Joe Pompei founded Watertown, MA-based Holosonics on what many acoustics experts called a crackpot idea: directing sound in a narrow beam. But Pompei says his attitude was, “this is too cool not to work.” He developed the initial technology in 1999 at MIT, the only school that gave him the green light on his PhD proposal. Within three months, he had a prototype of Audio Spotlight. The system includes a processor and a pizza-pan-sized transducer. First, the processor compresses the ling wavelength of an audible frequency into millimeter-long wavelengths of inaudible ultrasound. Then the transducer transmits the now tightly focused signal. In the air, the ultrasonic waves begin to elongate and regress to the original audible frequency, but only within the confines of a narrow beam. At distances of up to 200 meters, listeners in the beam’s path hear sound clearly, but those mere meters away from them do not. Dozens of groups have adopted the system: rock band U2 is experimenting with it to direct music at concerts; it’s used in museum and trade shows; and DaimlerChrysler installed it in Maxxcab truck models to provide personalized audio zones for passengers.