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This is a very large and complex topic and far beyond the scope of a short post…

First, density, while a convenient attribute, is not an appropriate factor in absorption. We simply inherit this characteristic as it is how thermal insulation is commonly characterized.

But for a given density, this tells us Nothing about the constitution of the material! For a given density, it may be one single piece of material per unit volume, or it may be a million small pieces featuring any physical characteristics in the same unit volume. …Or anything in between…. They are Not equivalent in terms of their acoustical impedance.

One should note an important and oft ignored aspect of acoustical impedance is not only resitivity which is constant over the entire frequency range, but the reactance – the frequency dependent variation of resistance.

The pertinent variable in absorption is flow resistivity. And a commonly referred characteristic is called gas flow resistance.

Factors that influence absorption are porousity, the ratio of total pore volume to the total volume of the absorbent material, and tortuosity (the complexity or structural form factor of the paths within the material)

Now, you are probably thinking: “Thanks allot! You have taken something that was relatively understandable and rendered it totally incomprehensible…”

And perhaps I have. The point being that the behavior of such materials is relatively well understood where various models vary in terms of decimal places. But it is also the case where the models are more complex than those that lend themselves to simple generalizations.

Hence, while I truly do admire and encourage experimentation, I might suggest first concentrating on understanding the basic ‘best practices’ guidelines that have been established, understanding the basic models for velocity based porous absorption, and after these are well understood Then attempting to ‘invent a better mousetrap’.

General guidelines:

For broad band absorbent panels, the best material density characteristics are either ~3lb/ft^3 Fiberglas or ~4 lb/ft^3 mineral wool.

And the minimum configuration that should be considered for a broadband panel is 4" thick with a 4" boundary gap. (Any thinner and the effective low frequency absorbent extension suffers. In other words, they are effective against high frequencies but their ability does not extend low enough to address all of the problem specular energy.)

This configuration effectively behaves similarly to an 8” thick panels placed flush to the wall (effective to ~420 Hz)


Low Density Pink Fluffy Stuff for THICK Porous Bass traps:

The light pink fluffy stuff is ONLY useful for thermal insulation and for very thick low frequency bass traps (e.g. “Superchunk” style corner traps with 2- 2 foot wall facings and a 34” front face) where it outperforms the denser more commonly suggested materials.
 

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Closed cell EPS is a reflector, not an absorber. And unless you can grind it up extremely small and avoid it becoming packed, the porosity will be insufficient to render it of much use. This panel's acoustical impedance will exhibit low resistivity and high reactance. Not a good combination at all.

And cellulose at the same or similar density will not function similarly as it is much 'clumpier'. This is a classic example of similar densities not having translatable gas flow resistivities.

Thus, I can't imagine why you would instead source the much less effective cellulose fill instead of the more effective Fiberglass or Rockwool - which are made of sand (silica) or basalt rock respectively.

Thus, the easy part is the 4" standoff gap for a panel that will not offer much advantage at all. One can use either retail L brackets or simple commodity shelving brackets to hang the panel(s) while using dowels or rigid door stops as standoffs.
 
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