Once I saw a demo where multiple speakers were outputting inaudible sounds (I assume very high frequency) that would interfere at certain locations to be audible. It has fascinated me for some time but I can't figure out how it works. Right now I have some use for something similar, so I guess it's time to understand.
Can I output two audio signals (either inaudible or appearing to be something else) that, when superposed, will interfere to create a third? Imagine, say, some music, where each channel on its own seems normal but when both channels are played together you hear a secret message (ooooh spooooky).
I know some basics of Fourier analysis and DSP, enough to find my way through with a few pointers on how this technique works, if it exists.