# What reflective material for basstraps?



## omholt (Jun 5, 2007)

I'm planning to build some basstraps. Probably triangle ones stacked with rockwool. Let's say I want them to be reflective of migh mids and the highest frequencies. What reflective material do I then use? 

I would also like to know what different types of reflectives can do as to which frequencies they will effect.


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## bpape (Sep 14, 2006)

The easiest way is to get some thin plastic and stretch it tight over the front. You can also use FSK scrim if you can find it and bond it to the mineral wool with spray adhesive.

FSK when bonded will reflect about 50% starting at around 400Hz and more as you go up.

Bryan


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## omholt (Jun 5, 2007)

FSK scrim will be similar as aluminium foil, right?

Is it possible to do something else to increase the absorption of low frequencies?


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## bpape (Sep 14, 2006)

FSK is 3 layers, one like heavy brown shipping paper. The other side is heavier foil. In the middle, it's a mesh of nylon. It's all bonded together.

Adding the membrane will give you a hump in absorbtion at a given range of frequencies depending on what you bond it to and how thick it is. 

To get lower, you simply need more thickness.

Bryan


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## omholt (Jun 5, 2007)

Thanks Bryan. This is great info.

A few more questions:

1. You're mentioning to strecth tight if I use plastic. Is that important? I just filled a couple of "pillows" with rockwool. I used some plastic bags from the grocery store around the rockwool, but didn't care about strecthing it tight. 

2. For the tritrap; Will I loose any effectiveness when using some thin timber on the sides? I need to get some sturdiouness so I can move it around.

3. I see some are saying that for a tritrap and for best bassabsorption, it's an advantage that the material one is using is not too dense. That lower gas flow resistance will actually give a better result. Is that true? We don't have 703's here in Norway (to my knowledge), but there are different kinds of both fiberglass and rockwool.


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## bpape (Sep 14, 2006)

1. It depends on what you want. If it's going to be bonded and hence a damped membrane, then it's not a big deal. If you're trying to do more of a tuned absorber, then it should be tight.

2. On the sides that face the walls, that's fine, just not on the angled face that's going toward the room.

3. That's pretty much correct to a point. For a typical sized triangular chunk absorber (17x17x24"), something around a 3lb density for fiberglass or 4lb density for mineral wool is usually optimal.

Bryan


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## lglura (Aug 25, 2008)

omholt said:


> Thanks Bryan. This is great info.
> 
> A few more questions:
> 
> ...


Hi omholt. It's nice to meet fellow countrymen in the forum. You can try this link to find the insulation to build bass traps. Ecophon is sold by Glava, check the Extra Bass, its look simular to 703, I`ll try it in my basstraps.
http://www.ecophon.no/templates/WebProductFamilyPage____17898.aspx


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## omholt (Jun 5, 2007)

bpape said:


> For a typical sized triangular chunk absorber (17x17x24"), something around a 3lb density for fiberglass or 4lb density for mineral wool is usually optimal.
> 
> Bryan


Are we talking lb/ft3? Before I convert to metric, I need to know correct measures....


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## bpape (Sep 14, 2006)

Yes. Pounds per cubic foot.


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## omholt (Jun 5, 2007)

Thing takes time. But I'm getting there.
If I want to increase the absorption at low frequencies, is FSK the recommended material for a membrane? And how do I wrap it to get best result? Are using more layers in between better?

Is it also important then that the top and bottom are totally sealed?


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## bpape (Sep 14, 2006)

You're addressing several different things there. The FSK is used as a facing and bonded directly to the absorbent material. This gives a small hump in absorption at a given frequency depending on thickness and what it's bonded to. It will not extend the bass absorber capability. It will however, limit the amount of upper mid and high frequency absorption occurring.

A taut membrand stretched over an absorber is also more narrow in scope whether sealed or not. Sealed ones can be tuned pretty low without a great deal of thickness but then the 'membrane' you're talking about is more something like plywood over a sealed air cavity of 4" or so and some 1" absorption inside spaced about 1/2" back from the plywood.

Bryan


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## omholt (Jun 5, 2007)

Yes. I'm not planning on making a tuned Hemholtz resonator. I'm aiming for a broadband basstrap. Since I'm using absorbents that go all the way up in first reflections, I think it's ok that the these basstraps will reflect mid and high frequencies. 

So, I guess what I'm asking is what material will give the highest hump and how to place it for maximum effectiveness. And how can I address different frequencies (by thickness)?

Someone indicated that sealing the top and bottom would increase bass absorption, but I don't know if that's correct.


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## bpape (Sep 14, 2006)

Generally in the rear of the room is the best place so you allow the surround field to be propogated and diffuse. It also gives you a good place to deal with the length modes of the room. The hump will not be terribly wide. If you want it relatively low, I'd use 4" of 703 with the FSK bonded to it.

Bryan


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## waldo563 (Apr 26, 2009)

I hope I'm not hijacking this thread but any suggestions for what to use for thin plastic? Would something like 3 mil plastic sheeting work for cutting down on some of the mid and high absorption?
Thanks.


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## bpape (Sep 14, 2006)

The thicker and more massive you go, the lower things will start to be reflective. 3 mil is pretty thin. Think something more like the thickness and density of heavy brown wrapping paper that's used to wrap packages to mail. Some people have even used thin poster board.

Bryan


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