John Sayers' Recording Studio Design Forum

Hi guys.

Part of my bass trapping Plan in my room currently under construction is the following.

5 1/2 inches of space between joists on the interior ceiling to be filled with 32kg/m rockwool (Bradford Soundscreen r2)

I will also have a screened off rear wall with 7 inches of he same stuff with a one inch gap to the wall.

More traps will be added as needed

Ideally I would like to use some 48k/g m material as most of you are. However In Australia that would cost me more than Four times as much !

Would be correct to assume the thicker size of 5 or 7 inches of this stuff would be equal to 2 or 3 inches of the 48k/g material ?

As it is I have a 7.6 x 10.6 x 19 ft long room. Even the 32 kg rockwool will cost Over $600 to do the back wall and ceiling. The 48 kg stuff is out of the question though I will get a bit of this stuff for a additional cloud and traps. Thoughts? Thanks.

Not really, unfortunately. In fact, the issue is not really density at all: the issue is "gas flow resistivity". That's the technical characteristic of the insulation that actually matters, acoustically. It refers to how the material resists the flow of gases (read "air") through it, because at the microscopic level, that's sort of what is happening: Sound waves have two components to them: velocity and pressure. Absorption works on the velocity component not the pressure component. As the sound waves move through the material, the air molecules are vibrating backwards and forwards in place, and sort of "bumping into" the insulation fibers around them, transferring some of their energy to the fibers, turning that into low-grade heat energy, and thus removing sound energy from the wave. So it is the characteristics of the fibers themselves and how they react to air molecules moving past, that matters.

The unit of measurement for gas flow resistivity is the rather daunting "pascal-second per meter" (Pa·s·m^-1), or alternatively "newton-second per cubic meter" (N·s·m^-3). That doesn't help much at all! Meaningless scientific techno-mumbo-jumbo. Plus, most manufacturers of insulation products (fiberglass, mineral wool, etc.) don't bother measuring or publishing the figures for their products, firstly because it doesn't matter for the main purpose (thermal insulation), and secondly because most people wouldn't have a clue what it means!

However, fortunately there is a very rough relationship between density and gas flow resistivity for each type of insulation, and since most manufacturers do publish density figures, you can sort of figure it out. That relationship isn't very linear, and isn't very constant or precise, but it is good enough for most purposes. 48 kg/m^3 is about right for mineral wool, 32 kg/m^3 is about right for fiberglass.

OK, so the issue is this:
(Not sure where that graph came from originally: if someone knows, please tell me so I can add the source)


As you can see, there is an optimum gas flow resistivity, and therefore an optimum density, at which you get maximum absorption. If you go too dense, or not dense enough, then you don't get good absorption. So you cannot just use twice as much stuff that is half the density, nor half as much stuff that is twice the density. The relationship between density and resistivity is not linear, and neither is the relationship between thickness and absorption.

Then there's the issue of frequency, as well: Gas flow resistivity changes with frequency (it is more like impedance than resistance), so for each frequency range there is an optimum density. It turns (counter-intuitively) that higher density is NOT better for bass frequencies: on the contrary, lower density is better for bass. So if you use more dense insulation, you will get better absorption in the highs, less good in the lows. And there is also a relationship between thickness and frequency: thicker insulation improves absorption of low frequencies, for several reasons that I won't bore you with.

And just to help things get even muddier: it depends on the manufacturer as well. Different manufacturers use different processes, binders, and packaging, and therefor there are differences between the behavior of one manufacturer's 48 kg/m^3 mineral wool, and another manufacturer's 48 kg/m^3 mineral wool.

So, going back to your original question: you can't compensate for 50% lower density by making it twice as thick, since the relationship is not linear. But in reality lower density is a GOOD thing for bass traps, and thicker is always better. Your 32 kg/m^3 mineral wool is probably fine for bass traps. It might not be the perfect density, but is still well up on the curve, and should work quite decently. And 6" inches of thickness is also very decent, so you should be OK there too.

There's also the issue of how you build your traps: Since all room modes terminate in corners, maximum effectiveness in a bass trap comes from the "superchunk" type of design (fill up the entire corner with insulation, diagonally, 36" across the front face, and floor to ceiling), but a close second is the "corner absorber" design, which is just a floor-to-ceiling frame, with 6" thick insulation, placed diagonally across the corner. They both work down to quite low frequencies, and are very effective. So that's would probably be a good option for you. Put one of those in each vertical corner, and also in some of the other corners if you can, for good bass trapping.


- Stuart -

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I want this studio to amaze people. "That'll do" doesn't amaze people.

Very informative as always Stuart, thank you very much.