The harmonic structure, such as speech, can be divided into two parts of envelope maxima and minima. The former carries semantic information; the latter is regarded as inaudible period according to the forward masking effect. In this study, we recalculate phase error and amplitude error of minute extrema between envelope maxima, and visualize the robust structure of amplitude envelope in the light of noise allowance, corresponding to subjective thresholds. Compared with 9% phase error and 40% amplitude error in the previous study, the allowances of both indices are quite large in the concave part of amplitude envelope, estimating at 14% and 100%, respectively. Taken together with the adaptation of firing rate in the auditory nerve fibers, we have hypothesized that the marginal parts play a physiologically indispensable role in recovery from the refractory period in the auditory nerves, allowing the modification of silence with zero-amplitude.
