Production and perception of glottal stops

Author(s): Garellek, Marc | Advisor(s): Keating, Patricia A. | Abstract: This dissertation investigates how glottal stops are produced and perceived, and why they occur so frequently before word-initial vowels in languages of the world. Specifically, the goal of the production chapter was to determine whether glottal stops are truly glottal sounds. High-speed imaging of glottal stops uttered by five phonetically-trained English speakers was obtained using trans-oral videoendoscopy. When produced as plosives, glottal stops always had some form of vocal fold incursion, though full glottal closure is not always observed. Moreover, glottal stops were usually - but not always - accompanied by ventricular incursion. Glottal stops are thus necessarily glottal but not necessarily ventricular sounds. The timing of ventricular incursion suggests that it may be used to sustain, rather than produce, glottal closure.The specific goal of the perception study was to determine whether glottal stops are perceived distinctly from phrase-final creak, which shares similar acoustic features to glottalization. Sixteen English listeners were asked to identify words with glottal stops as allophones of /t/, e.g. 'button,' 'atlas,' and 'dent. ' They were also presented with near-minimal pairs with no glottal stop, e.g. 'bun,' 'Alice,' and 'den. ' The target words either had creak or no creak. The results indicate that words with glottal stop allophones are more accurately and more confidently identified when no creak is present, when the glottal stop is acoustically longer, and/or when the glottal stop is word-medial. Moreover, glottal stops are more accurately identified in creak when they occur in 'button'-type words, compared with worse identification in 'atlas'- and 'dent'-type words. For words with no glottal stops (e.g., 'bun,' 'Alice,' and 'den'), the presence of creak does not render them confusable with words with glottal allophones, except for 'den'-type words where word-final creak is sometimes mistaken for a glottal stop. Thus, glottal stops are generally harder to detect in creak, but are mutually confusable with creak only word-finally after nasals.To determine why and when glottal stops occur word-initially, the occurrence of word-initial full glottal stops in an English corpus was analyzed using logistic mixed-effects regression modeling. Prominence and phrasing are overwhelmingly the most important factors in predicting full glottal stop occurrence in English. Moreover, prominent word-initial vowels that are not preceded by full glottal stops show acoustic correlates of glottal constriction, whereas non-prominent phrase-initial vowels do not. Rather, phrase-initial voicing (even for sonorants) is less regular, but in a manner inconsistent with glottal constriction. These findings were subsequently confirmed using articulatory measures from electroglottography, and extended to Spanish. Based on the results, a prominence-driven theory of word-initial glottalization is proposed and motivated, with higher phrasal domains responsible only for the relative strength of the glottal stop gesture. Glottalization before word-initial vowels is thus a marker of prominence and is used amplify cues to prominence when they would otherwise be weakened.