Calculating the Optimum Reverberation Time and Absorption Coefficient for Good Speech Intelligibility in Classroom Design Using U50

Acoustical comfort in general, and the quality of speech transmission from teacher to students in particular, are crucial in determining the quality of educational processes in schools. The acoustical treatment of interior surfaces, the sound insulation of surrounding constructions, the vocal capacity of the teacher, student activity noise, children's abilities to hear and concentrate and the educational circumstances, are among the many factors in the complex mechanism of speech understanding. Managing the rich variety of measures for optimising classroom acoustics demands a multidisciplinary approach. In this article we propose a predictive model for speech intelligibility, as expressed with the parameter U50, based on objective acoustical values, i.e. the reverberation time RT, signal-to-noise ratio SN, and the relative proportions of sound arriving early and late at the listener. The present article introduces the model and derives guidelines for architectural design. The model is illustrated using a typical Dutch classroom as an example. Architectural guidelines are derived for maximum RT values, and compared with existing recommendations. The possible additional need for minimum RT values is also investigated in relation to the phenomenon of 'overdamping` in classrooms.