An improved STI method for evaluating Mandarin speech intelligibility

Speech Transmission Index (STI) is a physical metric predicting the speech intelligibility. It shows a good correlation with the subjective intelligibility when used for English, but it has not been reported to be applicable to such tonal language as Mandarin. In this study, the traditional STI method is improved by adopting an accumulative normal distribution assumption between the signal-to-noise ratios and intelligibility, in which some characteristics of Mandarin speech are taken into considered. Results from psychoacoustic perceptual experiments show that the improved method is more suitable for objective evaluation of Mandarin speech intelligibility in noisy and reverberant listening conditions compared with the traditional method. The correlation coefficient between the subjective intelligibility and the objective STI is improved from 0.97 to 0.98; and the standard error decreased from 15.67% to 6.58%. This method is also expected to be applicable to other languages.