# Attempt at at a Bass Trap



## f0zz (Dec 1, 2011)

While framing my HT room I decided to put the back wall 16" away from the concrete wall. I filled that space with Roxul in hopes that it would act as a bass trap. At the very least it will give my old home some serious R value! The remaining walls use standard 2x4 studds and are filled with one layer of Roxul. The walls were also treated with 1" of spray foam against the concrete wall.

Was this a pointless venture? Or will extra thick layer of insulation be beneficial for bass absorption? The wall is 12 ft wide and 7 ft tall, and is covered with drywall. Large sections of drywall have been removed and were covered with cloth to help absorb higher frequencies.

Thanks,

John


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## SAC (Dec 3, 2009)

Generally speaking,like with almost all things, there are positives and negatives depending upon one's ultimate goals..

Let me briefing explain how this works using simple examples (of which reality is a bit more complex, but of which measurements can quickly determine the actual behavior), while providing a few relatively easily implemented options that will allow you to further fine tune the response in accordance to your goals.

Assuming a VERY simplistic linear normal (perpendicular) incidence model totally ignoring gas flow resistance, 16 inches of absorbent material (assumed to be 100% absorbent) will totally absorb energy that has a wavelength less than the 1/4 wavelength frequency. 64 inches corresponds to a frequency of ~211 Hz. Thus you have an excellent broadband specular absorber.

This will tend to deaden your room quite substantially as specular energy above the modal region that is incident on the surface will, in large measure, be absorbed. This is a mixed issue that _might_ be considered detrimental depending upon your desired room response. 

On the other hand, frequencies ranging between 1/10 and 1/4 wavelength will be mitigated to some degree, although it is difficult to determine the precise degree without measurements. Just be aware that the absorbent material in the first 8 inches from the wall is doing very little, as porous absorption is a velocity based absorber and the velocity goes to zero at the boundary surface. Thus the reason that a gapped absorber utilizing a gap equal to the thickness of the material is essentially equivalent to a solid absorber equal to the depth of the gap plus the material thickness. In other words, a 4" panel with a 4" gap is essentially equivalent to an 8" thick panel. Thus what you have applied is essentially equivalent to an 8 in panel with an 8" gap.

Also, as has been discovered, as the trap becomes increasingly thick, lower gas flow resistance results in increased low frequency absorption. Hence why with large porous corner traps we have discovered that the fluffed cheap 'pink fluffy stuff' has a performance advantage over the more dense, higher gas flow resistance materials. 

Thus, the exact degree of effectiveness in your particular room can be determined with a program like RoomEQWizard using the waterfall or spectrogram display. 

If I was going to critique the approach, here are areas of comment. 

First is the previously mentioned issue of the efficient use of material. You could have essentially saved 1/2 of the material (or used it to make a thicker panels gapped a little further from the wall (at the expense of room space) but also lowering the low frequency extension and effectiveness) by using a gapped panel. 

If you find that too much of the broadband energy is being absorbed, you can face the material with a more rigid facing, the thinnest of which should be a 6 mil plastic layer. This layer will effectively begin to reflect energy above 600 Hz, thus retaining more of the specular energy while still absorbing the low frequency energy. This facing can be even more substantial if desired, as it will not stop the transferal of the low frequency energy while also increasingly reflecting the higher frequency shorter specular wavelengths and this will lessen the 'deadening' effect upon the room while still helping with the mitigation of the low frequency modal behavior.

One option that you might consider (if you are 'into' this) is to set up a measurement platform such as RoomEQWizard and to cover the rear wall with salvaged paneling (available at salvage yards for about $1 a sheet and simply tacked up to prevent its falling) and to run some ETC responses. If you have high gain focused reflections off the rear wall, you can remove the paneling in the spots determined to be the points of incidence, allowing the rear absorption to do 'double duty' as both LF and a broadband absorption. Retaining the solid covering in the other regions will help retain the remaining specular energy that adds to the sense of space in the room as it will contribute to the later return of lower gain energy. You can also add diffusion to this reflective surface if you like as well.

So, over all, IMO what you did has more positive benefits than negative effects. It can be fine tuned a bit further as indicated using a reflective surface, ideally applied selectively.

In any event, I would suggest using the measurement platform to further evaluate your actual low frequency modal response. In this way you will know for certain what behavior exists. And based upon this, if modal problems remain, you might consider installing front corner 'superchuck' style corner traps using the cheap 'pink fluffy' Fiberglass as fill.

And while you are at it, you can generate an ETC response for each speaker source and identify any exceptional early arriving high gain reflections and surgically either broadband absorption only at the specific points of incidence. Later arriving high gain reflections can be addressed as previously mentioned by allowing he rear wall absorption to be exposed, or a bit more optimally with the application of diffusion that will retain and redistribute the energy in the room rendering it with a sense of greater spaciousness - depending of course on your preferred room response.

As always, realizing that this is a very quick 30,000 foot perspective, if you have questions as to the conceptual perspective or with the practical implementation of how this can be done, PM me and I would be glad to help with more specificity.


