Audio Book vs. Silence reading

I was really surprise how this two methods of reading is very different in the term of understanding. I don’t know about other people, but when I read this book, I normally read it and listen to the CD at the same time. When I read the book without a CD, I feel that there is no imagination that ran through my head. When I listen to the CD at the same time, I have a much easier time understanding it and giving me to be able to imagine the picture out.