Need to upgrade homemade absorbsion boxes

beech · Post by **beech** » Thu Jan 19, 2006 3:40 am

Hi,
This is my first post here so I hope it comes across ok. Firstly I would like to thank those who moderate on these forums...you do a great job!

Now for my little dilemma. I'm in the process of upgrading the acoustics in my recording room which is approx 20x16x9 high. I am replacing the floor with real oak mounted on battons which are directly fixed to the concrete. The walls are being given some extra stiffness too but I don't need to elaborate any more on that, just wanted to give you a little detail.

Below, hopefully you will see some panels I made about ten years ago. This happens to be the clearest picture of one that I have but hopefully you will get the idea. Each wall box is 8 x 4 ft and about 9 inches deep. The fronts are covered with battons with gaps of various lengths. The rear is simply 6mm plywood.

This is the part I need help with. Inside at the moment is loose rockwool with heshin to stop dust. I've been told that this is really not doing much for the absorbsion so I was thinking of ripping it out and refilling with maybe some 50mm compressed rockwool and a layer of heavy carpet. Or is there anything else that that I should consider?

Many thanks,

Daire

AVare · Post by **AVare** » Thu Jan 19, 2006 4:16 am

Welcome to RSD! Lovely absorbers!

This is the part I need help with. Inside at the moment is loose rockwool with heshin to stop dust. I've been told that this is really not doing much for the absorbsion so I was thinking of ripping it out and refilling with maybe some 50mm compressed rockwool and a layer of heavy carpet. Or is there anything else that that I should consider?

Like a good piece of info, your question develops new questions. Lucky us.

Are you currently having any dissatisfaction with the room acoustics? If so, describe it please.

From looking a the picture it is not clear if the absorbers are slats with a space between them or solid front. If the former (slats) thent he current design works as a tuned system where the slat/slot spacing determines the high end cutof of absorption and the material inside the low. Very generally, the thicker and further out the material inside, the lower the effective low frequency of absorption. 9" of ~45kg/m^3 material will yiled a flat random aborption of 1 down to around 50-60 Hz.

The more info the better.

Andre

beech · Post by **beech** » Fri Jan 20, 2006 2:50 am

Thank you Andre. The acoustic in the room has been considered too low midrangey over the years... I guess from about 800hz down. The fact the the floor was carpeted may have been killing the highs too much.

I've attached two more images which might give you a more accurate view of the construction. The slats do have a gap between them. The boxes are 8" deep (not 9" as was stated in earlier post).

"Very generally, the thicker and further out the material inside, the lower the effective low frequency of absorption. "

This is exactly the kind of info I was looking for. So how about putting a layer of heavy industrial carpet inside the slats, then maybe 4" of RW3 compressed rockwool. That would leave an airgap of approx. 4".

Just to give you a little more info on the overall plan. The old floated floor has been removed and a more solid real wood floor will be laid on top of concrete.

The walls will be covered with 6mm rubber matting and then finished with 3/8" plasterboard. This is being done to add rigidity to the original stud walls (which are in front of block walls).

My fear is that when all of this has been completed, the overall reverb time in the room will be too much. Hence the idea of revamping the absorbers.

I've attached a floorplan of the studio. The rooms that are getting an overhaul are the main room, vocal and iso booth. The main room has four 8 x 4' absorbsion boxes and the other two booths have one each.

Thanks again.

Daire

beech · Post by **beech** » Fri Jan 20, 2006 2:52 am

I'm sorry, I should have made sure the images would have fit on the screen! Nothing bigger then 750 across I guess.

len-morgan · Post by **len-morgan** » Mon Jan 23, 2006 3:50 am

One thing that jumps out at me on your current slotted resonator (they're NOT absorbers per se) is that while you have varying slot widths, the wood strips all seem to be the same width. I believe in order to build one of these correctly, you need varying slot widths AND varying slat widths.

len

beech · Post by **beech** » Mon Jan 23, 2006 7:16 pm

Thank you Len. I appreciate that observation. I would surely change some of the slats...question is, should I leave say two boxes the way they are and maybe make alterations to the other two (4 boxes in main room)? Or make each box have various size slats perhaps?

Also, I'm looking for some advise about whether or not to use some robber backed carpet behind the 100mm rockwool.

Cheers,

Daire

PS. Just came in this morning to find the floor flooded over the weekend.

len-morgan · Post by **len-morgan** » Mon Jan 23, 2006 10:25 pm

If I understand the theory correctly (the formula is on the site somewhere), those absorbers are TUNED based on the spaces and slat widths. That being the case, if you only change one or two of them, the others will still be tuned to a single frequency. My suggestion would be to have multiple widths AND variable spacing on all of them.

