Sounds convey information by changing in time, and by being different from one another, i.e., in their subjective representation. It is not a priori evident how such changes and differences can be adequately measured. In a number of experiments, subjects made estimates of the extent to which pairs of successive sounds were different. By and large, it turned out that it were not the basic auditory subjective scales, such as that of loudness and pitch, that optimally accounted for the differences, but the number of JNDs included in the differences. It was concluded that the subjective difference between sounds of any kind can be described by the number of JNDs that are included in the step from one sound to the other. This implies that the subjective difference between one particular pair of sounds is "the same as", or equivalent to, the subjective difference between any other pair of sounds, if the number of JNDs included in the two pairs is the same. This was termed the equivalence principle , ,  p.394-395, Suchowerskyj (1975a, 1977a, 1977b, 1977c).
Author: Ernst Terhardt firstname.lastname@example.org - Mar 1 2000