Use of G and C50 for classroom design

Abstract

Reverberation time is the primary descriptor for the acoustic response of a classroom, and is typically the only parameter which is described in regulations for acoustic response. However, it has been demonstrated by others that reverberation time and classical theory is neither a good predictor of speech intelligibility, nor ambient noise levels during classes. Furthermore, there is a high degree of uncertainty in predicting reverberation time in classrooms, which invokes risk and cost in the design. This study presents new acoustic measurements of the acoustic response in the rooms used previously for the Essex Study, for which there are much in-use acoustic data and user evaluation. Recent revised and modified theories by others for the spatial variation of sound level in well damped and furnished rooms, leading to Strength, and Clarity, are presented. These are used to illustrate the conflict for achieving preferred acoustic conditions in classrooms to control background noise but enable speech intelligibility in terms of Signal to noise ratio. Speech Clarity is also necessary to enable sufficient speech intelligibility, and provides a constraint to enabling higher signal levels in larger classrooms. It is illustrated that for a 56 m2 primary school classroom with a background noise level not exceeding 35 dB(A), the minimum value for Strength should be 15 dB to achieve a minimum SNR of 20 dB from speech at normal levels at a distance of 7.5 m. This requires a minimum reverberation time of 0.47 seconds according to the methods outlined. The minimum value of Clarity with intelligibility weighting should be at least 3 dB to correlate with "good" speech intelligibility, where STI = 0.6; this entails a maximum reverberation time of 0.65 seconds in a room of any size. However, the reverberation times derived should be used a guidelines only; the design should be assessed on the basis of the Strength and Clarity required, as these are the parameters that affect the use. It is demonstrated that measurements of G and C50 beyond a distance that is half the square root of the floor area may be arithmetically averaged, to describe the room conditions.