On the perception of imperfect periodicity

Imperfect periodicity in speech sound is the rule rather than the exception. Experiments are described that were set up to determine in what order of magnitude perturbations of perfect periodicity are relevant to the human ear. In the first place, it was found that stochastic imperfections (jitter) render pitch imperceptible when they become stronger than some ten percent of the pitch period. On the other hand, jitter ceases to be heard as such when it gets weaker than a few tenths of a percent. This threshold rises steeply, when the sounds are shorter than about one tenth of a second. It is also higher in sounds with a nonstationary pitch. We found that jitter is heard in such sounds when it exceeds the smallest systematic increments of the pitch period.