International Standard ISO/TR 3352:1974 “Acoustics – Assessment of noise with respect to its effect on the intelligibility of speech” provides methods for measurement and assessment of noise with respect to intelligibility.
Example
For a speech interference level of 35 dB, the maximum distance at which raised voice is considered to be satisfactorily intelligible is 15 m, while it would be 8,4 m for a speech interference level of 40 dB.
Comment
This indicates that from a strict Signal to Noise ratio point of view, teacher’s voice in classrooms will easily reach the furthest pupil position, generally located 8 m from the blackboard.
