Ok Mark understood that this is from the drive element itself, but is this an effect only with HF and tweeters, and with respect to all tweeters?
The reason I am asking is that when I checked this out with the manufacturer of the ribbon tweeter, the response was that such incorporation was unnecessary due to the cavity design, but if I had other reasons to do so (which I did) then proceed. I supposed it is a form of waveguide...