Researchers at the University of Chicago have developed a bracelet which can be used to jam microphones in a number of mainstream consumer devices.

The jammer employs 24 speakers which are pointed in multiple directions and transmit ultrasonic sound to interfere with the audio recordings on microphones.

“Our device is based on a recent exploit that leverages the fact that when exposed to ultrasonic noise, commodity microphones will leak the noise into the audible range,” the researchers said.

According to the team, this leakage is the result of the inherent non-linear property of microphone hardware.

“While these ultrasonic signals are imperceptible to human ears, they leak into the audible spectrum after being captured by the microphones, producing a jamming signal inside the microphone circuit that jams (disrupts) voice recordings.”

Better than stationary jammers

The team claimed the build and use-case scenarios for their device offer an advantage over stationary ultrasonic jammers.

While a stationary device can block out microphones, it is extremely directional and requires the user to point the jammer straight at the microphone’s location to work.

To create larger jamming coverage, several devices are needed. However, where the signals from two or more jammers overlap, they cancel each other out, preventing the microphones from being blocked out and creating a blindspot.

The team explained that the omnidirectional placement of the speakers, and synergy between ultrasonic jamming and the natural movements of the wearer – such as walking or gesturing – helps the bracelet address these inefficiencies.

“We demonstrate that these movements can blur jamming blind spots and increase jamming coverage,” the researchers said.

The bracelet’s functionality was demonstrated in a video, which showed it blocking out the microphones of a smartphone, smartwatch and smart speaker during a conversation.