Acoustic correlates of the syllabic rhythm of speech: Modulation spectrum or local features of the temporal envelope

TL;DR

This study investigates the acoustic features of speech envelopes related to syllable rhythm, finding that local envelope features are more reliable than modulation spectrum in correlating with syllable timing, with implications for neural speech processing.

Contribution

The paper demonstrates that local features of the speech envelope, rather than the modulation spectrum, better correlate with syllable onsets across large speech corpora.

Findings

Speech envelope's center frequency correlates with syllable rate over minutes.

A component of the speech envelope phase-locks to syllable onsets.

Syllable onsets explain about 24% of variance in the speech envelope.

Abstract

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.