The plot below shows traces of different (wooden) horns applied to a Carver 60" ribbon in an effort to extend the high response above 16 kHz. As can be seen, some of those implementations affect response negatively because of diffraction effects at the external perimeter of the "horn". The high response did get a small boost however ;-). More on this technique in the article on building a dipole baffle for a Carver 60" ribbon.
Red, is flat driver. White is with 3/8" horn, yellow is an example of a model that didn't make it.;-)