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Low Density vs High Density for Superchunks

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Low Density vs High Density for Superchunks

Postby Lloydon1 » Mon Nov 13, 2017 12:24 am

Hi Everyone, I've recently started work on some broadband panels for my studio in Malvern using 100mm Rockwool RWA45. I am now moving onto the basstraps this week but I have a couple of things which are still confusing me even after searching the net. As a long standing reader of SOS I hoped someone might be able to point me in the right direction.

I wanted to build some superchunks but I seem to find most people are putting the more semi-rigid battens like RWA45 in the UK or OC703 in the states. Dividing the material into triangular sections and stacking from floor to ceiling. However, I keep reading from most pro audio guys on gearsluts that for traps it is better to go with something less dense like the fluffy loft insulation to trap the low end.

This is what is now confusing me, is it better to go down the semi-rigid Rockwool route for triangular traps or is there a better lower denser material?

I have a friend in Worcester that has access to two different types of insulation which I can get for a pretty decent deal, do you think any of these will be suitable for building my traps? I have tried to search to find the more details to see if these are suitable regarding their absorption coefficient but I can't seem to find much on one of the two Knauf links - Photo attached with one of the absorption details for the DriTherm slab.

Knauf Eko Roll Loft Insulation, (L)4830mm (W)1140mm (T)200mm | Departments | DIY at B&Q

Knauf Full Fill Cavity Insulation 1200mm 455mm 100mm | Departments | DIY at B&Q 100mm Earthwool DriTherm 37 Slab.

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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Mon Nov 13, 2017 10:24 am

Having discussed this at length recently (and read earlier discussions) the consensus for 'normal' bass traps and broadband absorbers is that Rockwool RW3 or Knauf RS60 60kg/M3 is the best compromise. You definitely don't want less dense rock wool as the mass plays a large part in its ability to absorb low frequency energy. Some suggest 100kg/M3 but that is, apparent;y, less efficient at higher frequencies.

Remember as well that insulation between two rigid (block or brick) walls will not perform in the same manner as that in free air (as in acoustic "treatment" rather than "sound insulation")

I don't know how this applies to 'Studiotips Super Chunks' but I imagine it will be the same answer, RW3 semi rigid slabs are the best compromise. That's what I'm going for, off to buy the Rockwool and timber today, covering fabric will be ordered when I have a carpet fitted so I can at least pay lip service to colour coordination :headbang: .
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Re: Low Density vs High Density for Superchunks

Postby Wonks » Mon Nov 13, 2017 10:41 am

As these will be corner bass traps, I would suggest that the denser Rockwool would be best in this application. Yes it is less efficient at high frequencies, but is better at low ones. Your broadband absorbers will be covering the mid and high frequencies and you want your corner traps to be best at soaking up bass, so I'd go for the 100kg/m³ just for the corners.

Also, it's easily possible to absorb too much high frequency, so if your current broadband absorbers have got that tamed, then you just want to deal with the low end.
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Re: Low Density vs High Density for Superchunks

Postby Lloydon1 » Mon Nov 13, 2017 12:12 pm

Thanks for both of the responses.

I have found (I believe anyway that the b&q knauf slabs I linked up are 40kg/m3. However, I have gone on gearsluts and typed 40kg in and alot of those guys (even GIK Acoustics) are saying around 40kg/m3 or lower density is what it should be when it comes to superchunk traps?

Why is this so confusing, seems conflicting advice and nothing solid?
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Re: Low Density vs High Density for Superchunks

Postby Lloydon1 » Mon Nov 13, 2017 1:24 pm

Here is just one thread online from john sayers forum on avoiding 100KG!

High density? For BASS TRAPS??? Are you SURE about that? :shock: How is that going to work? 100 kg/m3 is waaaay too dense for bass trapping, if you ask me.

Contrary to popular belief, higher density is WORSE for bass trapping than low density, not better. Bass trapping is usually accomplished with lower density, while high frequency absorption benefits from higher density, but 100 kg/m3 is overboard.

