From the thread DIY Diffusors to the Max on 26th Feb 2009
I'm upgrading the acoustic of my studio since a few months now (making bass-traps, polycylindrical diffusers, and frames for Auralex foams in a modular way)
I was linked to this thread at the begining of my project and i've decided to move to a rubber membrane for my bass-traps instead of ply-wood.
The design of mines seems to be similar to yours:
-30 cm depth
-frame made of 18 mm pine wood
-back made of 12 mm MDF (quite rigid, according to me)
-membrane made of 4 mm Rubber (SBR, density of 1.55, Shore A about 60/70)
-membrane dimension 60 cm*120 cm
I've made 2 of this type (should resonate at 44Hz without damping, 37Hz with damping)
I've also made 4 with 20 cm depth and 3 mm rubber membrane (should resonate at 62Hz without damping, 52Hz with damping)
According to the dimensions of my room, I've got modes at 42.5Hz, 48Hz and 64Hz. When I designed the bass Traps, I only had the formula without damping so they might be tuned 1.2* to low but according to your experience, they should work pretty well if they where designed like yours !
But they don't ! No need of ARTA to tell you that I still have a 120 dB null (at least, no kidding
) at 64Hz at listening position ! (quite the same at others modes)
What's wrong with mines ?
-Are your membranes made of 4mm rubber (or 6mm) ?
-Did you joint and sealed everything (mines are not, but yet fermly and precisely fixed)
-What did you use for damping (I've used Auralex foam, thinking it was Hi-Density Foam, but I've found that you can buy 3 to 4 times more dense foam, since Auralex is about 20 Kg per cubic meter)
Please, let me know about that precisely tweeked after long long research and all the controverse about velocity/pressure, Hi-density or not, rigidity or not, big or little air flow resistivity performance of porous absorbers, and especially when used in bass-traps !!!