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Questions regarding DIY Absorber around my Drums
Old 13th February 2020
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Questions regarding DIY Absorber around my Drums

I recently found some nice old boards in my wood stash and now I'm thinking of building some acoustic absorbers using those. Reading into the topic a bit i discovered that rock wool / mineral wool works quite nicely as absorbent material, so my plan is to build some nice wooden frames of 160cm x 60cm (ca. 62 x 24 inch or 5,2 x 2 feet for all you non-metric people out there :P), all of them 16cm deep (or 6,2') and stuff them with some rock wool.

Now reading a bit further into it i learned that the flow resistivity measured in Pa.s/m2 of the absorbent material is quite important when it comes to designing these things and as far as i understood it from various threads on the internet the general advise is to use rock wool with a higher flow resistivity (≥16kPa.s/m2) in less thick absorbers (under 15cm / 6inches) and rock wool with a lower flow resistivity (≥5kPa.s/m2) in thicker, deeper absorbers (over 15cm / 6 inches).

So then i discovered this calculator from acoustic modelling and tried some different combinations of rock wool (and their respective flow resistivity) that are easily available at my local hardware store and found that ROCKWOOL (the company) has two types of products that seem to work out as far as numbers go. ROCKWOOL Sonorock with a flow resistivity of ≥6kPa.s/m2 and ROCKWOOL Termarock with a flow resistivity of ≥16kPa.s/m2).
Unsure of whether it's better to use the one or the other made following calculations:

Besides having two curves for only Sonorock and only Termarock i also tried different "sandwich-conifgurations" where i have an outer and inner layer of Termarock and Sonorock respectively.
The thought behind those sandwich combinations of Sonorock+Termarock was to minimize reflections. Termarock being the material with the higher flow resistivity and therefore higher density should - as far as my understanding goes (which usually isn't very far) - be more absorbent in lower frequencies but more reflective at higher frequencies. So to minimize the reflective property of Termarock i thought about countering this effect by putting a layer of Sonorock on top and therefore trapping higher frequencies first (sonorock) before they hit the Termarock layer beneath (being more effective at lower frequencies).
And just to be complete i also let the calculator do a version where Termarock is the outer layer and Sonorock the inner on.

And after all that it seems like (as far as the calculation from acousticmodelling goes) the combination of Sonorock (low flow resistivity) as outer layer and Termarock (high flow resistivity) as inner layer gets me the "smoothest" curve.

Now my question is, is there something obviously wrong in my thought process. All i have is the above calculation and very limited information on this topic so the probability of me being just an idiot trying to figure this out is quite high it seems. Also Termarock mineral wool is lot more expensive than Sonorock. So I'm also unsure whether it's really worthwhile using the more expensive Termarock when the differences between using only Sonorock vs. using a combination with Termarock doesn't seem to be that big? Idk.

Maybe someone has some good tipps on this.

Thank you all and cheers!
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