How they work
These emit ultrasonic sound above the range of human hearing. Microphones pick it up and it overwhelms their input, so audio recorded in the room comes back unintelligible. People in the room carry on talking normally, which is what makes it practical for a conversation you need to keep private.
The approach is preventive rather than investigative. Rather than sweeping a room before every meeting and hoping you found everything, you make recording ineffective regardless of what is present or where it is hidden.
Choosing one
Coverage area decides which model suits the room, and several are built into ordinary objects, including speakers, power banks, tissue boxes and car air fresheners, so they can sit on a boardroom table or in a vehicle without drawing attention. Portable units suit meetings in changing locations.
Related
To find a device rather than defeat it, see listening device detectors and spy camera detectors.
Common questions
Can people in the room hear it? No. The output is above the audible range for adults, though some people report a faint sensation of pressure.
Does it affect phone calls? A live call in the room will be affected the same way a recording would.