Headphones can be made with built-in microphones, touch-sensitive buttons, motion sensors and even heart-rate monitors. But researchers have found a way to bring all these features to any existing pair of standard headphones, regardless of age or cost. The technique could replicate the advantages of smart headphones without the cost and environmental impact of producing new devices – and they could even be used to eavesdrop on a conversation or verify the identity of the wearer.

The tiny speakers inside headphones convert electrical signals from a smartphone or laptop into sound waves that we can hear. But the process can also work in reverse. Vibrations near the speakers – such as those from a heartbeat or from speaking – create electrical signals. Xiaoran Fan at the Samsung Artificial Intelligence Center in New York and his colleagues have leveraged this effect to measure a wearer’s heart rate and count their steps.

By playing a short burst of sound, it can even identify the wearer by analysing the reflections from within their uniquely shaped ear canals.

The signals created using this technique are extremely weak compared with audio signals coming from a smartphone or computer and could be easily lost among the noise. So the team created a small electrical circuit to filter out the incoming audio signal and allow the fluctuations to be recorded while still letting the wearer listen to music.

Read more: Smart speakers could hear your heart beating from across the room

This circuit can be built with nothing more than two resistors. It uses no power and could be incorporated into a smartphone or built into an adapter for use with existing phones at a cost of just 50 pence. Fan says that simple signal processing was then carried out on a computer to extract information, but that any standard smartphone would be capable of the same task.

The system achieved more than 96 per cent accuracy when identifying the wearer among a group of 27 participants. The heart rate of the wearer was detected with more than 96 per cent accuracy when compared with a pulse oximeter. The researchers were also able to detect two different gestures, a tap and a stroke, on the outside of the headphones with more than 98 per cent accuracy.

What’s more, they demonstrated that audio of the wearer’s speech could be extracted even if they were listening to music. This means that headphones without microphones could be used for two-way phone calls.

Fan says that advanced headphones offering these features are more costly than standard headphones and often aren’t as long-lasting. “The trouble is the batteries have a two or three-year lifespan, like after two or three years you have to replace them with another one,” he says. “But these standard headphones are just purely passive electronics. They’re built to last and there are so many of them.”

Reference: MobiCom ’21: Proceedings of the 27th Annual International Conference on Mobile Computing and Networking, DOI: 10.1145/3447993.3448624