One way to describe the desired acoustical quality in a classroom is to specify an acceptable maximum ambient noise level. This level is measured in terms of A-weighted sound levels or octave band sound levels that can be used to determine other measures such as noise criterion (NC), room criterion (RC), or balanced noise criterion (BNC) values. By combining the effects of sound at different frequencies in a manner similar to that which takes place in the human hearing system, these measures rate the loudness of sound to listeners. A second way to describe acceptable room acoustical quality is to specify the reverberation time, which is approximately the time it takes for a loud sound to die away to inaudibility after the source is turned off. Reverberation times increase with room volume and decrease as sound-absorbing material is added to a room. However, excessive sound-absorbing treatments will have the negative effect of reducing speech levels and degrading the intelligibility of speech in a classroom.
People’s ability to understand speech is influenced largely by how loud speech sounds are relative to ambient noise or any other competing sounds, hence the importance of an adequate signal-to-noise ratio (i.e., speech to background noise ratio) for a classroom to function well. Reverberant sound causes one word to smear into the next and can decrease the intelligibility of speech. Acoustical design should be aimed at improving the recognition of speech sounds in the classroom. The focus should be first on reducing unwanted noise and then on controlling excessive reverberation. Good acoustical design can facilitate learning by allowing for more accurate verbal interaction and less repetition among teachers and students because spoken words are clearly understood. There is also evidence that good acoustical design may have a health benefit for teachers