That is, data transmission using an ultrasonic (sound) carrier - a non-RF wireless link.

Google is not helping me here, because searching "Ultrasonic Data Transmission" just gives loads of stuff about transmitting the data obtained from (medical) ultrasound procedures!

Can anyone suggest any references on the subject, or better keywords to focus my search?

I have an idea along the lines of having the link running down a corridor (not necessarily straight), so that it would be broken if a door is shut - thus enabling the obstruction to be detected.

I'm looking for something more than just a simple "ping" - so that it could work with branched corridors, multiple doorways off the corridor, etc.

Something like Zigbee over ultrasound...!?