I found some very helpful information on the web about dog hearing. I hope this helps you.

Dogs hear at a wider range of frequencies than humans. The low end of the range is similar to humans (humans hear down to about 20 Hertz, or cycles per second, dogs are at about 40 Hz). But at the high end, they are quite different. Humans hear to about 23 KHz or 23,000 Hz (kilohertz = 1000 x Hz) but dogs can hear up to 45 KHz! Although as dogs age they may become deaf and some dogs are born deaf.

When dogs become aware of a sound they've heard their ears will prick up and move around. Part of the reason why dogs can hear better than humans is that their ears have more mobility which enable it to maximize its ability to ear. The shape of a dog's ear also helps it hear more proficiently. As we cup our ears, some dogs ears are already in that position. A German Shepherd, for example, has upright curved ears which helps it to maximize sound.

The following link to Wikipedia has interesting information about the dogs' anatomy and their different senses:

Dogs hear at a wider range of frequencies than humans. The low end of the range is similar: 67 Hz, but dogs hear noises up to 45 kHz, while humans only hear sounds up to about 23 kHz. This means that they could be hearing and responding to sounds that we can't hear at all.

For some reason, humans have not had to in their evolutionary history hear as well as dogs do, so humans don't. Why, in terms of our organ structure? I would guess because of the way that a dog's ears reflect sound that our ears do not, but that is just a guess.