The sound is so painful for this 10-year-old that her family tries to laugh quietly. Lucy suffers from hyperacusis, or over-sensitivity of hearing. Other sounds that cause her distress are the singing of hymns, applause, the vacuum cleaner, washing machine, lawn mower and electric carving knife.
'Lucy doesn't mind us talking, but if we are all laughing, she sits there in tears. We try not to argue in front of her either,' says her mother, Ann. 'I don't like shouting,' says Lucy, who inadvertently has become the family's mediator.
She has Williams Syndrome, a genetic condition caused by an abnormality on chromosome 7. WS children are difficult babies, often failing to thrive. They may have raised blood calcium, heart problems, learning difficulties and a pixie-like expression. Despite their learning difficulties, they handle language well and have a bright manner. But nine out of 10 have a pronounced aversion to certain everyday sounds.
Mrs Clark's first recollection of Lucy's reaction to noise dates back to before she was born. In an advanced state of pregnancy, she was relaxing in the bath when the door slammed. 'The baby jumped so hard inside me that I was convinced it was an extreme response to noise,' she said.
As a baby, Lucy became so distressed by the singing at the prayer meetings her family attended that they had to take her outside. 'She spent most of her toddler years with her hands over her ears,' says her mother. 'As she gets older, she seems to be able to cope better, particularly if we warn her that we are about to use the Hoover.'
Although there may be an element of super-sensitive hearing in WS children, the problem is thought largely to relate to the way messages are handled by neurotransmitters in the brain. This means sounds that are acceptable to most people are perceived as something painful.
The problem appears at its most extreme in the pre-school years, with the severity lessening by adolescence. But some adult sufferers have told researchers they have 'moved from place to place to try to find silent areas of the country'.
New research sponsored by the Hearing Research Trust reveals that while WS sufferers are particularly affected by hyperacusis, a wide spectrum of the population who have no genetic abnormalities experience similar problems.
The research shows abnormally sensitive reactions to sound could be making life miserable for many more people than was previously believed. Results indicate people of all ages, from small children to pensioners, experience these problems. In childhood, as many as 6 per cent find everyday sounds disturbing and the incidence doubles in children with learning difficulties.
Josephine Marriage, an audiologist at Addenbrooke's Hospital, Cambridge, is in charge of the study. 'Children can be particularly badly affected. Even the sound of a vacuum cleaner might be upsetting. The sensitivity appears to upset their concentration and may cause them to restrict their leisure activities,' she says.
Ms Marriage has even encountered parents whose babies are so sensitive to noise that they have to go into another room to talk. One mother always had to find a 'silent' room to feed her child. When she tried to wrap presents the ripping noise as she stripped the sticky tape from the reel made her baby 'howl and scream and go floppy as though it had been shot'.
But hyperacusis does not always lessen with time and can continue or begin in adult life. There is a tendency for it to occur with several other conditions, including migraine, depression and sleeping difficulties, which are also associated with a deficiency in a specific brain chemical, neuroamine.
A woman who read about the condition in a magazine said she was 'delighted to know that I am not going barmy as my husband kindly puts it'. A retired foundry worker from Sheffield, who says the noise of his workplace never caused him problems, wrote: 'My wife is always saying she can't hear the television while to me it seems too loud, so much so I can hear it upstairs. Next door's wireless is too loud and has caused friction between us.'
Time after time sufferers have told Ms Marriage that noise from vacuum cleaners, strimmers, lawn mowers, tumble dryers and washing machines are the commonest sources of offensive noise. She would like to analyse the constituents of these electrical sounds which feature so highly on the problem list to see if she can find a common factor, although the ongoing study indicates that the problem does not, in fact, appear to be 'frequency specific'.
Loudness does not seem to be an explanation either - mild hearing loss does not prevent hyperacusis. 'Often, people will play music very loudly. It is possible, for example, to have otitis media, middle-ear infection or impacted wax and still suffer from hearing over-sensitivity. This is partly why ear plugs give only small relief,' Ms Marriage says. 'It is not at the ear that the over-sensitivity occurs, but the way the brain translates and hears the sound.'
Hyperacusis may sometimes be the first symptom prior to the onset of tinnitus, noises in the ear or head. This has been observed during withdrawal from the tranquilliser Valium after long-term use.
Unusual sensitivity to sound is a recognised symptom of several disorders, including Meniere's disease, which causes vertigo and deafness and tinnitus but, until now, has received scant professional recognition as a problem in its own right.
New objective methods of assessing hearing, including oto-acoustic emission tests, which record signals produced by the sensory cells in the inner ear, are being used to throw more light on the condition.
Treatments that can be helpful include tinnitus maskers obtainable through local ear, nose and throat and audiology departments. Ear plugs may help, but not always. A programme of desensitisation can also prove beneficial.
As the chemistry of the brain, and particularly the role played by specific neuroamines in the brain's processing of hearing, becomes better understood, the way may be opened for development of a drug that turns nasty noises into pleasant ones. Certain nuerotransmitters - substances produced naturally by the body - play a part in modulating sensory output, and one neuroamine, serotonin, may be the key. It has a complex action on the brain and nervous systems and is also involved in sleep patterns, migraine and depression. 'We are just beginning to understand its role and it will be many years before we see a prescribable therapy for hyperacusis,' says Ms Marriage.
Hearing Research Trust, 330/332 Gray's Inn Road, London WC1X 8EE (071-833 1733).