Speech intelligibility and the subjective assessment of speech quality in near real communication conditions

"Subjective intelligibility", "concentration", "annoyance" and "coping" were examined as relevant speech quality features for describing the burden on a listener under adverse communication conditions. Test subjects with normal hearing were presented samples of spoken language (monosyllables, sentences) against a constant noise background with different signal-to-noise ratios. These language samples - spoken by native speakers and non-native speakers - were either modified in order to create additional different levels of difficulty or were presented together with a parallel task. The collection of speech intelligibility data (as a percentage of correctly reproduced speech stimuli) and subjective assessments of the four speech quality features on rating scales (from 1 to 5) revealed the following: Speech intelligibility and subjective assessment (on scales from 1 to 5) reflect the whole range of the communication situations, but despite high speech intelligibility the listener gives speech quality a lower rating. The assessment statement formulated for the four speech quality features have proven valid measuring instruments. The gradient of the curves for the assessments of the four speech quality features is still clearly positive under the simplest test conditions (signal-to-noise ratio of +10 and +15 dB), and so it can be supposed that the speech quality can be further enhanced appreciably. Conclusions are drawn for the quality of speech communication.