I have been contacting a few companies to get perforated mesh for some speaker prototypes I have been building. I have so far used clip on grilles per driver, proprietary stuff that you can buy. However, looking at perforated mesh and settling on either a 10mm square hole or 10 mm circular hole mesh, I am still getting an overall 'open area' of 50 - 60 %. These are the better percentages I can get, most seem to be 40% or less.... Can the speaker bldg gurus here comment on this? At what percentage coverage does the grille impede performance?