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Rockwool rigid slabs - suitable for acoustic treatments?

Discussion in 'Home Cinema Building DIY' started by NicolasB, Oct 6, 2003.

  1. NicolasB

    NicolasB
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    Still vaguely exploring acoustic treatment solutions.

    We'll put the reduction of bass on one side for the moment, and concentrate on >120Hz.

    The first problem is: way too much direct reflection off the walls. So I figure the logical place to start would be to cover the whole of the front wall (behind the front speakers), and the side and rear walls up to ear height, with some sort of absorber.

    Not altogether happy at the idea of paying >£8 a square foot for 4" Auralex pyramids, so I've been hunting around for alternative absorbers, and have come across Rockwool "rigid and semi-rigid slabs". Extrapolating from Rockwool's figures, I reckon a 100mm thick slab of RW5 would absorb virtually everything above 150Hz, with an absorption of 0.75 at 120Hz, and still above 0.5 even at 100Hz. Not too shabby.

    Does anyone have experience of using RW5 (or perhaps RW3) slabs as an acoustic treatment (NOT as soundproofing)? If so, can you tell me if it has to be set in a wood or metal frame, or if it's possible for the more rigid versions (e.g. RW5) to be glued to a wall?
     
  2. Gary Lightfoot

    Gary Lightfoot
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    I used cheap carpet as an absorber for front wall and side walls below ear height, and that seems to work fine, so I've not needed to do anything else to reduce higher frequencies.

    If you want to use the slabs, what you can do is fix wooden battoning to the walls at something like 1m intervals, then fill the gaps with the slabs (you can use tile cement IIRC). Staple and stretch clothe over the battons to give a cloth wall efect, then cover the staples using a decorative wooden trim of some sort.

    In the US, they use a 1" deep material to do this, so I think what you want to do may be overkill. Have you looked over at avsforum for info on this? The accoustically transparent cloth they use is Gothic of Maine IIRC, so that could be useful in a search.

    What sort of bass problem do you have? A non rigid wall can act as an absorber, so if you've added a partition wall that uses less tha 4x2 wood and only 1 layer of plasterboard, that may be the cause. Resiliant bar can be even worse for this.

    If you have bass peaks and troughs, a Behringer Feedback Destroyer can help with this to increase or reduce the frequencies that are too high or low.

    HTH

    Gary.
     
  3. avanzato

    avanzato
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    Hi NicolasB :hiya:

    I bought 3 sq m of rockwool 30mm slab for £20. It's arranged along the bottom third of my front wall and the full width of the room.

    The difference is not massive but is noticable especially to the imaging. More improvement to the sound than any other £20 I've spent. As this experiment was succesful I went to get some 100mm slab but on the way my car was rear ended so never got to the merchant. I think I will get some and glue it to the back of my car. :) I have read that the 100mm slabs mounted accross a corner will work to below 100hz but can't confirm that yet.

    The slabs can be glued to the wall with construction adhesive and they look horrible. As Gary suggests framing and covering them is a must.
     
  4. NicolasB

    NicolasB
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    Gary,

    Bass (as in below 120Hz) I think needs to be handled separately. I do actually own a BFD, which certainly helps, and there are all sorts of possiblities for handling bass, but I want to look at 120Hz and upwards first.

    From Rockwool's own figures, a 30mm slab of RW5 has total absorption from 630Hz upwards, 0.85 at 500Hz and 0.62 at 400. I calculate that a 100m slab should have an absorption of about 0.75 at 120Hz, so using that means that I can rely on the Rockwool to handle everything except what is specifically dealt with by bass traps (or whatever I subsequently decide to use). Using thinner material risks there being stuff going on the midrange that isn't handled by either.

    I know Rockwool slabs are ugly - are the "tissue faced" versions any less horrible? :)

    My main problem here is that I am just awful when it comes to DIY, so even something of the sort of complexity you are talking about is probably beyond me. :(


    Avanzato,

    Sorry to hear about your car. Do you think maybe 100mm Rockwool slabs are subject to a voodoo curse? :eek:
     
  5. pemberto

    pemberto
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    I am just doing the same thing with Rockwool 50mm RW3. I have created a wooden panel and tacked to back using 10mm staples a sheet of 3mm MDF. Placed the Rockwool inside and then stapled a backing sheet over the frame.

