Membrane on back of absorbents?

[bwo]

I remember reading that someone said that adding membrane on back of the absorbents increased bass absorption. Is there a consensus on this?

I considering adding plastic (0.20 mm) on the back of my broadband absorbents. And is it important that it's straddled very tight?

[avare]

Your question is incredibly vague. Yes there are certain ranges of insulation depth/density where ading a thin membrane will improve low end absorption. Yes you can put insulation in front of a low frequency membrane absorber to increase the low end absorption.

Yes, there are a lot of variables. The question is how much absorption do you need and at what frequencies?

Andre

[bwo]

Originally Posted by avare

Your question is incredibly vague. Yes there are certain ranges of insulation depth/density where ading a thin membrane will improve low end absorption. Yes you can put insualtion in front of a low frequency membrane absorber to increase the low end absorption.

Yes, there are a lot of variables. The question is how much absorption do you need and at what frequencies?

Andre

I'm not talking about adding membrane on the front. I've already done that with panels which I want to reflect mids and highs.

The panels I'm talking about here are placed in first reflection on sidewalls. They absorp all the way up and are 10 cm thick. Rockwool with 60kg/m3 density. I'm just interesting in a general extra absorpion in the bass if that's possible by adding membrane/plastic on the back. Don't see why I need to know exactly what frequencies for added absorption in the bass region. More absorption in the bass is always better.

I can also do the same with my drop ceiling which I'm in the process of building. 20 cm thick rockwool here.

[avare]

Originally Posted by bwo

I'm not talking about adding membrane on the front. I've already done that with panels which I want to reflect mids and highs.

I can also do the same with my drop ceiling which I'm in the process of building. 20 cm thick rockwool here.[/QUOTE]
Great! We are getting somewhere now. I am not writing about material on the front of insulation either!

The panels I'm talking about here are placed in first reflection on sidewalls. They absorp all the way up and are 10 cm thick. Rockwool with 60kg/m3 density. I'm just interesting in a general extra absorpion in the bass if that's possible by adding membrane/plastic on the back. Don't see why I need to know exactly what frequencies for added absorption in the bass region. More absorption in the bass is always better

So you are asking about the first reflection point absorbers and how get greater low end absorption for the room treatment, as opposed to making the RFZ. With the 60 kg/m3 insulation 10 cm thick, you have good absorption down to ~125 Hz. Adding a membrane with the insualtion flush against the ceiling, or any hard surface, will do nothing acoustically. If you want to increase the low end abosption space the insulation out from the ceiling. This is commonly called gapping here. The amount of gap is not critical. Professionally designed spaces use up to slightly over twice the thickness of 10 cm mineral wool.

I can also do the same with my drop ceiling which I'm in the process of building. 20 cm thick rockwool here.

Fantastic. Note that you can use 10 cm material with a gap and get just about the same acoustic effect.

Good luck with your studio!

Andre

[bwo]

Adding an airgap isn't an option here. You can see the panels on the sidewalls. The frame is 2 cm thicker then the rockwool panels as you can also see. If I add plastic as close to the rockwool as possible, there will be a small airgap between plastic and wall. Shouldn't that imply that the bass meets more resistenst both coming from the front and also when it bounces back from the wall? Which again should imply that the bass absorption should increase?

I believe DanDan was the one who said that adding membrane on the back of the panels increased bass absorption and he had taken measurements of this. If he reads this thread, hopefully he can comment.

Of course my coming suspended drop ceiling will have a large airgap. Also interested to know if adding plastic on the back here would be good.

[Glenn Kuras]

Yes Dan Dan has claimed it and he might be right, but I still would like to see lab testing to really see. Needless to say I don't think you are "wrong" for doing it. Just note it might not yield really much difference in your room response. You would have to try it to see I guess.

[bwo]

Ok. I can try it and post measurements.

[DanDan]

I did say that, due to a Before and After test. Unfortunately I took a notion that the FRK on the back of HF MiniTraps was resonantly sucking all the warmth out of my room. NO EDT at 250Hz, plus when you thump them, which I do all the time , they have a a very easily audible tone.
With some deal of trouble I removed the FRK from eight or so, the RFZ and Cloud. After all this trouble the LF modal behaviour and even the Frequency response measured worse.
So much for 'notions' and intuition.

So as test trumps theory, I am pretty sure that FRK on the back of absorbent panels increases the LF absorption. In much the same way as on the front.

Either way requires an air gap to function at all. If the FRK is on the back, flush to the wall, it effectively does not exist, it is another layer on the wall.

In a lab test, FRK faced panels with an equal airgap underneath have shown increased LF absorption with the membrane facing up.

In that case the FRK is twice the distance from the boundary, plus it is facing a very large airgap, i.e. the height of a test lab. No wonder it measures better.

I believe that test may be the main reason why forward facing FRK is a given. But for reasons stated above, I believe it is also very effective at the back. A useful option if you do not want that little HF bounce returning to the room.

I have mused a little, so just for clarity for the OP, a back membrane will have no effect without an airgap. However, in conjunction with an airgap, well worth it to hit those side and vertical modes.

DD

[Yiannis]

What about the paper (2mm is what I have) in front of the absorber like this.

Is it good or bad?



Uploaded with ImageShack.us

[Glenn Kuras]

Looks like poly to me.

[DanDan]

Me too Glenn. Please explain yiannis.
DD

[Yiannis]

Ok,

I have these paper sheets and I am trying to see if I can get some life back in my tracking and control room.

The problem is that the paper is not touching the absorber and I am afraid that there will be some vibration.

I have to take some before/after measurements.



Uploaded with ImageShack.us

[Glenn Kuras]

I have never done it with paper but I can't imagine it resonating any kind of tone back into the room.

Edit:Diffusion generally is not very effective in corners, so if those are in the corners I would just lay it flat. It is not going to hurt anything with it bent but just pointing it out.

[DanDan]

That's a new one on me. I have seen pseudo polys which were just bent thin ply or hardboard, open at each end. This design is a much lighter version of that, with significant damping behind. Card or thick paper seems quite damped to me, so I concur with Glenn's view that this 'panel' will not rebroadcast into the room. I am guessing that it works very well indeed, in terms of throwing back HF in a sort of scattered way, plus normal or perhaps even enhanced LF.
Nice one y.

DD

[Glenn Kuras]

I do wonder though how much is actually absorbing vs diffusing. Thus the reason I don't like diffusors made of cardboard. But in this case it should be fine.