Google has conducted research into audioplethysmography (APG), a technology that adds heart rate sensing capabilities to active noise canceling (ANC) headphones and earbuds through a software upgrade. This approach involves sending a low intensity ultrasound probing signal through the headphones, which triggers echoes that are received by feedback microphones. These echoes are then processed into heart rate readings and heart rate variability measurements. Google’s studies found that APG achieved consistently accurate heart rate and heart rate variability measurements, even when impacted by body motion. Google believes this technology is a better alternative to traditional sensors, as it eliminates cost, weight, power consumption, and design challenges. However, the implementation of APG in Google’s Pixel Buds is not yet guaranteed.
https://9to5google.com/2023/10/27/google-anc-earbuds-heart-rate/