I read an article about Bruce Lee a couple weeks ago. Apparently, he used to train his hearing and perception. He would put on a set of headphones. In one ear would be loud city noises - traffic, honking horns, construction, people yelling, etc. In the other ear, an extremely soft sound would be played at intervals. He would try to focus and pick out the soft sound from the loud noise. A friend walked in on him while he was doing this, and he was sweating profusely despite sitting still. When asked why he was doing it, Lee said he needed to be able to react to the slightest sound - if he were to be attacked in the street, he didn't want anybody sneaking up on him.
"Don't waste your time, or time will waste you."