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Phonetics of period doubling


The human voice is the most common `carrier' of speech, but how does linguistic voice quality affect speech production and perception? Typical `modal' voice possesses a single fundamental frequency (f0), identified as the voice's pitch. Period doubling, known as a commonly-occurring type of creaky voice, consists of alternating glottal pulses with different periods and/or amplitudes for which multiple fundamental frequencies (f0s) co-exist. Thus, the pitch during period doubling is often indeterminate, and so it is unclear whether linguistic tone is identifiable, and how linguistic tone is identified, in this voice. Although period doubling has been mostly studied in voice disorders and singing styles, it frequently occurs in non-pathological voices, and its defining characteristics remain to be determined.

This dissertation contributes three studies to characterize the physical, distributional, and perceptual aspects of period-doubled voice. Simultaneous electroglottography (EGG) and audio recordings of a Mandarin read speech corpus were analyzed to capture properties of the articulation and acoustics of period doubling in Chapters 2 and 3; artificial language learning and shadowing experimentation were used to probe perception of period doubling in Chapter 4.

The EGG study in Chapter 2 finds that period doubling is articulated as two alternating pulses with distinct pitches as well as voice qualities. Specifically, I show that the glottal cycles in period doubling are not generally constricted, but instead oscillate between degrees of constriction shown by alternating contact quotients, pulse shape, and speed of vocal fold contact. This in addition to the alternating frequencies likely leads to the indeterminate pitch and quality percept in period doubling. The results also pose challenges to the existing taxonomy of creaky voice subtypes based on the established acoustic attributes.

The acoustic analysis in Chapter 3 finds that period doubling is characterized acoustically via lower spectral tilt due to a stronger second harmonic from the original f0 (the first harmonics is derived from subharmonics), which distinguishes period doubling from vocal fry (another creaky-like voice quality) and modal voice. The results of the prosodic distribution show that, in Mandarin, period doubling occurs most frequently at the ends of utterances whereas vocal fry occurs at a post-focal position. This suggests that period doubling reflects vocal instability at the beginning and end of phonation, whereas vocal fry may be marking a weak prosodic element.

The perception study in Chapter 4 finds that both Mandarin and English listeners hear a `low-tone' during period doubling, which is driven by the strength of frequency modulation more than that of amplitude modulation. When frequency modulation is at extremes, pitch is heard unambiguously as a lower tone. When frequency modulation is weak, pitch is often heard as ambiguous -- both high and low tones are possible. Further, listeners are able to imitate the period-doubled tones not only by adjusting f0, but by also modulating their voice quality. It is predicted that period doubling is used to signal low tones and could interfere with perception of tone of a high pitch.

Together, this dissertation establishes period doubling not only as a phonetic category distinct from other voicing types such as modal voice and vocal fry, but also serves a distinct linguistic role based on its phonetic aspects and role in perception. The findings provide insight into speech production, perception, and processing, with implications for how period doubling can be synthesized and used to convey linguistic meaning.

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