This works by placing the listener in the intersection of two ultrasonic beams. The beams are individually inaudible, but the difference in their frequencies can be an audible sound that is heard only inside this small region of intersection.
So it’s like set theory?
I don’t want to know how their are going to sell condoms with this.
The beat goes on.
Are you sure? (About the cross-section piece of your explanation...)
I read some time ago about how something like this worked in the laboratory.
When you play two frequencies together, you also produce the difference of those two frequencies, that is - so how it works is that they use ultrasound, say they play a 30,500HZ signal and a 30,000HZ signal - both of these tones are above the human range of hearing, but you will hear the difference - a 500HZ tone.
The added feature to the ultrasound method is that the audible tone(s) will not have a direction - like ultrasound, but will be easily audible - even though all tones transmitted physically are in the ultrasound spectrum and above the range of human hearing.
So they have built a simple speaker system to use this effect, and it's highly directional but requires only one transmitter, and not a 'cross-section' like stereo speakers would.
We did this back in the ‘70s with a fire control radar dish (power off). We could speak softly or whisper across the front of the dish, and people on the deck or pier over 200 feet away could hear the voice as if it were originating right beside them (or inside their head). All it requires is a correctly shaped dish pointed at the intended recipient.