This is done through the use of laser beams that can send these audio signals from a distance through the “photoacoustic” effect. The lasers will be absorbed by water vapor in the air which then forms sound waves. So there is the question of, what if a person happened to walk by the direction of the laser?
Based on their research, by sweeping the beam at the speed of sound, it created a sound that can only be heard at a specific distance from the transmitter. This means is that someone is close to the laser beam or further away, they wouldn’t be able to hear it. The researchers also claim that these lasers are safe so it’s not as if people will get burnt or blinded just by walking past it.
It is unclear what this technology could be used for, but for now it does seem like it would come in handy for spies or law enforcement who might be trying to get messages and signals to each other covertly.