Data over Sound
Send a text or a small file to a nearby phone or computer as a sound, audible or near ultrasound: a speaker, a microphone, no network
Or send a small file, up to 2 KB once compressed
Band
Speed
Open this page in Send mode on the other device, then listen here. Keep the two within a meter or so, with the sender's volume up: both bands are listened to at once, and a message is read whole once its last tone is heard.
Or read a recording: a WAV made here, a voice message…
Signal
Audible
Near ultrasound