Just Noticeable Difference

Just noticeable difference (JND): minimum physical change detectable by a human observer (see JND.pd).

Fechner:

• used just noticeable difference (JND) in his experiments;
• accumulated JNDs from the point where no change was detected up to a given stimulus level
• number of JNDs used to express the psychological level of a given stimulus
• found that the first approximation to his perceptually measured scales was a logarithmic scale.

Doesn't yield quite the correct answer for pitch.