Audibility:clearky,teachets need to be audible.They must be sure that the students at the back of the class can hear them just as well as those at the front.But audibility cannot be divorced from voice quality:a rasping shout is always unpleasant.
Teachers do not have to shout to be audible.In fact,in most classroom there is a danger of the teacher's voice being too loud.Good teachers try to get this balance between audibility and volume just right.