Relation between sound absorption and speech intelligibility in room, part 1: Theoretical investigation

In Japan, requirements and design guidelines on sound absorption of a variety of ordinary spaces are currently under consideration. In general, sound absorption works for reverberation suppression and noise reduction, both of which contributes to improve speech intelligibility performance. However, its combined effect is not fully clarified, especially on the relationship among of reverberation time, average absorption coefficient and speech intelligibility indices. In this paper, two theoretical case studies are performed based on the formulations of the clarity C50 and the useful-to-detrimental ratio U50 in the conditions without and with background noise. For rectangular rooms with an aspect ratio of 1:2:4, those indices are calculated at three source-to-receiver distances, changing the room volume widely under different noise levels. The results demonstrated that the absorption coefficient required for a target C50 increases with increasing the room volume, but it decreases beyond a certain volume. Moreover, it was shown that under high background noise, U50 is not effectively improved by increasing the absorption coefficient if the communication distance is much longer than the critical distance.