Speech intelligibility in multilingual spaces
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This thesis examines speech intelligibility and multi-lingual communication, in terms of acoustics and perceptual factors. More specifically, the work focused on the impact of room acoustic conditions on the speech intelligibility of four languages representative of a wide range of linguistic properties (English, Polish, Arabic and Mandarin). Firstly, diagnostic rhyme tests (DRT), phonemically balanced (PB) word lists and phonemically balanced sentence lists have been compared under four room acoustic conditions defined by their speech transmission index (STI = 0.2, 0.4, 0.6 and 0.8). The results obtained indicated that there was a statistically significant difference between the word intelligibility scores of languages under all room acoustic conditions, apart from the STI = 0.8 condition. English was the most intelligible language under all conditions, and differences with other languages were larger when conditions were poor (maximum difference of 29% at STI = 0.2, 33% at STI = 0.4 and 14% at STI = 0.6). Results also showed that Arabic and Polish were particularly sensitive to background noise, and that Mandarin was significantly more intelligible than those languages at STI = 0.4. Consonant-to-vowel ratios and languages’ distinctive features and acoustical properties explained some of the scores obtained. Sentence intelligibility scores confirmed variations between languages, but these variations were statistically significant only at the STI = 0.4 condition (sentence tests being less sensitive to very good and very poor room acoustic conditions). Additionally, perceived speech intelligibility and soundscape perception associated to these languages was also analysed in three multi-lingual environments: an airport check-in area, a hospital reception area, and a café. Semantic differential analysis showed that perceived speech intelligibility of each language varies with the type of environment, as well as the type of background noise, reverberation time, and signal-to-noise ratio. Variations between the perceived speech intelligibility of the four languages were only marginally significant (p = 0.051), unlike objective intelligibility results. Perceived speech intelligibility of English appeared to be mostly affected negatively by the information content and distracting sounds present in the background noise. Lastly, the study investigated several standards and design guidelines and showed how adjustments could be made to recommended STI values in order to achieve consistent speech intelligibility ratings across languages.