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 can be generated such that it will eventually be heard due to distortion as it passes through the atmosphere, and that the audible sound thus produced will be heard in only a small area, often a circle a couple of feet across.
There's an interesting article in Advertising Age magazine, but you need a paid subscription to read it. I have a copy of the article someplace but I don't have access to it at the moment. It talks about some of the things noticed with this technique, namely, the operators of the Audio Spotlight devices fucking with people while on the job, which isn't a terribly nice thing to do to someone. In my younger years I messed around with a similar device that was cheaper and had a much shorter range to good effect. You can, quite frankly, scare the almighty hell out of people with one of these. With a microphone run through a harmonizer or some audio processing software on a reasonably fast computer you can make people doubt their sanity and often get them to act irrationally in public places, ala a certain scene in the movie Real Genius. The two major problems are 0) not being seen by the people you're driving mad (so they can't find you and shove the device up your nose) and 1) aiming the device reliably. The latter problem was the one I ran into - considerable trial and error were required to resolve the situation during a time when laser pointers were frighteningly expensive (so no laser-designator-from-the-basement-lab). As for the sound that was used, I was never any good at playing God so I used track number thirteen from Peace and Love, Incorporated by Information Society.
Another question that should be asked is how much sound can be heard outside of the beam's zone of influence? The answer is, I don't know. The tuned sound cone speaker I rigged up when I was a kid had the annoying problem of leaking sound around the reflector used - when it was fired up you could hear the sound in question at the point of origin (sometimes quietly, sometimes not), even when it was packed in a box full of insulation, so to prevent the person sniped from putting two and two together a "hit hard, hit fast" technique had to be used - turn up the volume, nail them, kill the power, and duck. I would think that a corporate R&D lab with a budget several orders of magnetude than my own back then would have solved that problem by using ultrasound.