Applicability of the theoretical formulations of C50 and U50 for predicting speech intelligibility in lecture halls

Theoretical formulations of the clarity C50 and the useful-to-detrimental ratio U50 for speech intelligibility have been derived under the assumption of diffuse sound field, which could be useful for room acoustic design. However, their applicability to a variety of daily rooms is still unclear. In this study, room impulse responses are measured in several lecture halls of various sizes with and without a sound reinforcement system, and the distributions of C50 and STI are obtained in each lecture hall. Then the theoretical values of C50 and U50 with and without Barron's revision are compared with the measured values, considering a level of HVAC background noise. Additionally, the relationship between the measured STI and C50 or U50 is examined, and the proportion of seats in each grade of speech intelligibility is evaluated in each lecture hall.