The finding came from a study of pupils in Canada, where schools have resorted to installing surround-sound systems in classrooms and giving teachers microphones.
The Canadian Association of Speech-Language Pathologists and Audiologists studied 1,162 students, aged five to nine-years-old, half of whom were in traditional classrooms and half in acoustically enhanced classrooms. They found that more than 90 per cent of the normal classrooms had inadequate listening conditions.
Lessons typically took place in a noise of around 60 decibels, which was produced by the chattering of the children and the humming sound from fluorescent lights, heating, air conditioning systems, fish tanks and computers.
The researchers found that, to make 95 per cent or more of their speech intelligible to the children, teachers needed to speak at least 15 decibels louder than the background noise. This meant talking at a volume of 75 decibels, a level which is hard to maintain as it is far higher than normal speech, which is around 50 decibels.
Linda Rammage, president of the association, said: "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 to 'fill in the blanks' when they don't hear a word or part of a word."
Canadian schools may not be as noisy as many in the UK. A 2002 study of London pupils found they were exposed to an average of 72 decibels at school. However, the levels in its classrooms exceed Canada's building regulations for workplaces. Some schools have tried to quieten their classrooms by installing acoustic soundproofing tiles and attaching cut-open tennis balls to the bottoms of table and chair legs. Others have installed surround sound systems that allow every student to hear the teacher.
Such a system is used at Bear Creek Elementary in Surrey, British Columbia. 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 sit back in their chairs, knowing they are not going to struggle to hear what the teacher has to say."
Meanwhile, a US academic has urged schools to allow pupils to be noisier. Angeline Lillard, professor of psychology at Virginia University, spoke at a conference in London this week which marked the centenary of Montessori education, in which pupils of different ages work together at their own paces.
Professor Lillard said children were able to work against a background hum of noise and found it difficult to learn from each other if forced to be silent.