Speech intelligibility in multilingual spaces
Abstract
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.