In the test parcours the speaker has to go through during development it was found that the 650 Hz sag and the unevenness of the FR below was not caused by the typical internal resonances alone but by stuffing issues with the housing part of the upper mid wooofer. The cabinet was over-damped and stuffed ineffectiviely.
Also, the stuffing of the lower mid-woofer cabinet was not optimal.
So the issues have been sorted out step by step supported by measurements of the electrical impedance, which nicely reflect resonance issues.
During the course of re-damping, a few re-designs of the housing had been implemented to further support the effectiveness of the damping. Some of which were:
- shortening the tweeter pipe inlet in order to make the pipe construction more compact and better follow the internal wooden structure of the cabinet
- provide more space around the pipe for damping material where it can be more effective compared to the pressure maxima towards the bottom of the housing.
The successive approximation and the final result can be seen in the posted diagrams.
They start with the undamped resonances all the way to the correct amount of damping, which is right at the border of too much and too little.
The remaining issues between 300Hz and 450Hz are due to fact that the current damping material has already been worn out a little. They come and go with every measurement and are simply a sign of the critical damping at play. I am nearly convinced that fresh material will take care of it.