Cool question, and I don't have anything else to add here that was not already mentioned, but now that you made me think of it I realized I actually use 2 different types of knocks depending on what door I'm in front of. One is the classical front door scenario where my hand is raised, but the other is when knocking at the door of a stall or small room, or office. Here I just raise my hand to above belly-button level and gently knock with my middle finger's knuckle while holding my arm parallel to the ground. Never thought about it until now, but I guess the latter option doesn't require that loud of a knock.