What she heard was her own breathing but because the device that gave her hearing was switched on to high volume, it must have sounded like a jumbo jet taking off, her father, Peter Ballard, says.
Katy, from Wimborne in Dorset, was the first British child, born deaf, to receive surgery for a cochlear implant, a revolutionary aid orginally thought to be useful only for those who have had hearing but who become deaf later.
The implant works by by- passing the cochlea inside the ear and stimulating the auditory nerve electrically. But now that government start-up money for the seven UK implant centres has come to an end, clinics such as the South of England Cochlear Implant Centre in Southampton, where Kay was treated, face an uncertain future. The funding has finished just as surgeons realise the potential of giving hearing to very young children.
Assessment, surgery and the first year of speech therapy cost pounds 26,000. In future in the NHS market, health authorities will have to decide whether they can afford to send children to the centres.
Katy's surgery was in January but the device was not switched on for two months, when her scalp had healed. Now she is learning to talk. She has mastered 'Mumma' and 'Dadda' and 'Ver-ver' for her nine-year-old sister, Victoria. She responds to the door bell and hums to herself.
It was not all plain sailing. Mr Ballard said: 'When the device was switched on Katy burst into tears, tore it off and rushed to us for comfort. Now she puts it on herself.'
Norman Haake, her surgeon, is pleased with her progress. 'She is having to start from scratch and will take a little while to catch up. Experience in America suggests that the congenitally deaf children take a bit longer to learn to talk.'
There has been resistance, he said, to using the new implant technology for very young children. 'There were fears about damaging any residual hearing a child might have and it was also felt that with special schooling and learning to sign, there was an alternative to surgery.'
Mr Haake says it is important in children born deaf to operate before they are five. If starved of sound stimulus for more than the first five years it seems that children are not able to develop their language skills, he said.
A flat receiver coil is put beneath the skin of the scalp, and connected to an electrode which is passed through the walls of the inner ear. When this has healed the patient is given a flat microphone that sits on the head, connected to a processor box worn on a belt.
The device works because the auditory nerve is normal but the cochlea in the inner ear which transmits sound from the outer and middle ear does not respond to noise. The implant does its work for it.