I was doing some research about Sound from Ultrasound. In short, this beams two inaudible ultrasound waves at your ear, which interfere with each other after hitting a solid (like you) to create an audible sound wave. In other words, only people standing directly in the beam can hear the sound. This has been marketed as "HyperSonic Sound" (HSS), such as the HSS-450, and the Soundlazer.
There has also been some work with using microwaves and radio waves to create a similar effect, but apparently the energies are too high to be safe for humans.
In 2008, there was some concern this technology would be used for obtrusive advertising.
From what I can tell, this field of research looks pretty dead. The Soundlazer only exists because of a Kickstarter, after all. In 1997, this was expected to become the Next Big Thing. Most of the information I can find about this technology is 10-15 years old, which makes me think there must have been some major drawbacks to this technology. What were they?
Its current use seems to be limited to creating a cone of sound around museum exhibits. What are the drawbacks that keep it from being used for live production audio more frequently? For example, a venue where you have multiple productions at the same time, and don't want the sound to overlap.