Semitone scale

Definition
Twelve semitones form one octave, which is defined as a doubling of the fundamental frequency. As the human auditory system perceives tones in a logarithmic way (instead of in a linear way), the logarithmic semitone scale is a more appropriate measure of the perceptual consequences of differences in fundamental frequency. The following conversion transforms frequency (F) values into semitones (ST):

ST=39.87*log(F/50)

Links
Utrecht Lexicon of Linguistics