 Google has announced research into audio plethysmography. It uses hardware for active noise canceling to sense your heart rate. It works by sending a low intensity ultrasound signal into the ear and then the active noise cancellation mics can detect the echoes of the sound off the ear canal. Tiny movements of the ear canal skin changes how the sound echoes back in a very small way, but it changes them and the software can use the variations detected to tease out what your heart rate is and even your heart rate variability.