>if so, I'm thinking of making triangular boxes (to be stacked in corners) full of fibreglass with a thin plywood front as a membrane ? should i try different membranes - ie some with thicker ply and some with rubber sheeting or roofing felt ? Since this a bodged solution I wonder if I should try and cover my 40-160 Hz range better by using many different types of membrane ?<
I don't think you should do it that way (if I understand you right
). If you are making panel traps with a plywood front, those would go right on the wall in the corners. I think what you are talking about making is called a super chunk. All that is, is cutting rigid fiberglass to triangles and stacking them from floor to ceiling (no wood fronts to them). These will be the best for bass trapping if you have the money for them. If you want more of a limp membrane trap then you would us FRK on the fronts of them or build 4" or 6" panels and put them straddling the corners with FRK. I myself recommend using FRK when you can and non FRK for the panels that would go on the first reflections in the room.
Hope that helps
Glenn