If you hear mysterious voices in your head the next time you stroll down the street, they may be trying to sell you something. That was the case recently in New York when people walking beneath a billboard for the A&E show Paranormal State" suddenly heard a woman's disembodied voice whisper: "Who's there? Who's there?" and "It's not your imagination." The creepy effect was caused by technology called Audio Spotlight that projects sound in a focused beam so only people in a certain spot can hear it. "The idea of directing sound was a real uphill battle when we first started, but all of a sudden people are coming to us saying, 'We have to have directional sound. We don't want all this noise in our store,' " said Woody Norris, founder of American Technology Corp. in San Diego. Norris said he has sold many units for use with video screens in checkout lines in ... grocery stores so audio can reach waiting customers without constantly bombarding store workers. While some of the advertising applications are recent, directed sound is often used in museums and other places where sound must be focused on people standing in front of an exhibit or display without disturbing those around them. Smithsonian museums in Washington have used [such] systems [as have] the New York Public Library, the Boston Museum of Fine Arts and ... the observation deck of the Seattle Space Needle. Directed-sound devices ... use narrow beams of ultrasound waves that can't be heard by human ears. The beam distorts air as it passes through, generating sound people can hear along its length.