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Distance, background noise and reverberation

The sound level of the teacher’s voice drops dramatically over a relatively short distance and the students in the back of the classroom can find it difficult to hear the teacher’s lessons as well as the other students.

In a typical classroom, students in the first rows hear about 83% of what the teacher says. In the middle rows, this number drops to about 66%, and in the back rows, only 55% of the lesson is heard1. The problem becomes even worse, regardless of placement, when the teacher turns away from the class to write on the blackboard or check notes.

Background noise
Traffic noise, chatting classmates, clatter from chairs and tables, noise from the corridor, nearby classrooms and the like all contribute to create background noise in the classroom. Typically, the average background noise in a classroom is about 50-60 dB. This is about the same level as normal speech, so when the teacher speaks at a comfortable volume it becomes difficult for students to differentiate between the teacher’s voice and the background noise. To understand 100% of speech sounds, the teacher’s voice needs to be about 15 dB louder than the background noise.

Reverberation (echo)
High ceilings, large windows, uncarpeted floors and sound reflections from walls and tables escalate many distracting sounds. Reverberation in the classroom has a considerable negative influence on speech understanding and makes it difficult for students to understand what the teacher is saying.

1Carl Crandell “An Update of Classroom Acoustics” The Volta Review 96 (1993)