Hearing with Ultrasound using a Neurophone
 

By mixing an audio signal with ultrasound, you can hear the audio as if it's inside your head... even if the 'headphones' are nowhere near your ears. This even works if the "audio" signal is itself ultrasound, thanks to the principle of heterodyning.

The device is called a Flanagan Neurophone, and I posted the whole writeup here:
Hear with Ultrasound Using a TL494 as a Flanagan Neurophone ? DIY How-to from Make: Projects
some electronic projects

Enjoy!