The din of modern classrooms is making primary school children miss as many as one in six words spoken by their teachers.
Chattering children, heating and air conditioning systems, humming lights, fish tanks and computers created a 60 decibel background.
Normal speech is around 50 decibels. Researchers in Canada found that to make 95 per cent or more of their speech intelligible to children teachers needed to speak at least 15 decibels louder than the background noise.
The Canadian Association of Speech-Language Pathologists and Audiologists studied 1,162 students, aged 5 to 9. Half were in traditional classrooms; the rest were in "acoustically enhanced" rooms, some with surround-sound.
The researchers found that more than 90 per cent of the normal classrooms had inadequate listening conditions. Linda Rammage, president of the speech-language association, said: "Young children, whose auditory centres are not fully developed in the brain, require better signal quality than adults to understand speech well. They do not have the language quality or life experience or understanding to fill in the blanks when they don't hear a word or only part of a word."
A 2002 study of London pupils found they were exposed to an average of 72 decibels at school.
Bear Creek Elementary in Surrey, British Columbia, has introduced a surround sound system in classrooms. Anna Crosland, a teacher, told the Ottawa Citizen: "Even the youngest children say it's easier for them to hear the teacher.
"I see them settle back in their chairs, knowing they are not going to have to struggle to hear what the teacher has to say."
For tips on quiet classrooms go to: www.caslpa.caenglishresourcesnoise_ in_classroom.asp#materials.