These graphs are examples of the effects of baffle diffraction and the results of adding felt to ameliorate it. This baffle is 7" wide by 12" high. A picture of the felt applied can be seen at the bottom of this page.
ALIGN=""The tweeter is flush-mounted with a small (lateral) offset of 7/8". Its center is 3¼" down from the top edge. I had just received some new felt (½" thick), SAE Number F-13, Firm Gray Felt. It is available from McMaster-Carr under Pressed Wool Felt. F-11 would probably be slightly better as it has a higher wool content, but I wanted some with adhesive backing. Of the softer felt only F-13 is available from them with this. It's available in strips up to 2" wide. I bought a 10' length of 2" wide ($16.80) and one ½" wide ($6.30). The 2" is probably all which is needed, since it can be cut without much trouble.The ½" thick, ½" wide strip actually is about 5/8" thick. I tested it but then used cut strips from the 2" wide roll because of aesthetics. I didn't like the protruding of the smaller felt. Plus, a 10' roll of 2" wide felt will go a long way!
The tweeter offset had helped to reduce (spread out) the diffraction effects, but I wanted to see just how flat the measurement could be. I am extremely pleased with the results.
One thing to note is that all measurement graphs in this section have no smoothing applied. The inherent smoothness of these graphs is more surprising when this is taken into account.