The education sector is undergoing major changes when it comes to teaching styles and pedagogical methods. New patterns of interaction between teacher and pupils are taking place. More and more, learning is done in small groups and a high emphasis is put on incidental learning. The typical, traditional "blackboard teaching" situation of the static teacher speaking uninterruptedly to the classroom represents a decreasing use of the classroom. This asks for classrooms that are designed both to maintain low ambient speech levels as to convey teacher's speech to all pupil positions. Complementary descriptors need to be developed, besides traditional classroom descriptors like reverberation time (T) and speech transmission index (STI). Rooms with ceiling treatment are not diffuse. Traditional, Sabine based relationships between reverberation time and sound pressure level do not apply. Based on extensive classroom acoustics investigations, the descriptor "sound pressure level decrease" is proposed and discussed.