Back when he still worked as a speech therapist and audiologist, Yasuhiko Fukuzono used to observe an interesting phenomenon. When deaf patients were fitted out with hearing aids for the first time, they complained that everything was just noise. "Even when they were at home, not doing anything, it was still noisy," he says. "You know what it was? It was the sound of the fridge. We don't even notice it, right? When you don't recognize a sound, you hear it as noise, but once you know what it is you're okay: your noise threshold changes."