Dogs and humans can hear from the same distances - about a mile away and about the same but it is the frequency ranges that are different. Human frequencies are about 20,000 vibrations per second compared to 40,000- 100,000 per second for dogs. The more vibrations there are the higher the sound pitch is.
Every dog ear has different shapes and sizes but their ears can move to a position that can go with the sound. Dogs with their ears that are up or kind of floppy can hear the best. A dog can sense the noise like a door slamming or footsteps before a human can hear it.
In my opinion a dog can hear better not only because they can hear higher frequencies but because they can sense or hear some sounds before us, humans.
Click on title for video clip on dog frequencies.
Information from: http://www.ehow.com/facts_6967509_dog_s-hearing-better-human-hearing_.html