I have listened to Umm's voice many times in my studio. But I did not know about tarab until I listened to a recent NPR show (links and quotes below-emphasis mine).
Tarab is "a unique quality of Arab music that translates best to the word enchantment.
Part of tarab is the idea that listeners are as important as singers; that there's a powerful, spiritual exchange between them that is crucial to the performance."
http://www.npr.org/templates/story/story.php?storyId=124612595
More on "The Voice of Egypt" on NPR:
http://www.npr.org/templates/story/story.php?storyId=90326836
