WATERTOWN, Mass. — 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."

Holosonic Research Labs (ENLARGE)

An Audio Spotlight system at the New York Public Library allows visitors to hear audio from different TV screens while the surrounding area remains quiet.

Blue Blast Media (ENLARGE)

A&E promotes its show 'Paranormal State' in New York by directing whispers from a disembodied woman to pedestrians.

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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. That "directed sound" system is the work of Joseph Pompei, an engineer who founded Holosonic Research Labs Inc. in Watertown.

"There is an interesting perception of the sound being inside your head," Pompei said.

Thanks to the spread of digital video signs and pressure on advertisers to reach consumers in new ways, the business of directed sound has begun to take off for a handful of companies.

"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 sells the HyperSonic Sound system, which uses the same principles as the Holosonic product.

Norris said he has sold many units for use with video screens in checkout lines in Kroger- and Meijer-owned grocery stores so audio can reach waiting customers without constantly bombarding store workers.

Court TV used Audio Spotlight in 2006 in Manhattan bookstores and a few Atlanta locations to promote a murder mystery show.

Customers who tripped a motion sensor would suddenly hear a voice whispering a 30-second message. Part of it said: "Don't turn around. Do you ever think about murder? Committing the ultimate crime? I do. All the time."

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 both systems, the companies said. Holosonic products also have been used by the New York Public Library, the Boston Museum of Fine Arts and, recently, on the observation deck of the Seattle Space Needle.

"The people who benefit most from the Audio Spotlight are the ones not hearing it," Pompei said.

At the University of Texas at Austin, the Harry Ransom Center is using Holosonic devices in the art exhibit "Jess: To and From the Printed Page."

The center uses audio and video to help tell the story of manuscripts without resorting to lots of text labels, said Cathy Henderson, associate director of exhibitions. She said she prefers the Audio Spotlight's directed sound over exhibition speakers that hang in big plastic domes and allow a distracting cacophony to spill out.

Directed-sound devices from the two companies 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.

"We're literally making sound out of thin air," Pompei said.

Pompei's standard Audio Spotlight is a small amplifier connected to a thin, square speaker 16 inches on one side. The A&E billboard used two more powerful 24-inch models.

An HSS system costs about $700, while a single Audio Spotlight is about $2,000, the companies said. Prices for both decline with bulk purchases.

Pompei built his first prototype while a graduate student at the Massachusetts Institute of Technology and in 1999 founded Holosonic, which now has about a dozen employees.

He said some consumers have bought the Audio Spotlight for home use, and he foresees an eventual marketing push for residential users. Two people on the same sofa potentially could watch and hear different TV shows without disturbing each other, Pompei said.

Norris said his company has had interest from Asian television manufacturers about licensing the audio technology.

Both men say they want to bring the technology to cars so each passenger can have a personalized soundtrack.

Paul Hummel of Saddle River, N.J., installed an Audio Spotlight above his bed.

"We have two televisions in the bedroom, and my wife likes to listen a little louder than I like to hear when I want to go to sleep," Hummel said. He said he learned about directed sound technology in a magazine and then sought out Holosonic.

Hummel said the effect is not complete silence, since sound waves bounce off the bed, but he is thrilled with the result.

"I remember the show 'Get Smart,' " he said. "They had the 'cone of silence.' I dub this the 'cone of sound.' "

Competition in the small directed-sound industry has led to some static.

Dakota Audio in Bismarck, N.D., uses more traditional loudspeakers in a controlled array to achieve focused sound and other effects. The technology has been used at exhibits nationwide, including in the National Museum of the American Indian in Washington.

That company argues on its Web site that its products are superior to ultrasound beam technology, with better sound quality and control and no problems with sound beams ricocheting off objects.

Tensions are even higher between American Technology and Holosonic.

Both Pompei and Norris criticize the other's technology and business practices.

"I'm surprised those guys are still in business," Pompei said.

While not mentioning Pompei by name, Norris said the two directed-sound firms don't have the best relationship, but "we like him because when we find out who he's selling to, we go after them."