Teachers should receive voice training to help them perform effectively in the classroom, a union suggests.
Teachers' voices can wear out through over-use
Philip Parkin, general secretary of the Professional Association of Teachers, said speech was the "main tool" for getting children to behave and learn.
Research showed a link between strength of voice and effectiveness, he added. But more than 50% of teachers suffered voice disorders at some stage.
Mr Parkin said training would avoid "the premature end to careers".
He told the association's annual conference in Oxford. "The conclusion has to be that the care and the use of the voice should be an essential and mandatory component of all initial teacher training - and increasingly a component of the training of teaching assistants.
"PAT would urge those designing such training to incorporate this into it."
Mr Parkin also expressed concern over the number of assessments facing teachers.
"I continue to believe that teachers are the most over-assessed profession there is, some being subject to weekly observation in some form or other," he said.
And he referred to the "plethora of initiatives" which had increased the level of work for school leaders and senior teachers.
"I see little sign of the pace of change slowing and remain unconvinced that those driving change fully understand the pressures on those required to implement them in schools."
Mr Parkin said: "I suspect that professionals in a number of fields in the public sector may well feel that their professional autonomy is undermined by an increasingly prescriptive government.
"Innovation and change from within the education sector is far more likely to be adopted with enthusiasm than the changes imposed by those who are perceived to be out of touch with the realities of school, nursery or college life.
"There must be concern that too much change is rooted in politics rather than in education."
Jemma Rogerson, a speech therapist from Chorley, Lancashire, conducted a study into the impact of a damaged voice on teachers' effectiveness in the classroom.
It involved playing 106 children different recordings of teachers with hoarse voices and other disorders.
The children were then given comprehension tests to see how much of the information they registered.
Miss Rogerson said: "Any form of disordered voice affects children's processing of spoken language.
"You can infer that any form of vocal impairment can effect children's learning. It has obviously huge implications."
She said teachers needed proper training in how to use their voices to prevent damage.
Unable to talk
Primary school teacher Mandy Morley, 34, from Buckinghamshire, has been off work with a damaged voice for five months.
"I have always been a singer so I thought I knew all about how the voice works," she said.
"But being a typical teaching professional, I got a stinking cold, had a sore throat, (carried on working) and had a parents' evening.
"I woke up the following day and could not talk at all.
Mrs Morley said she hoped to return to work part time in September.