Audioplethysmography (APG) is a physiological monitoring technique that uses sounds or sound waves to detect changes in blood flow. The method, similar to photoplethysmography (PPG), records changes in blood volume at a given point on the body. In particular, APG focuses on detecting changes in blood flow within the ear canal – why not create new ones earphones that measure heart rate and deliver devices capable of not only optimizing the listening experience but also providing immediate feedback on vital parameters into the hands of users?
That’s exactly what he did Google presenting his idea of earphones with active noise cancellation (ANC) and integrated APG system. This way, simply inserting a normal headset of new generation in the ear, it is possible to monitor the heartbeat and its variability, without adding additional sensors and without compromising battery life.
How earphones work to measure heart rate
Google technicians explain that relying on the APG technique and on low intensity ultrasound, it becomes possible to effectively keep the physiological signals of each individual under control. The monitoring process occurs through the sending of anultrasonic wave at low intensity (TX transmit wave) using the ANC earphone speakers and collecting the received wave (RX or echo wave) via the built-in feedback microphones. The APG signal takes the form of a pulse-like wave synchronized with heartbeats and reveals detailed information about the behavior of the heart.
The ear receives blood supply fromdeep auricular artery, which forms an intricate network of vessels within the ear canal. Variations in the shape of blood vessels caused by heartbeat and pressure can lead to changes that are easily detectable by a device inserted into the ear.
Google staff developed a cylindrical resonance model which helps understand the underlying physics of the APG. The phenomenon, which occurs on an extremely small scale, is suitable for detecting cardiac activities by simultaneously exploiting a bouquet Of frequencies on the band of ultrasound.
Studies conducted on a large sample of participants demonstrate that APG offers accurate measurement of heart rate and heart rate variability, overcoming the limitations of PPG sensorysuch as the variability of performance based on the chromatic characteristics of the skin.
How signals are measured
In the context of the simulation based on the analytical model developed by Google, the amplitude ( 𝑅(𝑡) ) and phase ( Φ(𝑡) ) of the APG signals reflect cardiac activities ( ℎ(𝑡) ). Amplitude refers to the value of the APG signal wave: changes in amplitude can indicate changes in blood flow or pressure inside the ear due to cardiac activity.
Phase represents the time delay or wave shift of the APG signal relative to a reference. Phase variations express changes in the timing of changes in blood flow and pressure.
Google has addressed the issue of noise generated by the body (which can make measurements inaccurate) using various strategies: the aforementioned low intensity ultrasound, so that the transmitted signal is more distinct than environmental noise; different frequencies (some frequencies may be more sensitive to cardiac activities, while others may be more sensitive to body movement); search for the best frequency; signal processing techniques which allow you to accurately isolate and analyze signals of interest.