Do dogs hear things we can't?
When it comes to hearing dogs have a distinct advantage over us. Not only is their frequency range of audible sounds much wider, but their mobile ears act to funnel in low volume sounds. This is why dogs are so great at alerting you that the mailman is on the way and almost impossible to sneak up on. Understanding your dog's super sense of hearing may provide insight to their behavior. So pay attention when your pooch barks, someone really might be coming.
For more information on understanding your dogs world, please read Understanding Your Dog Better.