flexCL as insulation for broadband absorber?

I am on the verge of building my first broadband absorber, just the simple type, a wood frame with some insulation material inside covered by a cloth. I am thinking about 100mm (4 inces) of insulation with a 50mm air gap behind it. It seems that there are two insulation materials that I can get my hands fairly easy at a local store, but I am looking for some advice which one to use. The first one (flexCL by Homatherm) has a pretty high flow resistivy (more than 70 kPas/m^2) and a gross density about 72 kg/m^3. From the readings I have done, such a high density and flow resistivity is too much for good sound absorption. And when I enter these figures into Chris Whealy's amazing Porous Absorber Calculator and don't get very promising results. However, the company publishes a very trustable looking measurement report which yields very nice absorbtion coefficients (for 50mm thick flexCL without any air gap): f/Hz 125 [removed] 2000 4000 alpha 0.2 0.8 1.0 1.0 1.0 1.0 Unfortunately the report is in german, but if anybody is interested the link is: http://www.homatherm.com/fileadmin/medi ... flexcl.pdf Some information in english can be found here: http://www.homatherm.com/uk/products/fl ... ption.html And the meassurement of the flow resistivity here: http://www.homatherm.com/fileadmin/medi ... flexcl.pdf So I wonder what you experienced guys think about the stuff, did anybody in the forum ever work with it by chance? Is there much more to the absorption than the flow resistivity? Or am I wrong and is the high resistivy a real advantage of that stuff? The other choice I have is a mineral wool called Termarock by rockwool which I guess comes pretty close to the famous OC 703 with a density of 50 kg/m^2 and a flow resistivity of 16 kPas/m^2. Again, I just have a german link to the given data: http://www1.rockwool.de/graphics/Testwe ... 150306.pdf I did not find any published absorbtion coefficients though. What would you recommend? Should I just go for the mineral wool and probably be fine (although I do not have any numbers regarding the absorption)? Or would flexCL be even better? Or am I just worrying to much anyways and should be the cheaper one? :-) I am sorry for the long post and I am really looking forward to any advice. Many thanks in advance!

Hi, download the software ZORBA by Marshall Day Acoustics as a trial version, it is slightly more accurate in these low absorption areas. If I enter 70 kPa*s/m² for the homatherm Flex stuff then the result is pretty much according to the measurement. However you will not get good bass absorption with it, not even with thicker sheets. Download the Rockwool data sheet for Thermorock at their site, the flow resistance is printed there. Bottom line the homatherm flex stuff will be worse, not better.

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