He reached over his head and knocked twice on the wall.

I knocked twice, and heard a familiar voice ask who was there.

He had to knock twice before anybody in there heard him.

I knocked twice, and she called for us to enter.

He knocked on the door twice, and got no reply.

He knocked twice on the back door, then climbed in.

He turned to the metal door, took a deep breath and knocked twice.

At the head of the steps he knocked twice upon a door that needed painting.

Roy's door was the first one on my side of the hall, so I knocked twice and got no answer.

Go to the door in the wall on your right and knock twice.