https://www.sciencedaily.com/releases/2021/06/210609084706.htm

But now, a team of University of Michigan researchers has developed a system that can inform a smart home -- or listen for the signal that would turn on a smart speaker -- without eavesdropping on audible sound.

The key to the device, called PrivacyMic, is ultrasonic sound at frequencies above the range of human hearing. Running dishwashers, computer monitors, even finger snaps, all generate ultrasonic sounds, which have a frequency of 20 kilohertz or higher. We can't hear them -- but dogs, cats and PrivacyMic can.