    Once completed I then covered this in fabric to match the colour of my room, all I did then was to use double sided Velcro and attached to the wall. The whole thing works and looks great.
     
  6. gvh

    gvh
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    Where do you get the rockwool ? What kinds of shops ?

    I was thinking of backing a piece of artists canvas already stretched round a frame. The frame is only an inch deep so I might need something else such as foam. Suggestions welcome
     
  7. Gary Lightfoot

    Gary Lightfoot
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    Wickes do a high density 30mm slab in packs of 6 IIRC, so that might do the trick.

    Gary.
     
  8. alexs2

    alexs2
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    the only proviso I'd make about Rockwool is to avoid any loose fibres being released into the room...it's nasty stuff and won't do your lungs much good...make sure the surface is adequately sealed.
     
  9. avanzato

    avanzato
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    You can find out a local stockist on the Rockwool site usually a builders merchant or insulation supplier. I tried Travis Perkins but the bloke behind the counter was very unhelpful so I am going to try Sheffield Insulation when I have time to get there. The slabs I have ATM are 30mm rigid floor insulation from Wickes. Knauf do slabs that work equally well if you have a stockist nearer to you than Rockwool does.

    If you are after High frequency absorbsion try to get a covering that is acoustically transparent at HF. If you can easily blow through the fabric it is probably OK or at extra expense get acoustically transparent speaker grill cloth.

    pemberto I'd like to see a picture of the install if you have one you can post here. :smashin:
     
  10. avanzato

    avanzato
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    Rockwool is safe, it doesn't cause cancer but is an irritant, gloves and face mask are recommended. A cloth cover will be enough to stop fibres getting out, the Americans tend to use some cotton batting between the slab and the cover just to make sure. Batting also hides the unattractive slab from view when using an open weave cloth.
     
  11. NicolasB

    NicolasB
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    I thought that the rigid and semi-rigid slabs were supplied "tissue-faced"- they even mention that you can get them in white or black. Does this not negate the need for wrapping them in fabric? (Possibly it doesn't make them look any less ugly, but I'd have thought it would at least stop fibres escaping).
     
  12. fufna

    fufna
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    They do indeed. but rockwoll is still aver tricky substance to work with cleanly.

    wrapping it in some sort of cloth, or installing it behind fabric panels is definitley the way to go
     
  13. alexs2

    alexs2
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    You will note that I didn't imply that it was a carcinogen,but having worked(admittedly some yrs back when I was a student)for a company both marketing and installing it as an insulation,I can certainly vouch for it's irritant properties,as can those who were actually installing it.
    Your comments regarding covering it suitably should suffice,as would mine.
     
  14. pemberto

    pemberto
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    avanzato
    I'll post some pics as soon as I am finished.
     
  15. deckard

    deckard
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    Nic

    You may be aware that I've recently installed some Auralex panels and have been thoroughly impressed. However, I did come across some cheaper alternatives whilst scouring the net. Have a look at the following links:

    http://www.studiospares.com/productdetails.asp?pid=36927&cid=2552&ctitle=ACOUSTIC TILES&p=1&gid=6
    http://www.studiospares.com/productdetails.asp?pid=36926&cid=2552&ctitle=ACOUSTIC TILES&p=1&gid=6
    http://www.studiospares.com/productdetails.asp?pid=36928&cid=2552&ctitle=ACOUSTIC TILES&p=1&gid=6

    Not as attractive as the Auralex IMHO, but the 110mm foam should compare reasonably well to 4" pyramids and at £15.69 for a 500x500mm tile they're considerably cheaper.
     
  16. NicolasB

    NicolasB
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    So that's, what, £5.65 per square foot (assuming 30cm to the foot)? I suppose that's a bit cheaper than Auralex, but 100mm RW5 Rockwool slabs are about £1.50 a square foot (plus VAT, I think) so that's a lot cheaper again, depending on how much money you have to spend on other stuff to actually make it work.
     
  17. deckard

    deckard
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    Mmm, that's right Nic once you do the maths. Not that much cheaper after all I suppose (4" pyramids £8.65/sq ft), although a damn site easier to install if you're not that hot on DIY.
     

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