As I know a horn just has a MOUTH and a THROAT. So What is that I showed in the below pic used in almost every megaphone???! Why the megaphones use that?
Its part of the horn. A reverse horn if you will. It makes the horn path longer. It also serves as a plug to help spreed the sound.
It is sometimes called a utility horn. I could not fine a good drawing on the net. Maybe someone else mite have on.
Some times it is called a folded horn. If you take off the piece pointed to, and rotate it 180 then attached it to the back side of the horn you will get a horn that is twice as long.