After doing some digging, here is what I can tell you. In reality, deer hear only about as good as we do. They hear approximately the same exact frequencies as we do. They are little better than humans on the higher end of our range, but all-in-all, they are not Superman or bionic by any means.
|Scanning for sounds|
The answer is not on the inside, but on the outside. Their ears work like parabolic microphones that you see on the sidelines of football games. A deer can rotate those big receptors independently of each other similar to the way a Chameleon can move it's eyes. These big microphones funnel sound in and help them to hear things directionally much better.
Once the sound has been pin-pointed, wary bucks have the nasty habit of circling back around and check the sound out from a different angle, especially when the wind is not in their favor. Their sense of smell is far greater than their hearing or eye-sight, so they will move to give themselves the biggest advantage.
Stated plainly, a deer is very aware of the sounds that their world makes. When they hear something that is not in their "normal", they become cautious and will flee. So much of what a deer hears is based on where they live. If a deer is accustomed to human sounds, then they will be less cautious when they hear human activity. It's normal. However, if you suddenly go to a much wilder hunting location and make the same noises you do in suburbia, then you are bound to get busted again and again.
I think the video below by Dr. Grant Woods sums up this article nicely and can helps us gain a better understanding on how we can mask the sounds that we make better. Enjoy!
Reference Credits for this article:
1. Tink's - How Well Do Deer Hear?