I made a prototype Pi midrange horn, but with a B&C 8PE21 driver and I have 2 problems. One is a huge dip in response at 1khz on axis. It moves to about 900hz and gets worse off-axis horizontally.

The other is an odd impulse response with a second positive pulse a short time(about .2ms) after the initial pulse. This may be reflection off the back of the enclosure, which is 4" deep, but I'm not sure.

I first measured the horn without an enclosure, but the 1khz dip seemed even worse, which I thought was due to backwave cancellation, but apparently not.