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DIY sound absorbers - advice needed please!

Discussion in 'Home Cinema Buying & Building' started by 0B1-1, Apr 26, 2005.

  1. 0B1-1

    0B1-1
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    :lease: Right then,

    I am now going to take the plunge and attempt to make some DIY absorbers for the side walls of my small HT (garage conversion).

    Can anyone tell me which products would be the most efficient absorbers?
    1) fibreglass loft insulation on a roll
    2) fibreglass cavity wall insulation slabs, (like fibreglass roll but is designed to be installed inside a cavity wall (usually 8' x 4' x 2" thick slabs I think)
    3) rigid insulation slabs (like kingspan-the stuff that fits nicely between roof joists and is about 2 to 3 inches thick I think)
    4) some foam
    5) any other stuff I haven't though of!

    The plan is once I have decided which of the above to use, to build wooden frames to size and then cover with some acoustically transparent material or fabric. Voila! I have read most of the posts on this subject but I just wondered which type of insulation product to buy!
     
  2. mattym

    mattym
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    Materials absorb at different frequencies depending on density, i can supply you with figures for the materials we use, and also some melamine foam absorption figures...

    This is difficult for me as i sell exactly the products you are going to make, but for DIY purposes, the denser the material the better, this is to minimise the amount of room you will be encroaching into. you wont get the thin 25mm high density that we use, the next best thing is likely to be the 2" thick slabs that are about 800x600 in size. I wouldnt recommend foam unless its specific acoustic foam, the absorbing qualities of foam vary, one type of foam will absorb far better than another so really you need to do research into the best type for your use..
     
  3. Killahertz

    Killahertz
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    0B1-1,

    I wouldn't personally use or recommend fibreglass. It's a reasonably effective absorber, but it's an awful product to use. Which, if you decide so to do, then you have to make sure that you cover the finished absorber, or the fibreglass will moult fibres. Rigid boards are safer to use than bagged rolls of the loose material, but need similar care.

    Rigid insulation comes in a variety of forms, from fibreglass to mineral wool, to expanded foam. What I would say is that this type of product can be the basis for a very simple absorbent panel, needing no framework, just covering for a reasonably straightforward professional finnish. What you have to watch out for though is absorbency. Not so much at the low end, as most of these type of products will absorb lower frequencies (notwithstanding the need for depth of material), but in the upper bass and mid-range frequencies. Put simply a panel that is too rigid will reflect frequencies extremely readily. So, how do you tell?. Well, you can look for absorption coefficient and all that, but that's not exactly straightforward. The easiest way is to just use common sense. If the board is very rigid (foil or wood backed foam, and heavily compressed fibre or mineral wools), and when struck 'pings' or resonates in any way, then it isn't going to be absorptive at the target frequency range. If the board is more semi-rigid (and absorbs knocks to it), using only modestly compacted absorbent materal (loose foam, fibre and mineral wool), then it will likely be OK.

    My preferred option is to build a simple rectangular frame, and use medium density rockwool slabs. You can get them pretty much anywhere - Wickes do their own brand (4' x 2' in diferent depths), and it is dirt cheap. Rockwool is a very effective absorber, and not in any way as invasive as fibreglass - although it is wise (if only for aesthetic reasons) to cover the panel. The frame should be sized to fit the rockwool slabs, both in terms of overall size, and depth. Remember that depth of material decides the effectiveness of the absorption offered, not the bandwidth. In other words, a simple 2" depth will work at lower frequencies, but it will not be as effective as 4", and so on. For space-limited generalised absorbency and echo treatment i'd say you be fine with 2" panels.
     
  4. mattym

    mattym
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    here is a little info on what we hear...

    Human ears respond to frequencies between 20Hz and 20,000Hz. The human voice produces frequencies between 500Hz and 2,000Hz. Below 20Hz and above 20,000 Hz sound cannot be heard but it can still be harmful. The ear is most sensitive to sounds between 1000 and 4000Hz.

    50mm(2" rockwool) gives the following absorption
    500hz = 0.96
    1k = 0.94
    2k = 0.92
    4k = 0.82

    as you can see its most effective at 500hz and stays failry even through to around 3k before it tails off

    The material we use(a high density slab)
    500hz = 0.93
    625hz = 0.99
    800hz = 1.06
    1000hz = 1.14
    1250hz = 1.15
    1600hz = 1.09
    2000hz = 1.08
    2500hz = 1.05
    3150hz = 1.02
    4000hz = 0.97
    these figures illustrate the point killahertz made about the thickness of the material absorbing lower frequencies.
     
