1

In Henning Reetz and Allard Jongman's Phonetics, they mentioned a way to increase the loudness is to increase the speed of closing the vocal folds. The points are summarized below:

The the most effective way to increase the loudness is to increase the vigor of the vocal folds (which can be done by stronger elastic forces of the vocal fold) such that the vocal folds can be rapidly closed. The rationale is that the largest peaks in the waveform of a speech signal are caused by the rapidly closing vocal folds, not by their (slower) opening. The advantage of this way is that less air and energy are needed because the vocal folds are open for shorter periods. This method is used by trained speakers and singers and can be used unconsciously by speakers when they stress individual syllables of a word. And the amount of stress can be regulated much more finely by adjusting the speed of opening and closure of the vocal folds.

I don't understand why "the largest peaks in the waveform of a speech signal are caused by the rapidly closing vocal folds, not by their (slower) opening".

2

This picture may be useful (likewise this article). The thing of interest is the glottal flow derivative, modeled in this paper (fig. 2 has an analogous graph). The flow derivative is held to be the best model of the pressure wave (i.e. the source that is filtered by the vocal tract). The greatest amplitude excursion in the flow derivative is associated with the closing phase, which is fairly brief, and thus rapid. The rightward-leaning asymmetry in the wave shape has to do with glottal and vocal tract acoustic mass, delaying flow increase (when it increases, at the beginning of the cycle), plus rapid decay (when it starts to close). I follow the Liljencrants-Fant-Rotheberg rule on relating glottal flow derivative to the acoustic source: if they agree that that that is the source, I'm gonna believe them. (95% of Fant is way above my head).

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