For Their Ears Only - New York Times

For Their Ears Only - New York Times: "The device, officially called the Audio Spotlight, was invented in the late 1990's by Joseph Pompei, president of a company called Holosonics; at the same time, the American Technology Corporation invented a rival speaker called HyperSonic Sound.
Directional speakers produce ultrasound: sound waves with wavelengths of a fraction of an inch that travel through the air in a narrow, columnlike beam. The ultrasound in turn vibrates the air, literally like a speaker, creating audible sound, which has wavelengths ranging from about half an inch to more than 50 feet. If you stand in the path of the beam, you feel as if you are wearing headphones."