The team started with a laser beam that fired at wavelengths absorbed by water. By sweeping the beam at the speed of sound, they found that it could be used to generate sound that can only be heard at a specific distance from the transmitter. That would allow a message to be sent to an individual, rather than just anyone who crossed the path of the laser.
"This can work even in relatively dry conditions because there is almost always a little water in the air, especially around people," said research lead Charles M. Wynn. And if you're thinking that a laser at head level sounds like a bad idea, apparently it's fine. "It is the first system that uses lasers that are fully safe for the eyes and skin to localize an audible signal to a particular person in any setting," added Wynn.
The scientists also found a method that works by modulating, rather than sweeping, the beam. "There are tradeoffs between the two techniques," said researcher Ryan M. Sullenberger. "The traditional [modulation] method provides sound with higher fidelity, whereas the laser sweeping provides sound with louder audio." The system currently works at more than 2.5 meters (8 feet), so for the next step, the scientists plan to scale it up to longer distances. After that, "We hope that eventually a commercial technology will develop," Sullenberger said.