For a couple of weeks now, people in major cities like New York City and Los Angeles have been experiencing something far more unusual: Voices in their heads that suddenly cause them to look up at billboards. It isn't auditory hallucinations causing this but snipers armed with tight-cone directional sonic projectors aiming recorded sounds at people on the street as part of an advertising campaign for a show on A&E called Paranormal State. The device in question is called Audio Spotlight from Holosonics and involves the use of carefully tuned ultrasonic speakers. The principle behind this is that ultrasound …