UltraData
Phones sending data to each other over near-ultrasound. No network, no pairing.
UltraData sends data as sound at about 18 to 22 kHz. That’s above what most people can hear, but still within what phone speakers and mics can handle. Any nearby phone that’s listening picks it up.
The first use is a text messenger. You set a tag, type a message, and it goes out over the speaker as modulated audio. Part of the project is comparing on/off keying with two-tone FSK.