Noise-cancelling headphones for the blind

Everyone sit down and make sure your mug of tea or any sharp objects are at a safe distance, because this is an idea that actually might have some practical application!

Some blind or partially sighted people find it helpful to make regular clicking noises as they walk along. The changing nature of the echo reveals a lot about their surroundings, including detail such as the proximity and type of material of a fence or wall they're walking past. Of course bats use this, but in a clever way, so that they close their ears while making the click or squeak (so that they don't deafen themselves) and open their ears to hear the echo.

So, this idea gives some of the advantages of the bat's system to blind people and adds a few convenient bits of technology too. You would wear a pair of headphones which would be extremely 'leaky' - i.e. they let through all the sound that is around you. They also emit periodic high-frequency clicks outwards, and a noise-cancelling version of this click into the wearer's ears. The headphones also listen for the echo of the click and can either amplify it for the wearer's benefit, or shift its frequency down to make it more hearable.