There's a small room in Minnesota that blocks out 99% of all external sound. That's an impressive number! Also impressive: nobody can take more than 45 minutes alone in the room before they go nuts.
When Apple is getting a new mobile gadget ready, they stick it in an anechoic chamber—a humungous room straight out of science fiction, where the entire external world is blocked out. The same goes for advanced aircraft.
Yesterday, while touring a new building on Microsoft's campus, I came across an anechoic chamber: A room designed to eliminate all noise from outside—and in. I spent about 5 minutes locked inside, and man is it freaky.