Headphones: these marvels of miniaturisation are worn by over 30 million people in the UK. Hannah Fry visits legendary headphone honchos Bose to find out how today's teeny earbud tech works and meets the human testers with 'golden ears'. She discovers how helicopters in the Korean War helped put the "shhh" into noise-cancelling, why it took a muted Mormon church service to get headphones onto people's heads, and how an opera singer's electric shock therapy led to the invention of the speaker.