Acoustic conditions for speech communication in classrooms.

The purpose of this study was to measure reverberation times, background noise levels and Rapid Speech Transmission Index (RASTI) values in occupied and in unoccupied classrooms. Probability distributions of sound levels were also measured during classroom instruction. The reverberation time was measured according to the ISO 3382-1975 standard and RASTI according to IEC 268-16. In one-third of the classrooms the reverberation times were found to be longer than 0.9 s. at one or more frequencies ranging from 250 to 2,000 Hz. In most of the unoccupied classrooms, the background noise level was 35 dB(A) or less. During classroom instruction the background noise levels (L90) were high, ranging from 40 to 58 dB(A). The equivalent continuous sound levels (LAeq) were also high, from 58 to 79 dB(A), indicating that teachers increase their vocal effort while speaking. Most classrooms had RASTI values that reflected fair intelligibility, but very few classrooms had RASTI values indicating excellent intelligibility. The acoustic design of classrooms should be based on achieving the highest possible degree of speech intelligibility for all the pupils.