‘Sound beaming’ 3-D technology from Noveto Systems tracks ear and sends it audio using ultrasonic waves, creating personal listening pockets
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If you ever hear of someone stabbing people on the train because "voices in their head told them to", now you know how it can be done. Every phone tower has a electromagnetic version of this.
Are you going to wait until you are the one being stabbed? May be too late for you by then.