Distance

Distance is one of the greatest obstacles in speech understanding. The further we move away from the sound source (the teacher's voice) the more difficult it is to hear and understand what is being said.

If we listen to the teacher in the presence of background / classroom noise, just as today's students do, then it becomes more difficult to identify and understand the teacher's voice as we move further away from them. This speech / noise relationship is often expressed as the Signal-to-Noise Ratio.

Independent research has shown that a Signal-to-Noise Ratio (S/N Ratio) of between +15 and +20dB is the optimum level in the classroom.

For example: if we assume that the teacher uses raised speech (65dB), the children in the front row are sitting 1 metre away and the classroom noise level is 60dB (average level) then:

Front Row Children sitting at 1 metre will hear the teacher at 65dB, 5dB above the level of the classroom noise. S/N + 5dB
Middle Row Children sitting at 2 metres will hear the teacher at 59dB, below the level of the classroom noise. S/N –1dB
Back Row Children sitting at 4 metres will hear the teacher at 53dB, well below the level of the classroom noise. S/N –7dB

What effect does Distance have on the speech signal ?

Using the above example, children sitting on:

Front Row Receive 83% of the speech signal and achieve 95% word recognition.
Middle Row Receive 66% of the speech signal and achieve 71% word recognition.
Back Row Receive 55% of the speech signal and achieve 60% word recognition.

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