There are millions of smart speakers out there, so even if you don't have one at home, odds are you encounter them on occasion. This prototype bracelet can ensure private conversations stay that way. It uses a ring of 24 speakers to emit ultrasonic signals in every direction. While the noise is imperceptible to most human ears, it will drown out any other noise heard by Alexa, Siri, Google, or any other ear-in-the-cloud. It was created by the husband and wife team of Ben Zhao and Heather Zheng, both computer science professors at the University of Chicago, after Mr. Zhao brought home an Amazon Echo to a disapproving Ms. Zheng. The bracelet remains a prototype for now, but the couple believes it could be manufactured for as little as $20.