Have a friend say something to you from across the room, now have that friend cup their hands around their mouth megaphone style and say something. The second could be described as "a horn loaded mouth".
The advantage is that horn loaded speakers are easy to drive loud however some (but not all) of us hear a slight megaphone like harshness to the sound. Again not everyone hears it but I do at least in the majority of the horns that I've heard.