In the UK Building Bulletin 93 is part of the Building Regulations. BB93 was last updated in 2015 where it specified the acoustic performance for new or refurbished primary and secondary schools for both normal and complex needs students. This paper describes a justification for a universal classroom acoustic criterion for educational spaces. This would include nursery, primary, secondary and further education colleges. The adult voice is the primary sound source for classroom-based learning. The sound spectrum of the voice is well known, as is the ratio of male to female teachers. This will form, along with familiarity, the basis for a universal single number classroom acoustic criterion based on reverberation time. This simple criterion will allow architects and acousticians to work together to specify appropriate materials at the earliest possible design stage.