each 'reference' tone (X axis in the diagram), there is a frequency difference
value (called Discrimination Frequency, 'Discrim. Freq.' label in the diagram),
under which the second tone is not distinguished from the reference tone; for example, the semitone difference (the blue
curve in the diagram) in the dodecaphonic system (12 semi tones Equal
Temperament) is, for each reference tone, under the 'Discrimination Frequency'
curve and can therefore not be detected.
In addition, for each 'reference' tone, there is a second important frequency difference value, called 'Critical Band'; if the frequency difference is below 25% of the Critical Band the two tones are judged 'clearly dissonant'; if it is between 25% and 50% of the Critical Band (the sky blue curve in the diagram) the two tones are judged 'partially dissonant'; if it is more than 50% of the Critical Band, they are judged 'consonant'; for example the tone difference (the violet curve in the diagram) in the dodecaphonic system is 'consonant' for reference tone frequencies > about 500 Hz, where as the minor third difference (the yellow curve in the diagram) in the dodecaphonic system is 'consonant' for reference tone frequencies > 300 Hz.
With permission by Francesco Caratozzolo, Electronic Engineer (Ph.D. in Bio-Engineering).