. Due to the low moving mass, the upper end of the bandwidth is limited solely by the acoustic design and not the speakers themselves. See below a comparison of the SPL between an earbud containing a single MEMS loudspeaker with an earphone containing a high-end ED loudspeaker and an earphone containing three BA speakers. As shown, a single MEMS loudspeaker can reproduce a higher bandwidth (higher SPL at high frequencies) than a BA-based earphone, even when using multiple BA speakers, while maintaining a more natural upper-midrange than the ED-speaker earphone.

As a general reference, the Harman In-ear target is added in the background. This represents the most commonly accepted target frequency shape an ideal in-ear phone should have. USound deviates from this slightly on purpose, based on our own research.

Harmann target for in ear and USound MEMS speakers

The results clearly show that a single MEMS driver can easily compete with even a multi-driver balanced armature configuration. This makes the MEMS speaker a compelling technology, especially for TWS-earbuds, where there is no space for multiple BA speakers.

USound’s philosophy is to supply our customers not only with the MEMS speakers but to offer in-depth support on the acoustic and electronic side if desired. Since MEMS-speakers require a different electrical driving stage than electro-dynamic speakers, USound also assists by providing reference designs. Furthermore, we are getting closer to launching our own ASIC, functioning as a dedicated smart audio amplifier to drive MEMS speakers optimally. It will not only work as an amplifier but also contain energy recovery, signal conditioning and biquad filters. With this, we can guarantee an optimized audio signal chain from the Bluetooth receiver to the listener’s ears. Furthermore, one of our upcoming products in our roadmap will contain these signal blocks — ASIC, MEMS speaker and acoustic design, within one pre-assembled audio module, which can then be used as an all-in-one audio solution for TWS.

In summary, the combination of a small form factor and the wide bandwidth provides unprecedented advantages for true wireless manufacturers and end-users when using MEMS speakers.

Contact us to learn more about which MEMS speakers version is more suitable for your target specifications.

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About the author

Jakob Spötl is the leader of the acoustic Development team at USound. Studied electrical and audio engineering with a focus on acoustics at the TU Graz.