Google could bring heart rate sensing capabilities to ANC earbuds in the future: here’s how

Google did just that and has incorporated a new acoustic sensing system in a pair of earbuds. Google has done this research in the field of Audiopletyhsmography or APG. It is a method for measuring blood volume changes in the peripheral circulation using sound waves.

Our ANC earbuds work by putting a feedback microphone inside the ear cup to sense the environmental noise and hear us. As this microphone hears the noise similar to the person does, the ANC circuit can create the anti-noise before sending the resulting signal to the headset speaker. To improve noise cancellation, the ANC headphones further leverage a feedforward microphone outside the ear cup to work collaboratively with the feedback microphone and to expand the ANC bandwidth. By taking advantage of these feedforward and feedback microphones, it is feasible to detect or record many bio-signals inside the ear canal.

Google says "The basic idea is to manipulate the speakers of ANC headphones to actively emit a set of low-intensity probing signals at different ultrasonic frequencies.

These probing signals work above the human audible band but can be well captured by the feedback microphone even in the presence of music playback and built-in high-pass microphone filters. "

A model that Google created works to process that feedback into a heart rate reading, as well as heart rate variability (HRV) measurement. This technique works even with bad earbud seals. Compared to existing heart rate sensors, it's not impacted by skin tones. Ear canal size and "sub-optimal seal conditions" also did not impact the accuracy of the study.

However, it was impacted by body motion, and Google countered with a multi-tone approach that serves as a calibration tool to "find the best frequency that measures heart rate, and use only the best frequency to get high-quality pulse waveform."

Google performed two sets of studies with 153 people that found APG "achieves consistently accurate heart rate (3.21 per cent median error across participants in all activity scenarios) and heart rate variability (2.70 per cent median error in inter-beat interval) measurements." 

This study was compared using multiple FDA-approved devices. The heart rate was collected by a Masimo MightySat Rx and a Nonin 9560 Onyx II Fingertip Pulse Oximeter whereas the ECG and HRV data were collected and compared by Zephyr bio harness 3 and Polar HIO chest strap.

Google believes this is a better approach than putting traditional photoplethysmograph (PPG) and electrocardiograms (ECG) sensors, as well as a microcontroller, in headphones/earbuds, as these sensors add a lot of cost, complexity, and weight to these devices making it impractical for daily use.

Google also says that the dicrotic notch that they found during the cardiac APG waveform might be used to determine the blood pressure of an individual using pulse wave analysis.

In the end, Google ended their paper with this statement:

"With a software upgrade, APG adds the cardiac monitoring feature to all ANC headphones with negligible battery life impact."

While this was a research study, we can't help but imagine a future where people will be using these tiny devices to monitor their heart rate consistently.