what is the mel scale?
To stand on @hotpaw2's answers, think of Mel as one kind of pyscho-acoustic scale, derived from a set of experiments on human subjects, others are Bark &amp; ERB

Why have such a scale? 


Imagine a 100 Hz sine wave playing in your head, .... wait for it to stick ...okay... Good.
Imagine now, a 200 Hz sine wave playing ... marinate in it. Good work.
Now compare the two, they have some "perceptual distance". Perceptual only because we're using your soft-grey matter as a measuring instrument.
Finally, if you were you repeat this experiment with 1100 Hz &amp; 1200 Hz, and again with 10,100Hz and 10,200Hz, your opinion of this percpetual distance might diminish, i.e. 100 &amp; 200 Hz can sound "farther apart" than do 10,100 &amp; 10,200Hz, even though in all cases the differences are equivalent to 100Hz.


These pyschoacoustic scales try to capture these distances from low (20Hz) to high frequency (20kHz).