>> reporter: his audologist explained how much he has to learn and how difficult it will be. >> he doesn't even realize the difference between the noise maker shaking and the sound of his mother's voice. until he can learn to discriminate those things, it's a pretty scary experience. >> reporter: unlike a traditional hearing aid, the cochlear implant sends coded sound waves into the hearing nerves. back in the testing room mom finally sees david's fleeting but unmistakable reaction. >> david. david. you hear me. >> it's going to change his life completely. i already see it. >> reporter: david's younger brother joseph with similar hearing and behavioral problems will be evaluated for an implant next giving this whole family the promise of a future full of speech, laughter and music. they'll all be able to hear together. >> what a sweet story. >> very cool. the reaction amazing.