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SaSi_SiDi · 9th February 2010

The primacoustic design is based around a 3" thick absorber panel mounted 5" away from the wall for a total thickness of 8" (as far as I understood the explanation and diagram).

The limb mass is suspended in the back of the absorber panel allowed to freely vibrate. That means that it should have a clearance from any surface (either the absorber panel or the back of the cavity) of 1" or so.

Now, as we all know, the wave velocity is higher as we move away from rigid boundaries, so the sheet is best suspended as close to the absorber panel as possible. At least that's how I understand it.

Regarding mounting, I am considering to use two pieces of 30x40mm hardwood planks and sandwitch the bitumen sheet between them. Then mount the sandwitch to the wall.

I am thinking of two scenarios. First scenario is for a corner trap and in such a case the sheet needs to hang at an angle to the wall-wall corner. My design for the corner traps calls for a 30 degree angle, so the bitumen sheet needs to hang at this angle behind the trap. Obviously I need to find a way to mount the wood planks at the two ends (angle cut to mate with the wall).

I am not sure if it's feasible to hang a single 250cm (8ft) long bitumen sheet. It has to do with the weight. The 10meter long roll is a bitch to lift and carry but manageable for me. I haven't weighed it but I am guessing 35~40kg (70~80lbs), so I am looking at a 7~12kg weight for the sheet plus wood support. It's easy to lift 10kg but not that easy to make precision mating with a 10kg inflexible sheet 250cm long. I may have to split it into parts.

I am also thinking that if I make a single sheet 250cm long, then the weight of the sheet itself will make the top part of it tension to the point that it defeats the point of "freely vibrate". I also need to think about ageing and the weight might elongate the sheet and deform it - perhaps to the point of failure. I have never worked with bitumen sheets before freely hanging. They are usually secured and glued in place.

So, the alternative is perhaps to combine the sheet in the back of each corner trap module. I am planing to make the floor to ceiling trap in parts, each module being 2ft high. Perhaps I need to re-design the top part of the frame so that the bitumen sheet can hang from there in the back of the trap.

9th February 2010	#842
SaSi_SiDi Gear maniac Join Date: Mar 2009 Location: Athens, Greece Posts: 172	The primacoustic design is based around a 3" thick absorber panel mounted 5" away from the wall for a total thickness of 8" (as far as I understood the explanation and diagram). The limb mass is suspended in the back of the absorber panel allowed to freely vibrate. That means that it should have a clearance from any surface (either the absorber panel or the back of the cavity) of 1" or so. Now, as we all know, the wave velocity is higher as we move away from rigid boundaries, so the sheet is best suspended as close to the absorber panel as possible. At least that's how I understand it. Regarding mounting, I am considering to use two pieces of 30x40mm hardwood planks and sandwitch the bitumen sheet between them. Then mount the sandwitch to the wall. I am thinking of two scenarios. First scenario is for a corner trap and in such a case the sheet needs to hang at an angle to the wall-wall corner. My design for the corner traps calls for a 30 degree angle, so the bitumen sheet needs to hang at this angle behind the trap. Obviously I need to find a way to mount the wood planks at the two ends (angle cut to mate with the wall). I am not sure if it's feasible to hang a single 250cm (8ft) long bitumen sheet. It has to do with the weight. The 10meter long roll is a bitch to lift and carry but manageable for me. I haven't weighed it but I am guessing 35~40kg (70~80lbs), so I am looking at a 7~12kg weight for the sheet plus wood support. It's easy to lift 10kg but not that easy to make precision mating with a 10kg inflexible sheet 250cm long. I may have to split it into parts. I am also thinking that if I make a single sheet 250cm long, then the weight of the sheet itself will make the top part of it tension to the point that it defeats the point of "freely vibrate". I also need to think about ageing and the weight might elongate the sheet and deform it - perhaps to the point of failure. I have never worked with bitumen sheets before freely hanging. They are usually secured and glued in place. So, the alternative is perhaps to combine the sheet in the back of each corner trap module. I am planing to make the floor to ceiling trap in parts, each module being 2ft high. Perhaps I need to re-design the top part of the frame so that the bitumen sheet can hang from there in the back of the trap.