Smart speakers and other microphone-equipped devices aren't supposed to listen all the time, but there might be a solution if you aren't willing to take any chances. University of Chicago researchers have built an experimental bracelet that uses ultrasonic broadcasts from 24 speakers to jam most microphones, no matter what direction they're in. The gadget takes advantage of non-linearities in its built-in amplifier to 'leak' ultrasonic noise into the audible range and render recordings useless.
The bracelet design isn't for the sake of fashion. It not only ensures omnidirectional jamming, it eliminates blind spots (where transducers cancel each other out) through your wrist movement. As a result, it's more effective than dedicated stationary jammers and can even scramble hidden mics.
You're not about to use the prototype bracelet. It's more than a little clunky, and the internals amount to a handful of boards and a battery. The scientists told the New York Times that investors have asked about commercializing the technology, though, and it's estimated that you could build this anti-mic bracelet for roughly $20. This could be viable for anyone who fears eavesdropping from voice assistants or spies, especially for people who tend to move from room to room.
Whether or not you'd see this widely available is another matter. There are a number of ethical concerns. If you wore this in public, you could play havoc with phone calls and other mic-dependent devices. And while this could help keep business meetings secret, it could also help politicians avoid accountability. As useful as the bracelet could be, it could prompt legal issues if it isn't used responsibly.