In Iran traditionaly there were special door knockers for men and women. They have different shapes and they sound different. So the woman inside knows if she needs to put on her veil or not, while opening the door to the visitors. Or the door can be even answered by the appropriate sex, helping to prevent the "immoral' mixing" of unrelated men and women. A man answers the door if a man is calling, while a woman answers the door if the caller is a woman.
Those knockers can be still found in old parts of towns.

And here is a knocking man :)