Activate This ‘Bracelet of Silence,’ and Alexa Can’t Eavesdrop

Last year, Ben Zhao decided to buy an Alexa-enabled Echo speaker for his Chicago home. Mr. Zhao just wanted a digital assistant to play music, but his wife, Heather Zheng, was not enthused. “She freaked out,” he said.

Ms. Zheng characterized her reaction differently. First she objected to having the device in their house, she said. Then, when Mr. Zhao put the Echo in a work space they shared, she made her position perfectly clear:“I said, ‘I don’t want that in the office. Please unplug it. I know the microphone is constantly on.’”

Mr. Zhao and Ms. Zheng are computer science professors at the University of Chicago, and they decided to channel their disagreement into something productive. With the help of an assistant professor, Pedro Lopes, they designed a piece of digital armor: a “bracelet of silence” that will jam the Echo or any other microphones in the vicinity from listening in on the wearer’s conversations.

The bracelet is like an anti-smartwatch, both in its cyberpunk aesthetic and in its purpose of defeating technology. A large, somewhat ungainly white cuff with spiky transducers, the bracelet has 24 speakers that emit ultrasonic signals when the wearer turns it on. The sound is imperceptible to most ears, with the possible exception of young people and dogs, but nearby microphones will detect the high-frequency sound instead of other noises.

“It’s so easy to record these days,” Mr. Lopes said. “This is a useful defense. When you have something private to say, you can activate it in real time. When they play back the recording, the sound is going to be gone.”

During a phone interview, Mr. Lopes turned on the bracelet, resulting in static-like white noise for the listener on the other end.