Speakers work basically by physically creating waves in the air, using a membrane or other mechanism to pump the vibrations out, which means they disperse in an ever-wider arc. In contrast, Holosonic Research Labs Inc. and American Technology Corp. disrupt the air with ultrasonic waves at about 100,000 hertz, a frequency not even dogs can hear.
This enables the companies to transmit a highly focused sound somewhat like how a flashlight beams a focused light. A person can speak in a normal voice and be heard privately up to 600 yards away yet anyone more than a couple of feet to either side of the intended listener won't hear a thing.
Chris Marlowe, "Sound technology for your ears only," The Hollywood Reporter, July 23, 2003
[S]o-called directed sound may soon be accosting consumers in all kinds of real-life settings. A small San Diego company, American Technology Corp. (ATC), has already begun shipping a product called the HyperSonic Sound system [HSS], which uses inaudible ultrasonic waves to create vibrant sound out of thin air.
Arlene Weintraub, "Here you hear it, there you don't," Business Week, February 17, 2003
"To the Shareholders of American Technology Corp." Business Wire, March 25, 2002
It became known to the newspaper "Mercury News" that the above-mentioned "promising ideas" alongside hundreds of others were discussed at an air force "brainstorming" conference held at Dayton, Ohio, in 1985. ...
[O]ne of the proposals that was highly assessed by airforce representatives and recognised as deserving to be developed envisages creating generators of long-range directed sound beams which could cause panic among enemy forces.
Vladislav Orlov, "Militaristic projects of American futurists," TASS, February 10, 1987
However, when pressed for further examples these inventors don't take long to get around to the marketing possibilities, which generally include standing in front of a product at the supermarket and immediately hearing an ad for that product. See example citation #2 for more on this new marketing low that's both creepy and alarmingly Minority Reportish.