Hypersonic Sound
Warren Ellis has a note about the Hypersonic Sound Beam that is being developed by the American Technology Corporation. References a 2003 USA Today article about the technology. Basically, ATC is developing a means of ultrasonic sound propagation that allows for delivery of sound to a specific point and only to that point. It's like wearing headphones but without being connected in any way with the source of the transmission.
Those kids at ATC are busy. "American Technology Corporation is Shaping the Future of Sound® by developing and licensing its technology and intellectual property portfolio which includes: the award-winning HSS® (HyperSonic® Sound Technology); NeoPlanar® Technology; PureBass® Sub-Woofer Technology; HIDA™ (High Intensity Directed Acoustics), LRAD™ (Long Range Acoustic Device) and SFT® (Stratified Field® Technology)."