I didn't mount the horn in a cabinet so I had no "rear cabinet" at all. I didn't have a contained compression area either. I just built the horn, attached it in front of the speaker, and got major amounts more sound. I didn't even do both speakers. If the speaker was in a music hall it would have probably had problems. In my living room it didn't.

Even with the little work I did it still makes a HUGE difference. It confirms what you say above. Horn shape and size make a big difference.

I figure the size has a lot to do with how low a frequency the horn affects since the lower the tone, the longer the sound wave. The shape controls dispersion and I would expect with most people having 90 degree horns on the top end they would like to match the midrange to them. If you don't I expect a seating position right in front of the speakers would match up well. Off center would have waves of high and midrange not matching well.

I'll go back to car basics. If compression isn't affected by the size of the hole in the mounting plate a "slightly" wider gap between the speaker and hole should lower compression, therefore SPL increase.

Have a good week-end.

Grant.