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Google scientists have successfully employed audioplethysmography (APG), a technique utilizing ultrasound to measure heart rate, enabling the monitoring of heart health through wearable technology without the need for additional sensors or compromising battery life.
The research involved two rounds of user experience studies with a total of 153 participants. The findings demonstrated that APG consistently provided accurate heart rate measurements (3.21 percent of the time, across all activity scenarios) and accurate heart rate variability measurements (2.70 percent of the time, between beats).
Many individuals use headphones and earbuds not only for listening to music, but also for activities like working out, focusing, or altering their mood.
According to Google researchers in a blog post, “We have introduced a novel active in-ear health sensing modality. APG enables ANC hearables to monitor a user’s physiological signals, such as heart rate and heart rate variability, without adding extra sensors or compromising battery life.”
APG must adhere to safety regulations by staying 80 decibels below the set limit, remaining unaffected by seal conditions, and functioning effectively across all skin tones.
The sound quality when passively listening in the ear canal largely depends on how well the earbuds create a seal. It’s challenging to add health features to commercial ANC headphones that rely on passively picking up low-frequency signals.
“APG bypasses the aforementioned ANC headphone hardware constraints by sending a low-intensity ultrasound probing signal through an ANC headphone’s speakers,” explained Xiaoran “Van” Fan, Experimental Scientist, and Trausti Thormundsson, Director, at Google.
This signal generates echoes, which are captured by built-in feedback microphones. The heartbeat and minuscule skin movements in the ear canal alter these ultrasound sounds.
“The final APG waveform looks strikingly similar to a photoplethysmogram (PPG) waveform but provides an improved view of cardiac activities with more pronounced dicrotic notches (pressure waveforms that provide rich insights about the central artery system, such as blood pressure),” they clarified.
In their initial tests, the team observed that APG performed effectively even when the earbuds were not properly secured and music was playing.
“However, we noticed the APG signal can sometimes be very noisy and could be heavily disturbed by body motion. At that point, we determined that in order to make APG useful, we had to make it more robust to compete with more than 80 years of PPG development,” the team noted.
APG can transform any Truly Wireless Stereo (TWS) ANC headphones into intelligent sensing headphones through a straightforward software upgrade and is robust across a variety of user activities, the researchers emphasized.
