By design, smart speakers have microphones that are always on, listening for so-called wake words like “Alexa,” “Hey, Siri,” or “O.K., Google.” Only after hearing that cue are they supposed to start recording. But contractors hired by device makers to review recordings for quality reasons report hearing clips that were most likely captured unintentionally, including drug deals and sex.

Two Northeastern University researchers, David Choffnes and Daniel Dubois, recently played 120 hours of television for an audience of smart speakers to see what activates the devices. They found that the machines woke up dozens of times and started recording after hearing phrases similar to their wake words.

“People fear that these devices are constantly listening and recording you. They’re not,” Mr. Choffnes said. “But they do wake up and record you at times when they shouldn’t.”

Rick Osterloh, Google’s head of hardware, recently said homeowners should disclose the presence of smart speakers to their guests. “I would, and do, when someone enters into my home, and it’s probably something that the products themselves should try to indicate,” he told the BBC last year.

Welcome mats might one day be swapped out for warning mats. Or perhaps the tech companies will engineer their products to introduce themselves when they hear a new voice or see a new face. Of course, that could also lead to uncomfortable situations, like having the Alexa in your bedside Echo Dot suddenly introduce herself to your one-night stand.

‘No Longer Shunned as Loonies’

The “bracelet of silence” is not the first device invented by researchers to stuff up digital assistants’ ears. In 2018, two designers created Project Alias, an appendage that can be placed over a smart speaker to deafen it. But Ms. Zheng argues that a jammer should be portable to protect people as they move through different environments, given that you don’t always know where a microphone is lurking.

At this point, the bracelet is just a prototype. The researchers say that they could manufacture it for as little as $20, and that a handful of investors have asked them about commercializing it.