The Lombard effect on production and perception of Northern Vietnamese tones

Le, Giang

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https://hdl.handle.net/2142/124307 Copy

Description

Title

The Lombard effect on production and perception of Northern Vietnamese tones

Author(s)

Le, Giang

Issue Date

2024-04-22

Director of Research (if dissertation) or Advisor (if thesis)

Tang, Yan

Doctoral Committee Chair(s)

Tang, Yan

Committee Member(s)

• Shih, Chilin
• Shosted, Ryan K
• Ionin, Tania

Department of Study

Linguistics

Discipline

Linguistics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Lombard speech
• tones
• Northern Vietnamese
• tone production
• tone perception
• speech in noise

Abstract

In adverse listening conditions, human talkers spontaneously adapt the way they speak to maximize the success of communication due to the Lombard effect. Lombard speech is observed to have many changes at acoustic level over normal speech produced in quiet, such as raised F0 and overall intensity, reduced spectral tilt, expanded vowel space as well as elongated vowel duration. While the Lombard effect on speech production has been studied quite extensively, not much research has been done about the Lombard effect on lexical tones. This dissertation attempts to fill the gaps in the literature by investigating Lombard speech of Northern Vietnamese, a language variety with rich tonal contrasts by not only pitch cues but also voice quality cues, by presenting a collection of acoustic analyses, laryngographic analyses, and a perception study. The first study investigates the Lombard effect triggered at different noise levels on the F0 contours of tones in Northern Vietnamese. An acoustic analysis and same-tone comparison of speech produced in quiet, 78 dB and 90 dB noise presents evidence for a lifting of the contours' central value and an exaggeration of F0 contours due to the Lombard effect. A reduced Lombard effect at the highest noise level, as measured by decreasing or no-change mean F0, F0 range, and F0 slope at 90 dB SPL, is detected in many significant contrasts. My work on F0 modifications contributed uniquely to our understanding about Lombard speech due to F0 changes as both indicators of signal-based changes and code-based changes. The findings showed that the falling creaky tone B2 resisted the Lombard effect of raising F0 for female speakers, a counter result from previous findings. The rising creaky tone C2 exhibited hyper-articulation by way of the greatest increase in the mean F0, but also demonstrated a reduced F0 range due to reduced creakiness. The findings suggested that the Lombard effect is likely modulated by tone intelligibility due to its varying effects by tone. Consequently, my study provides support for the hypothesis that some aspect of the Lombard effect is not automatic and speakers might adjust articulation from perceived intelligibility outside of an interactive context. By examining the Lombard effects on three elicited vowels, we found that different vowels induced different degrees of formant changes in noise: F1 increase was not observed consistently across all three conditions for the low vowel /a:/ and F2 changes differed between back vowel and non-back vowels. Given that formants are resonances intensified by filtering effects in the supra-laryngeal vocal tract, our findings suggested that filtering effects were impacted by hyper-articulation in varying degrees in Lombard speech. A reduced vowel space suggested that like loudness, an expanded vowel space was not always an automatic response to noise for better intelligibility. In fact, the vowel space analysis challenged the description of Lombard speech as "hyperspeech", for a contracted vowel space, a hypo-articulatory characteristic, was demonstrated. The high number of phonemic tonal contrasts in the language calls for greater intelligibility of Lombard speech, which was achieved by tighter clustering rather than an increased vowel area. From the time domain Lx signals, jitter was found to decrease at higher noise levels while the mean harmonics-to-noise ratio increased. Prior hypotheses predicted that creaky tones might demonstrate different degrees of glottal vibratory changes compared to the modal tones. Interaction effects were detected between tone and noise levels, and a detailed examination showed that differences in Lombard effects were even found within creaky tones themselves. Furthermore, the laryngographic spectral tilt flattened at increasingly higher noise levels in the frequency domain, analogous to the flattening of the spectral tilt of the acoustic signals. These findings point towards reduced creakiness in the voice quality of speech produced in noise, and provide evidence that hyper-articulation starts at the sound source. In order to investigate the similarity between tone pairs, I extracted the essential voice quality dimensions from the original 26 features by utilizing the PCA, resulting in orthogonal principal components helpful for a downstream classification task. Four principal components representing harmonicity of the voice, F0, duration, and voice break quality were retained for further analyses. A support vector machine model was applied on the extracted principal components in order to determine which tone pairs are similar to one another given their acoustic features. The classification performance for tokens produced in noise was better than the performance for tokens produced in quiet, suggesting better separability of tones when produced in noise. This finding enhanced the hypothesis about better intelligibility of Lombard tones experienced by human listeners. The perception study with human listeners showed better identification performance of tones produced in noise compared to tones produced in quiet, confirming enhanced intelligibility of Lombard tones. Tone confusability can be partly explained by acoustic modifications and partly dialectal background.

Graduation Semester

2024-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2024 Giang Le

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