It isn't the tweeter. It is how it is mounted. You have have some throat style cancellation on the front or back side.
Also, as soon as you make a full line of those drivers it will change everything. As frequency decreases you will get coupling that will cause a lot of output gain. However, as frequency decreases the wavelengths start to beam and coupling goes away. By 20kHz you have no more output than if you were using just one driver.
This is tremendously valuable Danny, thanks so much for chiming in. The drivers are flush mounted on the front baffle - I did mount various waveguides in front of them, but I also measured it with no waveguide at all - just flush mounted. That notch was still there, basically unchanged with the waveguide present. However, I do have a small amount of webbing connecting the tweeters on the backside. Similar to how Jaytor's are now - the new design CaptainHemo milled up. I do have the edges rounded over on the back, but it's a small roundover because there isn't a lot of material there. Are you saying that you expect the nature of that notch to change when the full array is present?
