HyperSonic Sound
Description
Hypersonic Sound is sound that propagates in a beam like light rather than propagating in all directions like sound usually does. This means it's only detectable in a narrow area in front of the emitter. When an observer steps out of the beam of sound it appears as if the sound is no longer being emitted.
Hypersonic sound actually travels at ultrasonic frequencies, which are frequencies that are above what humans ears can detect. It's because of this that Hypersonic sound travels in a straight line rather than spreading out like lower frequency sound. (High frequency sound doesn't spread out) Sound forms complicated interference patters as it interacts with objects. That is how Hypersonic sound actually works. When the high frequency sound hits a person or some other object the high frequency sound waves interfere in such a way to create sound of audible frequencies at the observer. So to the observer the sound appears to be emanating from himself/herself.
Uses
Hypersonic Sound has many uses, but because it's a new technology it has not been implemented in very many areas yet. Possible uses could be advertising directed at particular sections in a store. Like announcing that apples are on sale to anyone who walks in front of them while someone a few feet away looking at the pears hears nothing at all. Using Hypersonic sound two people in a car could listen to different music without headphones. An automatic teller machine or a vending machine could send instructions directly to the person using the machine.
History
The US government worked on creating Hypersonic sound for use as a weapon for many years unsuccessfully. Elwood (Woody) Norris, owner of American Technology Corporation, announced he had successfully created a device which emits Hypersonic Sound in 2002. In December of 2002 Popular Science named Hypersonic Sound the best invention of 2002, beating out the Segway scooter. Commercial adaptation of the technology has still not occurred, leading to millions in losses by ATC. At about the same time Woody was developing his technology F. Joseph Pompei PhD (MIT) developed a similar technology, which he calls audio spotlight. Pompei launched his own company, Holosonics, which claims to have sold more than 1000 of their "Audio Spotlight" systems.