I'm glad we've got the new guy here to straighten us all out on that issue!
Seriously, I'm no expert, but the data clearly shows that rigid fiberglass and rockwool do a pretty darn good job absorbing down to the 125 to 150hz range. I'm not sure what you consider the "low mid and bass range," but rigid fiberglass clearly does more than "reduce the reflection of upper midrange and high frequencies." Sounds like you are describing the performance of foam.
On Ethan's site, he explains: "705-FRK that is four inches thick and spaced 16 inches away from the wall can be quite effective to frequencies below 125 Hz." He also has data showing that higher density provides better absorption at the lower frequencies.
Check out Table 1 on Ethan's site (it lists absoprtion coefficients):
http://www.ethanwiner.com/acoustics.html
Check out Page 2 on this website for rock wool absorption coefficients:
http://www.fibrexinsulations.com/pdf...uationFire.pdf
I do believe that sealed bass traps like those described (and manufactured) by Ethan provide absorption below 150hz, but stating that rigid fiberglass panels only absorb the upper-mid and high frequencies is just plain wrong.
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