My Lab Sasha does that too, I believe that its also a sign of protection when that doorbell rings they know theres someone at the door and want to protect their home..When Sasha does that with the bell or a knock, to get her to stop I will very sternly call her name once to get her to look at me and if her attention doesn't shift to me where I can stare her down, i will step in front of her and give her "the look" and point at her. Sometimes actions do speak alot louder than words....If you get yourself inbetween your dog and the source (the door) it sometimes lets them know that you are cutting them off from it, that you are dominent and their attention will turn away from it and they will stop barking. You could try that if you want, but do keep in mind it took me a few weeks of doing that before Sasha started to catch on and it may take some time, but now anytime Sasha barks at anything all I have to do is get up and stand in front of her and stare her down and she stops....its kinda fun! lol...I hope it works for you if you try it!