FDNY Captain Jay Jonas, who survived in a 6th floor core stairwell of the north tower as the building collapsed around him, describes that experience:
Now, people have tried to get me to describe what it was like while the collapse was happening. It was a montage of different sounds and experiences. The sounds were a combination of sounds. This building collapsed in what’s called a pancake fashion. In other words, one floor would hit another floor and would collapse that floor and then collapse the next floor. And every time a floor would hit another floor, it created a loud boom and tremendous vibration.
The entire collapse of this 110-story building took 13 seconds. So it sounded like boom, boom, boom, boom, boom, boom, boom, you know, like that. And every time that happened, it shook the entire building. It shook the whole floor. So every time a floor would hit another floor, we’d be literally bouncing off the floor. We were being thrown around the stairway.
There was also this very loud sound of twisting steel all around our heads. These massive steel beams and girders were just being twisted around our heads just like they were twist ties on a loaf of bread. And a very loud, like a steel screeching sound, almost like a lot of trains coming into a subway station at the same time and all of them hitting their brakes at the same time.