My friend and i were discussing this device with which inly one person can hear what you say. It doesn't have much practical application, but still: we have a device, an oval disk or something which surrounds your lips but doesn't cover your mouth, and we place ultrasonic sound emitters on it. You then point your mouth to someone's ear, and then speak, then would it happen that only one person hears what you say? Would the ultrasonic waves confine the sound waves you emit? And would this have any practical application? i know the drawings are horrible.