Which actress provided the rasping unnerving voice of the demon inside Linda Blair in the "The Exorcist"?

I tried to see this film twice with a friend when it was first released. Both times, she ran out of the cinema after a few minutes and I had to follow her as I'd promised we'd both catch the bus together! We lived in a big city at the time which was a bit scary to be out alone in at night. To this day, I've never seen the film!