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Old 16th March 2008
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Hannes_F's Avatar

Originally Posted by Ydope View Post
No, but at least for corner traps.
The denser stuff doesn't work worse, even with increased thickness. There are diminishing returns, and thus at some point "more is not better", but 703 doesn't absorb more at 6" than 705, with ~ 45 KPa s²/m² for the latter.
It seems that the simulation is not accurate in the bass region and especially in the context of corner traps.
At least that's what Ethan's density tests show.

I see your point.

Actually if I look at the measurements in Ethan Winer's density report I do not really see a significant difference between the 6 pieces 6" 703 and 705 (without paper). For the 12 pieces á 3" I see better modal ringing reduction for the 42 Hz mode with 705. However some of the higher modes seem to be a little better with the 703 material.

This is actually typical for the curve the Porous Absorption Calculator gives if we compare different gas flow resistivities. Often there is a tipping point that remains unchanged, and the absorption below that moves in the other direction than the absorption above.

Also if for a rough check we insert 6" as a average distance and 3" as absorber thickness, 16.6 kPa*s/m² for 703 and 25 kPa*s/m² for 705 then the absorption should be 23 % for 703 and 26 % for 705 (42 Hz). Here the calculation and the experiment show conformity.

I admit this does not really work for the 6" thickness. I suppose there are additional border effects, or the gas flow resistivities are a little different actually. Or the fact that there are these angled surfaces behind the panel that radiate waves into all directions changes the circumstances.

Nevertheless we should understand this:

This is false: Bigger gas flow resisitivity is always better for absorbing low frequencies.
This is false: Lower gas flow resistivity is always better for absorbing low frequencies.

This is true: For each combination of air gap, absorber thickness, chosen frequency and geometry (wall, wall-corner, corner-corner) there is an optimal gas flow resisitivity. If we go higher or lower from there, we loose absorption.

So I think that OC 705 is a good material for low transmission (for what it was designed in the first place). And it probably is one of the best materials (or even THE best material) for absorbing certain low frequencies - ... but only as long as you restrict yourself to relatively thin panels.

Think about this analogy: We fire with rubber bullets on targets made from different material. Let us say the thickness of the target is relatively thin and the target gives very low resistance, like wool or paper. Then most of the bullets will penetrate the target, bounce at the wall behind and penetrate again. No much effect.

Now we take a material that gives more resistance. With that we can catch more of the bullets, but quite a portion will be reflected.

If we now allow ourselves to have a thicker target then the lighter material may become more useful again. Because of its thickness, more bullets will be caught than before, but because the material is soft the reflection is very low. The overall absorption we can achieve with that material is better than with the denser material.

This is only a rough picture of course because sound waves can not only be reflected at the surface of the material but also inside and on the backside of the material. But you get the idea.

Since several vendors of acoustical treatment are reading here I would not be surprised if they come out with a GigaTrap or something alike that is 12" thick and filled with low resistance material. This would be not a bad idea and if you watch it happen remember where you read it first.

Actually I am thankful for your comment Ydope, because it while thinking about it I found a way to calculate better corner traps for my own needs. Thank you!