A spectrogram is stacked FFT frames drawn as an image, and it is the most information-dense standard picture of sound. Kick drums appear as columns, vocals as ribbed bands of harmonics, hi-hats as fine spray across the top.
Engineers read them to find problems that ears locate slowly: a resonance is a horizontal stripe, mains hum a line at 50 or 60 Hz, a lossy encode a cliff where high frequencies end.
The waterfall variant scrolls time continuously, which is the form visualizers borrow. Photism’s signal scene includes one, drawn in the palette rather than the classic false-color gradient, because the data is the aesthetic in that mode.