MEMS Microspeakers Make a Loud Statement Moving Into 2022 and Beyond
While balanced armature speakers have been a leading technology in true wireless earbuds, MEMS microspeakers are beginning to peak in popularity.
According to audio company USound, MEMS speakers will claim over 50% of the portable audio market by 2025. A driving portion of this market will come from microspeakers—a device Yole experts expect will drive the industry from $9 billion USD in 2020 to $11 billion in 2026.
To accommodate this burgeoning demand, many audio developers are heavily investing in various MEMS technology. On April 28, 2022, for example, audio giant Bosch acquired Germany-based Arioso Systems, a specialist in MEMS microspeakers.
MEMS chip with a true-wireless earbud. Image used courtesy of Arioso Systems
Arioso Systems is known for its proprietary audio transducer technology, which creates sound by moving vertical lamellas within a silicon chip. In contrast to conventional membranes, Arioso relies on the chip's volume instead of its surface alone to develop a small MEMS microspeaker. The speaker creates competitive sound pressure levels (SPL)—up to 120 dB SPL of a 10 mm2 active area, the press release reports. This MEMS microsystem, which features an electrostatic actuator of all-silicon MEMS, preserves battery power in both hearables and wearables.
USound predicts that within a few years, MEMS microspeakers will overtake balanced armature and electrodynamic speakers in popularity. In this article, we'll examine the main differences between these two leading audio technologies and how MEMS developers intend to pull ahead in hearable, AR/VR, and automotive sound applications.
The Basics of Balanced Armature Speakers
Balanced armature speakers, which were originally developed for hearing aids, are now widely used in earbuds. These speakers consist of a coil of wire wrapped around an armature that is suspended between two ends of a permanent magnet. When the coil passes a positive electrical current, the current will pivot one end of the armature and push out the speaker’s diaphragm. A negative current will have the opposite effect.
Diagram of positive and negative charges affecting the diaphragm in a balanced armature speaker. Image courtesy of Audio-Technica
As the current continually changes, the subsequently changing magnetic field of the system causes the armature to move at speeds of thousands of times per second. This constant movement of the diaphragm (related to the driving signal) is what creates the system audio. As the name suggests, the armature remains balanced between the permanent magnet when not driven by a signal.
This approach creates speakers that benefit from extremely high fidelity, small size, and high output power, making them a common choice for small audio devices.
Scaling Challenges for Balanced Armature
While balanced armature speakers have been widely implemented, they are quickly reaching their limits. One downside of balanced armature speakers is that they still rely on relatively heavy and large components such as coils, armatures, and magnets. This reliance limits how small balanced armatures can scale down.
While balanced armature systems have provided an advantage over previous technologies such as dynamic drivers, as the industry pursues miniaturization, balanced armatures may begin to fall out of favor.
The Merits of MEMS
Instead, many developers are turning to MEMS speakers to shrink audio devices moving forward. MEMS speakers work by using piezoelectric thin films as active elements that deform when a voltage is applied. This mechanical deflection displaces the surrounding air and generates sound waves.
MEMS speakers can offer significant size advantages over balanced armature speakers. Image from USound
A major benefit of this technology is its compatibility with silicon processes, meaning it can be inexpensively produced at a large scale and at extremely small sizes. The MEMS motor in USound's loudspeakers, for instance, is less than half a millimeter thick—an order of magnitude smaller than its electrodynamic counterparts.
Beyond size, MEMS speakers have the benefit of extremely low power— a result of their high intrinsic impedance—as well as exceptional audio quality. MEMS speakers can also be built on the same PCB substrate as an amplifier, saving board space.
Earbuds, AR/VR, Automotive, and More
In a Digi-Key article on the advantages of MEMS loudspeakers, USound cites two common applications for MEMS speakers: occluded-ear applications and free-field applications.
Occluded-ear applications refer to most hearable technologies in which the speaker within an earbud is positioned inside the ear canal. This orientation creates a noise-canceling effect, where the ear only captures sound from the loudspeaker; the seal of the earbud attenuates all outside sounds. Low-power MEMS speakers are also appealing in true wireless headphones because their small form factor allows developers to include larger batteries, extending the life of the device.
Diagram of a MEMS speaker. Image used courtesy of MDPI (PDF)
In free-field applications, the speaker is outside the ear, so the user can hear external sounds beyond those from the loudspeaker. USound reports that MEMS speakers access bandwidth in the ultrasonic region (up to 80 kHz) with its high-quality tweeter. This creates clear sound from audio modules in AR/VR glasses and even from arrays in automotive sound systems.
This combination of MEMS speakers' low power, high audio quality, and scalability has positioned it to be a rising player in the audio industry.