Spectrogram
A picture of sound: time along one axis, frequency along the other, and loudness as colour.
The audio is chopped into short, overlapping windows and each one goes through an FFT — a short-time Fourier transform. Window length is an unavoidable trade-off: long windows give sharp frequency resolution but smeared drum hits, short ones the opposite. The idea predates the computer; Bell Labs built an analogue sound spectrograph in the 1940s to study speech.
Today it is an everyday tool. Spectral editors let you erase a cough from a recording as if it were a smudge on a photo, ornithologists read birdsong from them, and anyone who has downloaded a "lossless" file can spot the sheer cliff at 16 kHz that betrays its past life as an MP3. In 1999 Aphex Twin hid his own face in the spectrogram of a track — visible only to those who looked instead of listened.