Someone else will have to chime in about the rubber sheet behind the carpet.

len

knightfly · Post by **knightfly** » Tue Jan 24, 2006 2:23 am

The helmholtz formulas here are based roughly on cloth you can actually BLOW through, and Owens Corning 703 fiberglass insulation; the cloth needs to go up against the slats and the insulation against that, or it will shift parameters (and not in a PREDICTABLE way) - so using some type of carpet might work, just not in any way you'd be able to know beforehand.

(IOW, I wouldn't use that)

In any given slat absorber, changing width of boards and slots won't do as much as it seems, because the wavelengths involved are larger than the boards. This means that the entire absorber will "average" - doesn't mean it won't work, just means you won't know exactly what you did whether it's good or bad

One way around this is to place "separators" inside the trap to segregate the space into individual chambers, making (in effect) several small helmholtz traps in one box. IF you do this, then making different sections within the same "box" would widen the range of frequencies the trap works at - it's also more of a PITA to build... Steve

Post by **kendale** » Tue Jan 24, 2006 2:23 am

Aloha,

Here's a couple of links with info on acoustics, absorbers, and lots of other great info. Be sure to click on the "Acoustics" and "Absorber" tabs in left hand margin.

SAE: http://www.saecollege.de/reference_material/index.html

Wall Units: http://www.johnlsayers.com/HR/index1.htm

Hope these help,

Aloha

AVare · Post by **AVare** » Tue Jan 24, 2006 4:04 am

Several people seem to have gotten off the track with your absorbers. With the % area open greater than 3, the absorber acts as a combined system of: the low frequency absorption dependent on the absoption function of fhe material BEHIND the slats; and the high frequency reflection of the slats, being a function of the: width of the slats, percent open area, and depth of slats. The Helmholtz effect is not present at such large openings, or grater. Looking at your photos, the open area is greater than 3%, and that number is not an absolute.

Trying to think of what is an easy and economical way to determine what changes will help, try removing (in a non-permanent) way every fourth and then second slat and listen to the effect each change has tot he room sound.

Another cheap way to experiment, is to temporarily fill every fifth slot, then fourth, etc or some other repeatable and expandable pattern, and listen after each filling. Strips of wood, plastic, etc, anything for the testing. Sort of like building half a dozen different absorbers in an hour!

Good luck and keep us posted!
Andre

beech · Post by **beech** » Tue Jan 24, 2006 11:35 pm

Much food for thought here! I will 'absorb' some of this info and then no doubt come back to this forum before I commence. As the rooms are still in the final stage of carpentry, it's a little difficult to know where the acoustic problems are going to be. But I'm sure of one thing...there will be problems, as the dimensions have not changed, just the finished surfaces (floor and walls).

You folks are great!

Daire

beech · Post by **beech** » Wed Jan 25, 2006 1:35 am

Andre, your suggestion to make changes in a non-permanent way is the way to go. Before I mount the boxes again though, I feel I must change the loose rockwool inside for the denser, more rigid stuff.

Given that the internal depth from front to back is 8", would 4" of 60kg rockwool seem about right?

I guess I'll forget the carpet idea unless you think it would be worth hanging a layer loose inside. Would this not help with the lower frequencies?

Thank you,

Daire

knightfly · Post by **knightfly** » Wed Jan 25, 2006 4:55 am

Do you have any way of actually measuring the room response? Something like ETF for example? IF so, I'd first remove the boxes entirely and find out what the room actually NEEDS - then, instead of building something no one here can calculate I'd stick to proven designs that address the actual problems.

I've not seen any actual dimensions of rooms - this would help get a ballpark of what modal problems you may see, but with a non-parallel walled room measurement in critical locations (mix, producer's desk, client couch, etc) would help more.

I'd recommend first finding out what you have WITHOUT treatments, including RT of the room; then you'll know better what needs to happen to get an even decay time so the room doesn't color your sound... Steve

AVare · Post by **AVare** » Wed Jan 25, 2006 5:05 am

Great studff we are going through here. I will not repeat Steve's advice, just say that is an integral part what you should do. Consider mine as a part of your strategy.

Before I mount the boxes again though, I feel I must change the loose rockwool inside for the denser, more rigid stuff.

Given that the internal depth from front to back is 8", would 4" of 60kg rockwool seem about right?

I am getting confused here. What you currently have is an 8" depth absorber with loose (no density specified) rockwoll partially or completely filling absorber? IF not clarify.

The best for broadband absorption at those de[ths is continous light (eg 20 -30 kg/m^ rockwool or 15 -25 kg/m^3 fiberglass). From what I think I understand, this is what you have right now. Why do you wish to change it?

Keep the questions coming! An extra week of questions is more staisfying than a week of tearing down and rebuilding something that was improperly designed!

Andre