knock on与knock at的区别是什么?

knock on 敲(门、窗等);撞
knock at 敲(门,窗等)
Knock on the door before you enter.
进屋之前要先敲门.
When I fell I got a terrible knock on the head.
我摔倒时头撞得很厉害.
He got a nasty knock on the head.
他头部遭到重重一击.
Did I hear a knock at the door?
是有人敲门吗?
I was wakened by a knock at the door.
敲门声把我吵醒了

knock at也有敲门的意思。意义与用法与knock on没有多大区别。