  5. Flimber

    Flimber
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    There was a link on here t'other day regarding DIY absorbing panels, in the Hardware News bit or summat. Did you hava read of that ?

    Mike.
     
  6. avanzato

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    Can you clarify what you're saying here.
    I thought that as a thicker layer of Absorber 'X' absorbs better at lower frequencies the bandwidth will be wider than a thinner layer of absorber 'X'. ie the bandwidth does relate to the depth of the panel.
     
  7. mattym

    mattym
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  8. 0B1-1

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    Wickes have some 65mm x 1200 x 455 insulation slabs (6 for £13ish) I think I will get some at the weekend and experiment. with their effectiveness prior to making the timber frames.

    Just a thought, depending on the thickness and size of the timber frames, the fact that timber is solid/dense will reduce the effectivenss of the insulations' absorbent qualities to some degree?

    (guess what's coming next..) Any tips on the type of timber to use for the frames??!!

    wickes have plenty to choose from and I have seen some 2400x 630x380 softwood timber studs for £1.69 each with a nice curved profile on all faces. What do you think DIYers?
     
  9. mattym

    mattym
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    the wood frames will not affect the absorption of the panels, we use pine!
     
  10. Killahertz

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    I know what you mean, and I suppose we could call that 'functional bandwidth'. But for that to be the case we would have to state the lower level of functionality, in terms of absorption coefficient (1 or 100% for maximum absorbency), and rate all products in the same. In reality the functional aspect is user defined, in that the levels of absobency required by one person may differ from another.

    Thus, the operational bandwidth is the same for both thicknesses of Absorber 'X'. What differs is not the bandwidth, but the functionality, the level of absorption. In other words, both may have functionality down to 125Hz, but the thicker example with have increased action at that frequency. If we take it to the nth degree, to where we lose functionality (if we first determine where this point is) of the thinner example, then the thicker example could be said to have a wider 'functional bandwidth'. In reality, however, this point is more than likely below that where modest thickness absorbents themselves are ineffective, and give way to designed low frequency absorption, such as the tube trap, the Helmholtz resonator and membrane absorbent panel.

    Don't worry about the timber - it is a means to an end. And that end is an absorbent panel where once there was highly reflective wall. Sure, it will have some effect, but that is inconsequential next to providing a large surface area of absorbent within it's framework.

    I used the Wickes timber myself. They have three grades: a rough sawn and a planed site either side of a middle grade (the title/grade of which I cannot remember) - I used that. Sized to suit the depth of material, or just to hold the material in place prior to covering.
     
  11. 0B1-1

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    Guys,

    I have constructed 2 absorber panels, the final size excluding the timber frames is 1200 x 900. All I need to do now is cover them. I have to say though, the acoustics in the room are greatly improved, the focus from the front 3 speakers is a lot tighter, and the sound seems much better placed. I am most impressed.

    Total cost of project so far, excluding fabric to finish off = £23
     
  12. Killahertz

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    Excellent :clap:

    Acoustics (and room set-up), whilst occasionally complex, really do hold the key to audio system quality. In many ways above the hardware itself, and certainly above more specious tweakery.

    What are you going to do about covering?. One thing, make sure it isn't too dense a material otherwise it will spoil the action of the absorbent beneath. Something quite open weave is ideal, like muslin. The following should give you an idea, and there's a fair few colour choices too:

    http://www.whaleys-bradford.ltd.uk/products.cfm?catID=2
     
  13. romanav

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    Please would someone post a typical recommended absorber setup in a 5.1 and hifi scenario? Would be much appreciated. Also, in my scenario I have a wall on one my left and open space on my right, anything to balance out the sound distortion this creates? Ceiling, tilting, left wall, back wall?

    Thanks in advance,
    Roman:lease:
     
  14. mattym

    mattym
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    whats your room size Romanav
     
  15. Streetrod

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    OB1 -1, you are doing is exactly what I want to do. I was hoping to get some family photos printed onto canvas or something like that to cover the panels. Would I have a problem with this and does anyone know of a company that could transfer my pics onto the material?
     
  16. mattym

    mattym
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    you need to find the right material, otherwise you will have a really nice looking panel that has no mid/high frequency absorption at all
     

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