Impact of University Classroom Size on the Relationship between Speech Quality and Intelligibility

In this paper, five objective measures of the quality of speech signals distorted by reverberation are compared with the Speech Transmission Index (STI). The main aim of the comparison is to further test and explain the reasons for the previously discovered phenomenon of an increase in the speech quality and intelligibility with increasing room size. The comparison is performed for three university classrooms of small, medium and large sizes. The correlation coefficients between the quality and intelligibility estimates of speech obtained for 5-6 points of each room are estimated. Speech signal quality is assessed using intrusive measures such as segmental signal-to-noise ratio (SSNR), log-spectral distortion (LSD), frequency-weighted segmental signal-to-noise ratio (FWSNR), bark spectral distortion (BSD), and perceptual evaluation of speech quality (PESQ). For BSD, high correlation coefficients (0.57-0.99) are determined for rooms of all sizes and an increase in the correlation coefficient with the room size increase is found, which can be explained by a decrease in the density of early sound reflections. For FWSNR, high correlation (0.65-0.98) is determined for medium and large rooms. For PESQ, high correlation (0.96-0.99) is obtained for large classroom. SSNR and LSD are found to be uncorrelated with STI for rooms of all sizes.