I found this interesting article in one of my Google searches:
A team of researchers at the Fraunhofer Institute for Communication (Germany) published in the Journal of Communications, a new prototype to exchange data between the computer systems which uses sound inaudible to humans, able to "carry" data (e.g. passwords, etc.), without need to be physically connected to an infrastructure network.
Using microphones and speakers, researchers reported that it is possible to transmit passwords and small amounts of information at a maximum distance of 20 meters (for ultrasonic frequency of 20 kHz).