http://www.atcsd.com/tl_hss.html
Since the sound that we hear is created right in the column of ultrasonic energy, it does not spread in all directions like the sound from a conventional loudspeaker, instead it stays locked tightly inside the column of ultrasonic energy. In order to hear the sound, your ears must be in line with the column of ultrasound.
Hypersonic Sound. Kinda redundant (“hypermusical music?”), but the concept is actually cool. Besides the examples given in the site I was thinking on an installation where people would wander around an empty space with hidden speakers placed in the ceiling, pointing to spots in the floor and changing positions slowly over time. It would be interesting to see how people react for the first time to these unexpected sounds, and how they do interact with them (for instance, if they try to catch up with them as they move, or try to find new ones). I wonder if we would get accustomed to the effect (I mean, when not warned of the sweet spot) if this technology ever got into everyday life: walking down the mall could be like crossing a “sound mined” field. And I wonder what would happen if two sound beams crossed in space…