Hi Andre,
sorry, maybe I did not get what you mean.
I am using material with low flow resistivity, around 5000 Pa*s/m². Cheap stuff.
Test these values in the Porous Absorption calculator:
Absorber thickness 100 mm
Air gap 100 mm
Start graph at 30 Hz.
Absorber flow resistivity 5000 or 16800 or 45000 Pa*s/m².
First, we see that the absorption is definetely different in the bass range.
Then, if we compare the 5000 Pa*s/m² with the 16800 Pa*s/m² curve we see that below ca. 135 Hz the absorption is better for the the more resitive material. Tests have shown this as well like Ethan Winer's site shows.
However if this leads to a "more is always better" thinking this could be an error. If you watch the curves closely you will see that there is a frequency range between 135 and 700 Hz where the fluffy 5000 Pa*s/m² material absorbs better. Moreover, the 45000 Pa*s/m² material aborbs worse than the 16800 Pa*s/m² material.
This is definetively counter intuitive. But if you think about it then denser material should be good for insulation, and this can include a good portion of backscattered energy.
Now enter an absorption width of 300 mm and a gap of 200 mm. Surprise - the 5000 Pa*s/m² is best!
If this is true then my conclusions are:
- If you are restricted to panels in the range of 10 cm or 4 " then material in the range of 16000 - 25000 Pa*s/m² is probably your best choice. With other words OC 703 or 705. (But do yourself a favor and don't ignore the demand for an air gap behind them).
- If you have more space then consider thicker absorbers with bigger gaps, filled with fluffy glass wool of 5000 Pa*s/m² or even less.
Now how relevant are those simulated values?
#1 Download the software ZORBA by Marshall Day Acoustics for a double check. It says the same if you activate "alpha normal". If you check their website you see that they verified their simulations with experimental data and the two are very close.
# 2 I am not finished yet with my studio but the traps that I installed so far behave exactly as they should according to these simulations.
My conclusions:
- More is not always better.
- It always depends (i. e. on the frequency range, the absorber width and position, the gap width)
- I try to know what I am doing. Read, read, read (I mean books, scientific articles, papers - not necessarily forum opinions).The links to the literature are everywhere. Start at the bottom of this site:
http://www.bobgolds.com/AbsorptionCoefficients.htm . Read all the BBC articles about their acoustics tests.
- I try to understand the theory but also run tests in between.
Somebody who does not know what he/she is really doing may end up spending much money with mediocre results. Cruel but true :-)
Hope this helps
Hannes