Amazon comes up with noise cancelling headphones that listen for specific sounds
 

Amazon has been granted a patent for headphones that not only cancel out noise, but also listen for specific sounds or phrases and respond by automatically turning off the feature so the user can hear sound from their surroundings.

This will prove useful if you are listening in a noisy environments where you might actually need to pay attention to the occasional noise. If you are shopping hearing the sound of gunfire might be rather useful as would be screams of people in distress.

http://fudzilla.com/news/wearables/4...pecific-sounds