Earbuds are becoming platforms for health and wellness features. But every additional sensor competes for space, power, and design freedom inside a device already packed with audio hardware. What if the speaker could take on a second role?
Heart-rate monitoring is moving into true wireless stereo (TWS) earbuds, alongside established functions such as active noise cancellation (ANC), voice capture, and wear detection. For manufacturers, the challenge is not simply whether a new feature works. The challenge is whether manufacturers can integrate it without increasing complexity or compromising the core audio experience.
The Limitations of Adding More Sensors
Many hearables use photoplethysmography (PPG) to measure heart rate. LEDs illuminate the skin, and photodetectors measure changes in reflected light associated with blood volume. This approach is proven, but it requires dedicated optical components and a suitable position against the ear.
Other functions add further requirements. Infrared sensors provide wear detection; temperature sensors need an appropriate thermal interface; motion sensors help identify activity and movement. Each can be valuable, but together they add demands to the bill of materials (BOM), housing geometry, power budget, and production process.
For TWS designers, this raises a more practical question: can existing audio components support sensing?
How Audioplethysmography Works
Audioplethysmography (APG) measures cardiovascular activity acoustically rather than optically.
A speaker sends a low-intensity ultrasonic signal into the ear canal. Small changes in blood-vessel volume and surrounding tissue subtly alter the ear canal’s acoustic characteristics. An inward-facing microphone captures the returning signal, and an algorithm analyses its variations to estimate physiological information such as heart rate and heart-rate variability (HRV).
Unlike optical sensing, APG does not need to illuminate the skin or reserve space for a photodiode. In an ANC earbud, the feedback microphone already has a useful position inside the ear canal.
AuriSense: Turning MEMS Audio into Biosensing
USound’s AuriSense platform applies APG using a wideband MEMS speaker and an existing inward-facing ANC feedback microphone.
The process follows three steps:
- Emit: A USound Conamara MEMS speaker generates an ultrasonic probing signal while reproducing audible sound.
- Receive: The ANC feedback microphone captures changes in the returning ultrasonic signal from the ear canal.
- Interpret: The AuriSense algorithm processes that signal to derive cardiovascular information and detect whether the earbud is being worn.
The platform supports heart-rate monitoring, HRV, and in-ear wear detection, with audio playback and biosensing operating simultaneously.
In the AuriSense architecture, sensor-free means no additional dedicated optical heart-rate sensor, photodiode, or infrared wear-detection receiver is required for those functions. It does not mean sensing occurs without a microphone, electronics, or software.
For OEMs, the potential benefit is architectural. Reusing the audio path can reduce the number of dedicated sensing components, eliminate an optical window, and free space otherwise reserved for separate sensor hardware.
Why the MEMS Speaker Matters
APG requires a speaker that can generate a controlled ultrasonic signal without sacrificing its audible audio function.
USound’s piezoelectric MEMS speakers provide wideband operation that enables this dual use. Their semiconductor-based manufacturing also supports consistent transducer characteristics, which help signal-processing algorithms resolve small acoustic changes.
A MEMS speaker is not merely a smaller replacement for an optical sensor. It becomes an active acoustic transmitter, while the existing ANC microphone serves as the receiver. The value comes from using the pair as one audio-and-sensing system.
This dual role also opens opportunities beyond health tracking, particularly for wear detection: the earbud can recognize an in-ear state through its acoustic response rather than relying on a separate infrared path.
The next step in in-ear biosensing need not be another sensor added to an increasingly crowded earbud.
With AuriSense, the MEMS speaker already responsible for audio can also generate the signal used for heart-rate monitoring and wear detection. For manufacturers, that creates an alternative integration path: more functionality from the acoustic architecture already inside the product.
