If the midhorn has 90º flare and placed in the corner, the mouth will probably be less than a foot away from the walls. So the walls will act as flare extensions up to a few hundred Hertz, setting the horizontal pattern and smoothing the response. Above that frequency, the walls will no longer be significant but the horn itself will play the major role. At higher frequencies, horn response is smooth without boundary reinforcement and the horn is better able to control directivity.