> There is plenty of published data on fiberglass absorption. <
The problem with the published data is it ignores how rigid fiberglass is actually used to absorb low frequencies. Believe me, I've been around and around with a number of acousticians about this, and I remain convinced that density is a big factor in LF performance. Below are the points I made recently about this elsewhere.
Some people claim that low frequency performance of rigid fiberglass is not a function of its density, and use as evidence the data published by Owens-Corning that compares 701, 703, and 705. The density of these types of rigid fiberglass increase with their product numbers. That is, 705 is denser than 703, which in turn is denser than 701.
I have never performed lab tests using different densities, but there's a lot of evidence that increasing density does in fact increase low frequency absorption. For starters, the head acoustician at Owens-Corning (Dr. Godfrey) told me it does, and as proof he sent me a 20+ page technical article. Data faxed to me by Rock Wool Manufacturing in Leeds, Alabama also shows a direct correlation. Further, I believe making rigid fiberglass more dense also increases its absorption at low frequencies because it starts to behave as a membrane all by itself.http://www.ethanwiner.com/oc-data.gif
The data often cited, above, is incomplete because it's given in octave bands that go no lower than 125 Hz. Since I have personally measured fiberglass mounted across a corner to have a peak absorption well below 125 Hz, it's clear that octave data down to only 125 Hz is inadequate.
The Owens-Corning data above is also riddled with inconsistencies when viewed in this light. If we consider rigid fiberglass as having linear absorption due solely to gasflow resistance, then this data makes no sense. For example, why is two inches of 705 eight times more absorbent at 125 Hz than one inch? And why is two inches of 701 only a little better at 125 Hz than one inch? This shows that more is going on than plain gasflow resistance. Further, to really understand the relationship between density and low frequency absorption, the material should be measured as it's actually used - with an air gap rather than flat on the floor. By resting on the floor, which is how the above data was measured, any potential diaphragm behavior will be damped.