Not a joke, a spectrogram is where you split a sound signal into its frequencies and amplitude for each frequency using a Fourier transform. 2D images can be encoded that way, though they usually don't sound like anything significant.
It's become somewhat of a tradition among electronic musicians to embed graphics into spectrograms, the most widely known example being Aphex Twin's "Windowlicker" (1999). A few examples, including that one:
There must be precursors of the practice before the advent of computer music, or even digital/electronic music production altogether, but I can't recall any; I'd love to be pointed to them.
This is interesting, but fairly easy to confuse. Esp. would be interesting to see what results come up when you use modified "artistic" spectographs like that of Windowlicker by Aphex Twin [1]. One thing I've learned from years of having worked with audio and images is that image representations of audio are horrible representations of it (other than for temporal changes).
These days there are a number of audio synthesizers which do basically what you said, such as the Beepmap plugin that comes stock with FL Studio.
Among the first to use the technique artistically, Aphex Twin famously drew his own face in the spectrum of his track "Equation" (https://www.youtube.com/watch?v=wSYAZnQmffg - skip to 5:20 for the face).
I wanted to see where in the spectrogram the sound was as it was playing, so I threw together this quickie jsbin to make it happen: http://jsbin.com/amunug/10
Useful for anyone else who wants to be able to match the sounds and pictures, I hope!
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