Measurements and Comparisons of Two Types of Combined Measures for Predicting Speech Intelligibility in University Classrooms

Abstract

The present study is a follow-up to previous paper titled “Prediction of changes to perceived reverberance due to added absorption by occupants in classrooms.” This work reports on the two types of combined measures, speech transmission index (STI) and useful-to-detrimental sound ratios (U50), for predicting speech intelligibility in university classrooms. Acoustical measurements were made in 12 occupied university classrooms. The measured impulse responses of the classrooms were used to determine the modulation transfer function, m(F), for the STI calculation according to IEC 60268-16. U50 values were determined by averaging octave band values from 125 to 4000 Hz and using the frequency weighting from the STI measure. The present results demonstrate that both measures, mean STI and frequency- weighted U50 values, were highly linearly related and essentially describe the same properties of the classrooms. The results showed that classrooms having U50 values of about +0.5 dB have STI values of 0.60, which indicates ‘Good’ acoustical conditions for speech intelligibility. The U50 measure can be more practically useful means of assessing and understanding real active classrooms.