Quote:
Originally Posted by esaias  I had this idea of building basstraps so that I have the surface layers made from "real" acoustic slabs(~40kg m3, kinda like Paroc AKU or Echophon Classic A) and the inside would have the thicker material.
I'll do some plans right after new year and post them to GS fo your evaluation 
-Tomi |
Tomi:
I'd be careful about investing a great deal of time and effort in making sandwiches of different materials - such constructs can behave in unpredictable ways as the sound passes through media of varying gas flow properties and encounters multiple impedance jumps going from one layer to the next.
You generally get more benefit from shopping your local market for the best material [balance, cost, material handling ease, and the import that overall weight of the panel may have to you], and then making your panels of only that one material - of a thickness to match the abilities of the mineral fiber chosen - adding only an upholstery layer that is as sheer [breathable] as concerns of durability and aesthetics allow.
Making stacks of the same material [using two 2" panels of 703 to make one 4" panel] works out fine, as long as you stay away from panels with foil or other scrim facings.
The 4" thickness range is generally held to be the approximate efficiency "sweet spot" for panels made from 3 lbs. to 6 lbs. pcf fiberglass - or 60 to 130 kg/m3 rockwool. Making them thicker makes them better, but you get more Sabines per $ making for example - using 703 - two 4" panels versus one 8" panel. OTOH, with a lower density [gas flow resistance] fiberglass material the thicker panel would yield efficient improvement.
Getting denser than the ranges described above may work - but the cost per panel is bound to rise roughly in line with density and you may not get the results you want. Don't confuse density with performance capabilities. Its all about balancing thickness with gas flow properties - and price with ease in handling and overall panel weight.
I believe that Mini-Traps use 3" of 705 with a felt facing, and GIK's 4" panels use an 8 lbs. rockwool with canvas upholstery. Our standard 4" product, the RT424 has a 703 core - or if you desire we make them using 705 - but I wouldn't suggest you let any of those choices overwhelm your thinking. A manufacturer has a number of concerns arising from the supply process, the manufacturing process, and the shipping of finished unit that would matter little, or not apply to your circumstances. Any material from the ranges described above will work well acoustically - shop for price and ease of handling that match your local market and your project.
Good Luck!