We had moved from the big city to a quiet rural town when I first heard a Great Horned Owl. I had no idea what it was, but reveled in the new nature sounds all around us. I remember standing by our open bedroom window in the middle of the night, listening to the owl the first year we lived in our house (by then I had figured out what it was).  I was enchanted. The sound visually reminds me of a small bouncing ball, in which there is a big bounce, then a few smaller successive bounces as it moves away. Allaboutbirds.org describes the call as a deep, stuttering series of 4 to 5 hoots. HOO...hoo hoo hoo hoo.