Effects of Contextual Predictability Clues in Speech Materials on the Korean Speech Intelligibility Index

Purpose: This study aimed to derive band-importance functions (BIFs) and transfer functions (TFs) according to contextual predictability clues to determine the influence of contextual predictability clues in Korean speech material on the speech intelligibility index (SII). Methods: This study was conducted on 156 native speakers of Korean who had normal hearing. Korean speech perception in noise test material, which was composed of 120 high-predictability and 120 low-predictability sentences, was used for stimuli. To obtain intelligibility data, participants were tested for intelligibility in various frequency ranges and signal-to-noise ratio conditions. In order to derive the BIF and the TF, a nonlinear optimization procedure using MATLAB (MathWorks, Inc.) was used. Results: The BIF derived from the high-predictability sentences showed a peak in areas of 700 Hz (7.0%), 1,850 Hz (8.5%), and 4,800 Hz (7.6%). The crossover frequency for the high-predictability sentences was around 1,370 Hz. The BIF derived from the low-predictability sentences showed a peak in areas of 570 Hz (7.5%), 1,850 Hz (9.3%), and 4,000 Hz (8.0%). The crossover frequency for the low-predictability sentences was around 1,600 Hz. In the case of the TF, the TF curves derived from high-predictability sentences were steeper than those derived from low-predictability sentences. Conclusion: In the SII model, speech intelligibility differs according to contextual predictability clues. Especially, the more contextual predictability clues at the identical audibility, the higher the intelligibility predicted by the SII. Therefore, accurate speech intelligibility prediction requires the use of SII considering the contextual predictability clues that are characteristic of the stimulus.