  The syllable is a perceptually salient unit in speech. Since both the
syllable and its acoustic correlate, i.e., the speech envelope, have a
preferred range of rhythmicity between 4 and 8 Hz, it is hypothesized that
theta-band neural oscillations play a major role in extracting syllables based
on the envelope. A literature survey, however, reveals inconsistent evidence
about the relationship between speech envelope and syllables, and the current
study revisits this question by analyzing large speech corpora. It is shown
that the center frequency of speech envelope, characterized by the modulation
spectrum, reliably correlates with the rate of syllables only when the analysis
is pooled over minutes of speech recordings. In contrast, in the time domain, a
component of the speech envelope is reliably phase-locked to syllable onsets.
Based on a speaker-independent model, the timing of syllable onsets explains
about 24% variance of the speech envelope. These results indicate that local
features in the speech envelope, instead of the modulation spectrum, are a more
reliable acoustic correlate of syllables.
