DECRIPTION
A calm environment in a classroom can increase intelligibility and student attentiveness. This can help the teacher be clearly understood without stress or fatigue. The correct reverberation time combined with a speech (signal) level higher than the background noise become important factors.

CONSTRAINTS
Students near to the front of a classroom can generally hear the teacher well, whilst those to the rear of the classroom may experience problems hearing or understanding due to excessive absorption and/or intrusive noise. Disturbance from air-conditioning, external noise through open windows, particularly in summer, are just some of the factors affecting the listening conditions.

ARMSTRONG SOLUTION
The Building Regulations, through Building Bulletin 93, recommend a maximum reverberation for most classrooms of either 0.6 secs or 0.8 secs depending upon whether in a primary or secondary school. Long reverberation times favour the build up of background noise and can impair intelligibility, but short reverberation times can limit the strong reflections needed for those at the back of the classroom to hear clearly.

Ideally, for good speech intelligibility, the level of the voice needs to be at least 10 to 15dB above the background noise level. International surveys show that up to 15% of the students in a class have permanent or temporary hearing problems due to illness or allergies during the course of a year. In these instances audiologists recommend a difference of 20 - 30dB difference between voice and background noise.

Having too high a level of sound absorption can create an acoustically 'dead' space resulting in difficulties in communicating. A suspended ceiling with sound absorption in the order of α_w 0.60 generally provides most classrooms with the required reverberation time.

RECOMMENDATIONS
In addition to the required maximum reverberation times, Armstrong recommend a minimum reverberation time of 0.4 secs to prevent classrooms being too acoustically dead and maintain intelligibility at the back of the room: Dune Supreme, Dune Max, Perla, Ultima, Orcal Extra-Microperforated

In certain conditions, particularly in classrooms for hearing impaired children and those classrooms with poor sound insulation against background noise, passive acoustics may not provide satisfactory levels of intelligibility despite meeting the reverberation time requirements. In these instances Armstrong recommends a complimentary of active acoustics to reinforce the voice level of the teacher within the classroom: i-Ceiling Sound Panels.