21/06/2013 10:26 BST | Updated 22/05/2015 10:12 BST

Video: Moment Deaf Toddler Hears His Dad's Voice For The First Time

This is the heartwarming moment a three-year-old deaf boy's face lights up – as he hears his dad's voice for the first time.

And it's all thanks to a gamble Grayson Clamp's mum and dad tried after a cochlear implant failed to fix the little boy's disability.

Len and Nicole Clamp turned to an untested method that had never before been used on children in the United States:: an 'auditory brain stem implant.'

After the chip was implanted, Grayson - who was adopted by the Clamps as a newborn after he was placed in foster care - could hear for the first time in his life.

"We don't know exactly what it's like for him," Nicole Clamp told TV news.

"We don't know exactly what he hears. His brain is still trying organise itself to use sound."

Grayson is missing the cochlear nerves in his ears that allow humans to process the sounds that we hear.

After the cochlear implant failed, the Clamps decided to participate in a research trial at UNC Hospitals in Chapel Hill, North Carolina, where doctors were researching the use of the implant on children.

The procedure requires doctors to implant a microchip into a patient's brain. The microchip then helps him recognise and process sound.

The technology is similar to that of a cochlear implan - using electrical stimulation the brain stem implant uses electrical impulses to stimulate the brain, rather than the cochlea.

Around 1,000 people have undergone the procedure, with varying degrees of success - most recipients of the implant only have an awareness of sound.

It's described as being able to hear the beat, but not the melody.

For Grayson, the device seems to be working - his parents say his face lit up when he heard his father's voice for the first time about three weeks ago.

"It's been phenomenal for us," dad Len said.