This paper investigates the kind of work and communication behaviour in classrooms in two elementary schools. Using a database of 175 examined lessons an analysis is made of how different kinds of work (frontal lessons vs. differentiated lessons) have an effect on the sound level in the classroom. Parameters are discussed, which can describe classroom acoustics appropriately. Also discussed are how altered room characteristics (eg. increased absorption, shortened reverberation time and improved speech intelligibility) affect the sound level in the context of each kind of work. A methodical examination of the database allows an assessment of mean values but also of the detailed teaching phases, as characterised by certain pedagogical factors. The results provide the basis for discussion of stress and work demands of teachers.