Like Glenn and John said: 15 or 30 kg/m3 is much more like it for bass traps. Maybe even 45 kg/m3, depending on the type of insulation. But if you go much higher than that, then you start losing effectiveness for low frequencies
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Re: Low Density vs High Density for Superchunks

Postby Grappa » Wed Nov 15, 2017 6:17 pm

Hi,

I truly feel your pain with this.

In my recent research experience to treat my room I couldn't find any hard data to substantiate the most effective density for velocity based bass trapping. As for opinions there were plenty (in either camp)

I did however end up purchasing some panels from a commercial vendor as part of the overall solution and in discussion they confirmed that too high would limit effectiveness. Again, what was considered to be too high wasn't ever confirmed!

No help I know..

Regards,

Simon
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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Thu Nov 16, 2017 11:55 pm

I suspect the issues arise (as they so often do in this area) with the difference between 'sound insulation' (which is what most builders have some experience of) and 'acoustic treatment'.

I don't have an answer TBH but I believe GIK use 60kg/M3 Rockwool RW3. I'm going through the same conundrum as you (see above) :headbang: so I'm going for the same RW3 GIK use in two different thicknesses, 50mm for mirror points and a couple of specific places (over the drums and the vocal position) and 100mm for the bass traps (I think they use 67mm for their broadband absorbers).

If it doesn't work I'm bu99rerd.... :headbang:
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Re: Low Density vs High Density for Superchunks

Postby James Perrett » Fri Nov 17, 2017 12:59 am

As I understand it, a good compromise is to use a fairly high density for the front face and lower density for the filling. I've gone for RS60 across the front face and then filled the spaces behind with offcuts from other panels in a random way.
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Re: Low Density vs High Density for Superchunks

Postby Studio Support Gnome » Sat Nov 18, 2017 1:25 pm

The art of the bass trap.

Actually lower density does work fairly well, but is bloody difficult to build large bass traps with .
several issues present

1) it is structurally unstable, you cannot stack it and have it stay in place
2) it is highly compressible, so when you do stack it the bottom ends up denser than the top.

typically the best compromise is either 45kg/m^3 or 60kg/m^3.

I would not recommend you go higher.

the easiest to work with, that requires minimal framing and fuss is the 60kg , it is also stable enough to stack easily, and retains more consistent density when stacked.

beyond 60kg the gas entry impedance begins to make the absorption tail off, and that applies at both ends of the spectrum, but faster at the top


I have used higher densities in some soundproofing applications, when creating composite absorption/mass barriers , but never in a spectral control absorber , whether intended as a corner bass trap or broad band panel

a corner device with 1200 wide front face of 60kg , with rear fill of 30 works very well, but requires a bit of framing work to hold it together,

whereas stacking 60kg up, requires almost no framing other than a boundary to fix the fabric covering to , and just apply a teeny bit of pressure to the ends of the slabs ..

the difference in performance is minimal, but the stacked version takes half the time to build.

it's also worth noting that gas entry impedance of slabs stacked, is different from gas entry impedance of slabs with face out....

the side edges of slabs have much lower gas entry impedance than the front/rear large area faces , which makes stacked slabs more efficient absorbers than slabs with their faces out... as it were.
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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Sat Nov 18, 2017 3:06 pm

I'm planning to build 6 off 1200 x 600 x 50 mm panels (which will be spaced 50mm off the walls/ceiling) for mirror points, back wall and over the drums and vocal position then add some bass trapping. I have a space behind the monitors approx 1500 x 1000 mm of which I can use to a depth of up to around 300mm and intend to make two 1200mm high corner traps with a nominal front width of approx 400mm.

Plan A is for the bass traps to be 100mm thick 60gcm Rockwool GW3 with a softwood frame around the edge making the measurement from front face to corner around 400mm. Are there worthwhile gains to be made by stacking the Rockwool rather than using the panels in a conventional orientation?

