So after doing a bunch of reading about GFRs the last few days, I got to playing around with the Porous Absorber Calculator
. I thought about the materials I had available to me, my space limitations, and build complexity and decided there were a few different absorber layering combination possibilities that would meet my requirement of maximum of 6-6.5" of depth (including air gaps). Here's what I got... note the frequency range has been limited to 40-500Hz (the range giving me problems)... Porous Absorber Calculator - Results
I found the differences of having the air gap between the panels interesting, so I also compared changing which material was on top... Porous Absorber Calculator - Results
Do note, these calculations were done using a GFR of 16000 for the OC703 (per OC) and 14000 for the SAFB (estimation for 2.5pcf density). All other parameters were left untouched.