Speech intelligibility with speech level at constant signal to Noise Ratio

A lot of research has been carried out to find the relationship between the speech intelligibility and some other room acoustic parameters such as signal to noise ratio, reverberation time, useful to detrimental sound ratio and speech transmission index. Although it has been found that the signal to noise ratio is the major impact to the speech intelligibility, there is still lack of a study about whether the speech level can influence the speech intelligibility under constant signal to noise ratio conditions. The present work measured the background noise level and reverberation time in the classrooms of Nanjing University as well as their transfer functions. The Standard Coordinate Response Measure corpus was selected as the speech intelligibility material and was then convoluted with the measured transfer function to stimulate the listening environment in the real classroom. The speech intelligibility test was conducted via earphones by playing back the processed corpus at different speech level and signal to noise level. The experiment results show that the optimal speech level is between 60 dBA to 70 dBA, and the speech intelligibility continuously rises with the signal to noise ratio. Low speech level results in inattention and listening difficulty, while high one can cause people annoyed.