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Discussion Starter · #1 ·
Before doing anything to the more public parts of my house (my HT/Living Room), I thought I'd experiment with some home-made absorbers in an echo-y workout room in my basement. I was thinking of using a combination of cellulose insulation, cotton batting, and EPS foam (from packages) that I had left over from when this house was built.

With the goal of making the structure light, and assuming the basic starting point of a 4' x 2' x 4" rectangle, I was thinking I'd glue balsa to strips of the EPS to make an outside frame, with additional strips of EPS as a supportive grid within the frame. Then I'd fill the grid with cellulose and finally line the top and back with compressed cotton batt, glued down to the EPS grid and cover the surface with fabric (don't know what yet).

While the result isn't particularly critical, I'm having a hard time figuring out the net effect of increasing densities of the cellulose and necessarily of the overlying cotton batt (it would need to be compressed more to hold back the cellulose without bulging). I was thinking of just starting with--not fluffed up but not compressed--but that seems a bit lazy. Any thoughts on, for general echo control, what kind of density I should shoot for?
 

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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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Discussion Starter · #3 ·
Thanks for the detailed reply. I think I can find a decent reference for a lb/ft3 for the cellulose, and I'm pretty sure it'll be somewhere in the range you stated for the fiberglass and mineral wool.

The harder part will be figuring out how to achieve a 4" boundary gap. Almost all of the walls in that area are solid-SIP panels. It also begs the question whether the EPS grid of the absorber will act as a boundary or as a small membrane that would assist with low frequency attenuation.
 

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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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Discussion Starter · #5 ·
The cellulose was not sourced for this project. I have a couple of bails just sitting in the garage after filling my roof with it. Right now make a nice cushy stool when I'm working in there, but I was trying to think of something fun to do with them. It sounds like I'd be better off just using the cotton.

I thought of using the EPS because, aligned right, it could add some structure without adding much weight. Crumbled it has little purpose and will get everywhere. Basically anything that would serve the same purpose would be reflective.

Thanks for the idea on the brackets. Sometimes some really obvious solutions never fall into my head.
 
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