By 'diameter' I presume you mean thickness. For my panel traps I used 1/4" plywood. You gotta be gentle with it, but it worked well. I had a mix of 4" deep and 6" deep traps, and their center absorption freq. was between 70 and 100Hz. If you want your trap to absorb deeper, you can try using a heavier membrane or an even deeper trap. If you've got these built already, try a 3/8" membrane panel.what kind of wood and which diameter should i use for the membrane that closes the box. im going for reasonable absorption between 60hz and 200hz?
Oh, and you mentioned filling the box with insulation- but I'm assuming you know that you shouldn't fill the box completely.
For those absorbers, the back panel won't make too much difference. The idea is just to mount insulation with a space between it and the wall, so whether there's a back panel on the box or not it'll work about the same.my second question: i build a box for the hi/mid absorber from mdf (12mm) 1m x 2,3m and already bought some nice fabric to cover the front. i want to place the absorber in the two corners behind the studiomonitors. the question now is: can i close the back with a solid peace of wood (i already have the needed wood) or do i have to leave it open at the back and cover it with fabric like the front? would the absorber function with the solid-back-plate? building the thing with the solid-wood-back would be a lot easyer...
You could save money by leaving it off, but your box will be much sturdier if you put it on there. If you wanted you could cross-brace it or something- it'll all be hidden from view anyway.
Hope this helps.
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