John Whittaker showed me a good looking material suitable as "grille" for the ribbons. The graphs below show the acoustic difference. Noticeable is that the sharp peaks in the "raw" data on this indoor test is smoothen'd (less variance ) out somewhat while not showing any high frequency drop in response as often seen with materials that are deemed "acoustic transparant".
The resolution in the low end of the above graph is different than most other measurements on this website in that the test pas performed using a 4096 point MLS, allowing import in IMP/M. Most other tests performed use a 16 k MLS or a gated SINE instrument.