Bulgarian vowel reduction in unstressed position : an ultrasound and acoustic investigation

Vowel reduction in Contemporary Standard Bulgarian (CSB) has been variously claimed to involve raising, no change or lowering of the high vowels /iəu/. There is a general agreement that the low vowels /ɛaɔ/ are raised when unstressed. This paper directly measures tongue height using Ultrasound Tongue Imaging (UTI) and relates this measure to the acoustic correlate F1 at vowel midpoint. The six vowels of CSB were paired with respect to frontness (/ɛ, i/, /a, ə/, /ɔ, u/), and the overlap in height of the unstressed lower vowel in each pair was assessed relative to (a) its stressed counterpart and (b) the stressed and (c) unstressed realisations of the lower vowel. There was no evidence of the higher unstressed vowel in each pair being different from its stressed counterpart. The articulatory and acoustic results are not completely aligned, but both diverge from the traditional model of vowel reduction in CSB

Neural Network vs. Rule-Based G2P: A Hybrid Approach to Stress Prediction and Related Vowel Reduction in Bulgarian

An effective grapheme-to-phoneme (G2P) conversion system is a critical element of speech synthesis. Rule-based systems were an early method for G2P conversion. In recent years, machine learning tools have been shown to outperform rule-based approaches in G2P tasks. We investigate neural network sequence-to-sequence modeling for the prediction of syllable stress and resulting vowel reductions in the Bulgarian language. We then develop a hybrid G2P approach which combines manually written grapheme-to-phoneme mapping rules with neural network-enabled syllable stress predictions by inserting stress markers in the predicted stress position of the transcription produced by the rule-based finite-state transducer. Finally, we apply vowel reduction rules in relation to the position of the stress marker to yield the predicted phonetic transcription of the source Bulgarian word written in Cyrillic graphemes. We compare word error rates between the neural network sequence-to-sequence modeling approach with the hybrid approach and find no significant difference between the two. We conclude that our hybrid approach to syllable stress, vowel reduction, and transcription performs as well as the exclusively machine learning powered approach

The effects of English proficiency on the processing of Bulgarian-accented English by Bulgarian-English bilinguals

This dissertation explores the potential benefit of listening to and with one’s first-language accent, as suggested by the Interspeech Intelligibility Benefit Hypothesis (ISIB). Previous studies have not consistently supported this hypothesis. According to major second language learning theories, the listener’s second language proficiency determines the extent to which the listener relies on their first language phonetics. Hence, this thesis provides a novel approach by focusing on the role of English proficiency in the understanding of Bulgarian-accented English for Bulgarian-English bilinguals. The first experiment investigated whether evoking the listeners’ L1 Bulgarian phonetics would improve the speed of processing Bulgarian-accented English words, compared to Standard British English words, and vice versa. Listeners with lower English proficiency processed Bulgarian-accented English faster than SBE, while high proficiency listeners tended to have an advantage with SBE over Bulgarian accent. The second experiment measured the accuracy and reaction times (RT) in a lexical decision task with single-word stimuli produced by two L1 English speakers and two Bulgarian-English bilinguals. Listeners with high proficiency in English responded slower and less accurately to Bulgarian-accented speech compared to L1 English speech and compared to lower proficiency listeners. These accent preferences were also supported by the listener’s RT adaptation across the first experimental block. A follow-up investigation compared the results of L1 UK English listeners to the bilingual listeners with the highest proficiency in English. The L1 English listeners and the bilinguals processed both accents with similar speed, accuracy and adaptation patterns, showing no advantage or disadvantage for the bilinguals. These studies support existing models of second language phonetics. Higher proficiency in L2 is associated with lesser reliance on L1 phonetics during speech processing. In addition, the listeners with the highest English proficiency had no advantage when understanding Bulgarian-accented English compared to L1 English listeners, contrary to ISIB. Keywords: Bulgarian-English bilinguals, bilingual speech processing, L2 phonetic development, lexical decision, proficienc