Students will sense if the teacher does not understand them and may drop out. Since the strength of the unaided human voice is limited, ideal listening conditions are achieved by reducing the background environmental noise levels. Bare walls and hard ceilings are common features of today's cash-strapped schools. The Swedish standard specifies the amount of sound absorption needed to control reverberation for each type of space. This does not translate easily into terms that can be checked in American classrooms.
Many classrooms exceed that by a factor of two or three or more.
Reverberation does not only increase room noise levels. Reverberation reduces speech intelligibility even more for hearing disabled listeners than for normal listeners. Central heating and cooling are usually the quietest followed by rooftop installations. Quiet systems may be avoided because of their relatively high cost coupled with the absence of a clearly stated benefit of their quiet operation. Interior appliances such as the fans in overhead projectors and computers are another important source of interior noise.