Mel scale
A pitch scale based on how far apart notes sound to the ear, rather than how many hertz separate them.
The jump from 200 to 300 Hz sounds large; the jump from 8,000 to 8,100 Hz barely registers. In 1937 Stanley Smith Stevens, John Volkmann and Edwin Newman had listeners adjust tones until one sounded "twice as high" as another, and derived a scale on which equal steps are meant to sound equal. The name comes from melody, and 1,000 Hz is pinned to 1,000 mels. Below roughly 1 kHz the scale is close to linear, above it close to logarithmic; the usual formula is m = 2595 · log10(1 + f/700).
The original experiments have been criticised as hard to reproduce, but the scale survives because it is useful. Audio and speech models space their filter banks evenly in mels, and a log-mel spectrogram is the standard input for everything from MFCCs to Whisper.