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Hearing Things

One thing Pompei and Norris have in common is this: each quickly learned that in chasing directional sound he was heading down a path others had trod before. Norris says he first got the idea for what has become his Hypersonic Sound system in the late 1970s. He had recently raised money by selling stock in one of his startups and, flush with cash, was eagerly looking for his next big thing. “I was ignorant and nave enough to think I was the first person to think of this,” he says. But as he researched old patents and publications, “names came out of the wall. People have been trying this for fifty years.” Pompei’s own research revealed that major Japanese corporations had looked into the concept in the early 1980s but abandoned their quest, suspecting that the technique would produce distorted sound or require too much power to be of any use.

Both Norris and Pompei believe they have solved most of the problems that stymied their predecessors. Each of their systems contains a signal processor, an amplifier, and a platelike device that shoots out beams of ultrasound. At his suburban San Diego laboratory, Norris plugs his Hypersonic Sound emitter into an ordinary portable CD player for a demonstration outside in the parking lot. When he points the device at a visitor standing about 20 meters away, the visitor is able to clearly hear sounds ranging from a waterfall to jazz music. But when Norris points the emitter to the left or right or up to the sky, the visitor hears no sound at all. Inside the lab’s conference room, Norris directs the emitter at walls, bouncing the sound beams so that they seem to be coming from spots on the wall, rather than from the device itself.

It’s easy to see why directional sound is cool, but understanding how it works is far tougher. Traditional speakers generate audible sound waves that spread out in all directions like ripples from a pebble tossed in a pond. Norris’s and Pompei’s devices instead generate narrow, laserlike beams of ultrasound waves, which have a frequency above 20,000 hertz, the upper limit of what the human ear can detect. Both audible sound waves from traditional speakers and ultrasound waves from a directional-sound system distort when they travel through the air; in a traditional sound system, the distortion slightly degrades the sound a listener ultimately hears. But in a directional-sound system, the distortion is actually the mechanism that generates the audible sound, breaking the ultrasound waves into lower-frequency, audible sound waves along a straight, narrow path.

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