Those weird hemisphere things around tweeters in vintage speakers?

[shadyJ]

Yeah ok, what are those things around the tweeters, and what are they supposed to do?

I am sure they are there to help dispersion somehow, but I can't figure out how they are supposed to work. My google searches haven't turned up anything really explanatory. So what is it they were intended to do? It doesn't look like they would be great for creating an even off-axis horizontal response, and the vertical response has to be horrendous. I get that they are a horn of some type, and JBL uses a related geometry on this cinema speaker in this horn here:

But how that type of horn is supposed to focus sound is making any sense to me. On the cinema speaker, which is the 4675c, that horn is called a 'bi-radial' design.

 

[markw]

Pretty much everything you need to know about horns can be found here. If this explanation doesn't satisfy your curiosity, it certainly gives you a solid starting point for further research on this complex subject, particularly in the notes, references, and external links sections.