from a youtube commenter: “The device works by sending two ultrasound beams. One has the carrier wave and the other has the modulating wave. People don’t normally hear either, but when they cross, they add up and the difference of the two signals becomes audible e.g. 48 kHz + 46 kHz produces what is called a beat frequency of 2 kHz which people can hear.”
More info here: “The last key property of HSS is its ability to focus sound. The ultrasonic waves don’t dissipate, traveling pretty much straight ahead. So sound from an HSS speaker can travel 150 yards without distortion or loss of volume, while anyone outside the path hears nothing.”
Company site here
thanks for the tip Michael!
Funny, I just came across this concept a few weeks back, in a novel of all things.
http://en.wikipedia.org/wiki/Daemon_(novel)