The case would be if someone is recording you with a microphone placed somewhere unknown in the room. With the bracelet, and humans' tendency to move their arms while speaking, microphones around the room will be jammed at least part of the time.
Beam still has to be in the direction of the mic, which wouldn't help if your body is blocking most of the direct waves. You get some scattering, sure, but I think ultrasonic is pretty absorbable, right?
The bracelet does not have directional speakers, so the ultrasonic waves will kind of spread everywhere, bouncing off hard surfaces, and ultimately reaches the eavesdropping device.
