A proportional scale, in which equal intervals (measured in mel) correspond to equal perceived interval sizes. The mel scale is roughly proportional to the logarithm of frequency but becomes linear at low frequencies.
the inverse of this is
m = pitch [Mel]
Mel scale versus Frequency [Hz].
S. S. Stevens & J. Volkman, "The relation of pitch to frequecy: A revised scale", Am. J. Psychol. 53:329-353