I used a 6 1/2 inch driver in the mid-horn and I'm satisfied with the results. I followed Waynes advice about keeping the mouth about the same size, the lengths close to the same as Waynes design and proportioning the entrance at the throat to the smaller speaker. I suspect too large an opening leaves the driver incapable of going as low as the horn allows it and it will flap around causing distortion. I suspect too small an opening would restrict sound. I'm sure I'll find out if I'm right ;-).

From what I've read the length of the horn and size of the mouth have a lot to do with the frequencies the horn is capable of producing. A shorter horn, or smaller mouth would be suitable for higher frequencies. Longer horn, larger mouth, lower frequencies.

Grant.