I have a very small room (3.7 x 3.7 x 2.1 metres) so I don't expect to achieve miracles but I want to get the best I can out of the relatively small bass traps I can fit in.
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Re: Low Density vs High Density for Superchunks

Postby Wonks » Sat Nov 18, 2017 5:24 pm

I think Max would say yes. From what he's said, the edges allow sound in more 'easily' than the front face, so there's less reflection of sound and so more sound absorbance. If you've only got small traps, then you want them as effective as possible. (If you're building larger traps then you still want them as effective as possible, but you do have a bit more leeway).
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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Sat Nov 18, 2017 6:24 pm

Ta Wonky, I'd got that far TBH but, in my context, it makes the build somewhat more complex so if the differences are small it may not justify the extra effort involved. :headbang:
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Re: Low Density vs High Density for Superchunks

Postby Wonks » Sat Nov 18, 2017 11:25 pm

It's just a bit of extra cutting! :)
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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Sun Nov 19, 2017 12:18 am

True but it looks like it will make covering with fabric for a tidy appearance a little more difficult?
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Re: Low Density vs High Density for Superchunks

Postby Studio Support Gnome » Sun Nov 19, 2017 1:24 am

Stacked is more effective than panel


fabric covering,

put a 2x1 batten up the wall, SECURELY fixed in, at either end of the rock wool , , cut a top piece to fit across the 45 degree angle, and similarly a bottom piece.


fit the fabric over the top of the top piece, and then screw the top piece in to the side uprights...

stretch fabric evenly down the two side pieces, stapling on the outer edge as you go.


fit the bottom cross brace, before you finish the fabric ;)

space it off the floor by 1-2 mm, then use a curved spreading pallet knife, to poke the excess fabric under the bottom brace, using the weight of the rock wool and friction to keep tension on it.....

finish off the stapling.

then use some 19mm quadrant architectural mouldings to cover over the staples, clean , smart, and easy

or at least it is with practice, i imagine you'll get a bit frustrated with trying to make it completely wrinkle free the first time you do it .
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Re: Low Density vs High Density for Superchunks

Postby James Perrett » Sun Nov 19, 2017 1:43 am

Studio Support Gnome wrote: i imagine you'll get a bit frustrated with trying to make it completely wrinkle free the first time you do it .

Using Cara fabric will give you the best chance of being wrinkle free - even I can make acceptable looking panels with it.
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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Sun Nov 19, 2017 10:21 am

Cara fabric is on the way. I have two corners earmarked for bass traps for starters. I'll make the broadband panels first to get some practice with the fabric.

Thanks guys
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Re: Low Density vs High Density for Superchunks

Postby Studio Support Gnome » Mon Nov 20, 2017 12:55 am

James Perrett wrote:
Studio Support Gnome wrote: i imagine you'll get a bit frustrated with trying to make it completely wrinkle free the first time you do it .

Using Cara fabric will give you the best chance of being wrinkle free - even I can make acceptable looking panels with it.

actually it's easier with Lucia (also a Camira product ) , and panels are relatively easy, provided you fold and tuck neatly of the ends, but massive corner bass traps , not so much... you can't fold and tuck , you have to get the tension right all the way down to keep the panel straight..... the larger the trap, the harder that is....
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Re: Low Density vs High Density for Superchunks

Postby Sam Spoons » Mon Nov 20, 2017 10:46 am

I'm thinking I'll make a separate frame for the bass traps fabric covering. A couple of questions, do I need to have them sealed to the walls around the edges? Do I need a fabric covering (or even Poly sheeting in the case of the bass traps) on the back?
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Re: Low Density vs High Density for Superchunks

Postby Martin Walker » Tue Nov 21, 2017 8:36 pm

Sam Spoons wrote:I'm thinking I'll make a separate frame for the bass traps fabric covering. A couple of questions, do I need to have them sealed to the walls around the edges? Do I need a fabric covering (or even Poly sheeting in the case of the bass traps) on the back?


Hi Sam!

No sealing required - it's the air going into the absorbent material that's the important bit.

Similarly, you're unlikely to need a fabric covering on the back, and polythene might only be needed if you think the walls behind are damp.

Really all you're trying to do is get the sound from the room coupled into the bass trap without it leaking fibres back into the room - this is where spraying 'diluted PVA glue' helps, by stabilising the surface fibres without stopping the air getting in.

However, don't leave big gaps around the edge, as fibres might escape and be a throat irritant.

In the vast majority of cases a quick spray of diluted PVA followed by stretched acoustically transparent fabric over the top should be fine without any further measures.


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