Assessment of speech intelligibility during different teaching activities in classrooms with and without acoustic treatment

•The recommended SNR values of 15 dBA and 12 dBA could only be approached in Classroom-P during instruction situation. The SNR values were about 12-13 dBA below in Classroom-S compared to the SNR values in Classroom-P.•The influence of increase in SNR values on STI values is more significant in the Classroom-S for both teaching activities. This suggests that the classroom without acoustic treatment utilizes more from the increased vocal effort. An increment of SNR of 7 dBA provides an improvement of 0.02 STI in Classroom-P and 0.10 STI in Classroom-S for instruction situation; and an improvement of 0.13 STI in Classroom-P and 0.20 STI in Classroom-S for group work.•The STI values are in the range of “bad” and “poor” categories for normal and raised vocal effort in Classroom-S for both teaching situations. These results suggest that teachers and students would need to use raised vocal effort at all times; however, they might even opt for “loud” vocal effort in a classroom without acoustic treatment for better speech intelligibility.•The results also indicate that the STI levels during group works were more unfavourable in both classrooms except for the 1 m distance in Classroom-P. However, Classroom-S has STI values in the bad category which suggests that the student-to-student communication even in close proximity is harder in the classroom without acoustic treatment.•Recommended STI of 0.6 for both teaching situations could be only achieved in classroom which has a minimum RT of 0.8 s. The results indicate that the teacher might not need to use “raised vocal effort” during instruction situation in Classroom-P, while the students would need to use raised vocal effort at 3 m distance during group work. This finding suggests that minimum RT of 0.8 s would be optimal for secondary school classrooms up to 250 m3 with a capacity of 18 students. There is limited data for assessing speech intelligibility in real classrooms with realistic occupied noise levels and teacher’s and students’ speech levels for different teaching activities in Turkish secondary school classrooms. This study investigates the effect of reverberation time (RT), occupied noise levels and signal-to-noise ratio (SNR) on speech transmission index (STI) in real classrooms for instruction and group work. Noise levels were measured and STI values were calculated in two secondary school classrooms with RT of 0.88 s and 2.73 s. During instruction situation, calculated mean STI values were 0.63 (cla