I find this really interesting as I built an augmented reality headset that used only sound as output to the wearer as an undergrad project. It tracked the wear

I find this really interesting as I built an augmented reality headset that used only sound as output to the wearer as an undergrad project. It tracked the wearers orientation an position as well as using ultrasonic to detect obstacles. It then used 3D audio to convey this information to the wearer. It was built as a framework on top of which multiple applications could be built and three demos that we (I did this project together with another guy) made were: a guidance application that used 3D positioned audio as waypoints; a virtual band playing music so you could walk between the instruments as if the band were playing in the room and finally an application that used the ultrasonic sensor to detect walls. The wall detection would play a number of blips in quick succession and the tempo was relative to the distance to the obstacle. The cool thing was that you could turn your head and get a general sense of the shape of the room by how the blips sped up or slowed down as you scanned along the wall.