This work concerns the relationship of the two main factors, namely the room acoustics and background noise, with the vocal effort. Teachers and non-teachers have been invited to perform talking test under simulated classroom acoustic conditions of varying levels of speech shaped noise in order to understand this relationship. At the same time their subjective preference of teaching in classrooms by means of questionnaires has allowed refining guidelines for classroom acoustic design. Under simulated acoustic environments, talkers distort their voice linearly dependent on the voice support on maximum noise level, and the slope of this relationship is defined as room effect.