What determines the frequency of the hum? is it tube length, or characteristics of the vocal chords or something? I ask because I can just see someone trying to make the hum span the audible frequencies, so it can't be filtered.
I haven't played one, but I'm sure you could play numerous overtones like you can on a bugle.
They seem to be mass produced, resulting in most of them having the same length. That, along with the fact that the average sporting fan probably doesn't have much embouchure, would explain why there's a single tone that is much louder than any others.
http://www.newscientist.com/article/dn19041-what-makes-the-s...