Our house came with both an ugly door knocker and door bell. Visitors ALWAYS opted for the door bell. The bell had a repeating delay so it sounded like the person was pushing it over and over again. Human ears could only hear it if you were standing AT the front door. BUT, dog ears were incredibly sensitive to the sound, so Digby would be sent into a frenzy whenever we had visitors. His hysteria would inevitably get me so worked up that I think on some occasions I might not have seemed so happy to see the person on the other side of the door.