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## gtpsuper24 (Jun 24, 2009)

SAC said:


> Also, as has been discovered, as the trap becomes increasingly thick, lower gas flow resistance results in increased low frequency absorption. Hence why with large porous corner traps we have discovered that the fluffed cheap 'pink fluffy stuff' has a performance advantage over the more dense, higher gas flow resistance materials.


Don't want to hijack the thread but this might help the OP too. Whats the best way to make pink fluffly corner bass traps? I have about 10 huge rolls of it in my garage if I can do something with it that would be great. My right rear corner has a large amount of "boomyness" in it more so than any other area in the room.


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## scoot90210 (Dec 20, 2011)

Hi guys, I'm early in my research on HT acoustics and have not yet read anything about vapor barriers. I would recommend that people be careful about adding insulation on the inside of your living space. Warm interior air should always be allowed to circulate on interior walls to keep them warm enough to prevent condensation. If we put insulation on an interior wall we block the flow of heat to it but not the flow of moist interior air. Could be a recipe for a black, damp, moldy mess down the road. Has anyone come across this issue either in discussion or actual experience?


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## SAC (Dec 3, 2009)

There is a good reason that you will not read about "vapor barriers" used in regards to acoustical treatment. They are inappropriate for use with porous absorption used for in room treatment. Except as airtight seals are necessary for addressing flanking vectors in isolation - a far different subject that what we address with in the room acoustic treatment, we are not utilizing air tight seals for anything in the above discussions.

I think if you check, that no one has suggested installing a vapor barrier that only works if when installed it creates an air tight seal.

The installation of a properly installed vapor barrier in conjunction with porous absorption is a mistake.

We are referring to installing a reflective membrane surface that is NOT sealed. And plastic is simply the lightest practical resilient material that can be easily sourced and worked. You will note we also suggested more effective reflective surfaces such as MDF of 1/8" luan plywood. There is nothing in a porous absorber that is sealed airtight.


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## SAC (Dec 3, 2009)

weird double post


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## f0zz (Dec 1, 2011)

scoot90210 said:


> Hi guys, I'm early in my research on HT acoustics and have not yet read anything about vapor barriers. I would recommend that people be careful about adding insulation on the inside of your living space. Warm interior air should always be allowed to circulate on interior walls to keep them warm enough to prevent condensation. If we put insulation on an interior wall we block the flow of heat to it but not the flow of moist interior air. Could be a recipe for a black, damp, moldy mess down the road. Has anyone come across this issue either in discussion or actual experience?


In my case, all walls were spray foamed first. So moisture shouldn't be an issue.

John


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## f0zz (Dec 1, 2011)

SAC said:


> Generally speaking,like with almost all things, there are positives and negatives depending upon one's ultimate goals..
> 
> Let me briefing explain how this works using simple examples (of which reality is a bit more complex, but of which measurements can quickly determine the actual behavior), while providing a few relatively easily implemented options that will allow you to further fine tune the response in accordance to your goals.
> 
> ...



I don't have the equipment or knowledge to measure my room. Much as I would like to, it may be beyond me? This is my first attempt at room treatments.


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## f0zz (Dec 1, 2011)

I should also note that, my room is not working yet. I still have to finish building the speakers. Also, the top half of the rear wall will be diffusors. I will be attempting to build these as well.


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## SAC (Dec 3, 2009)

Finish the speakers.

Proceeding to apply room treatment without knowing what is actually going on is not a productive process.

Finish the speakers and take the few days necessary to understand the measurement process.
And yes, that is plenty of time to configure a platform and to generate waterfall plots for modal behavior and to properly generate ETC plots for each individual speaker. 

(And anyone who tells you that cannot be done should serve as an indication that they do not know how to do it - and should serve as adequate warning it and of itself regarding the source! And yes, they abound here and elsewhere!)

If at that point you need help interpreting the ETC plots, post the results here (the raw data files) or PM me and we can help you quickly interpret them and render them into usable information that will allow you to determine what treatment you need and where you need to place treatment depending upon the response you desire.


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## Prof. (Oct 20, 2006)

gtpsuper24 said:


> Don't want to hijack the thread but this might help the OP too. Whats the best way to make pink fluffly corner bass traps? I have about 10 huge rolls of it in my garage if I can do something with it that would be great. My right rear corner has a large amount of "boomyness" in it more so than any other area in the room.


Depends on the size of the rolls..If the diam. is not going to be intrusive in the room, leave them rolled up in bags and stack them floor to ceiling..
If they are too large to do that, then roll them up into smaller rolls and put them into a plastic garbage bag..
If they are tightly rolled, they should be reasonably affective as bass traps..


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## gtpsuper24 (Jun 24, 2009)

Prof. said:


> Depends on the size of the rolls..If the diam. is not going to be intrusive in the room, leave them rolled up in bags and stack them floor to ceiling..
> If they are too large to do that, then roll them up into smaller rolls and put them into a plastic garbage bag..
> If they are tightly rolled, they should be reasonably affective as bass traps..


Could do tightly rolled 15-20" cyclinder, place each one in garbage bag and wrap with a fabric.


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## Prof. (Oct 20, 2006)

Yes..or if you don't like the look of the cylindrical bags you could build a frame across the corners and stretch your fabric across the front of the bags..


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## scoot90210 (Dec 20, 2011)

My apologies, I think I got you off on the wrong track. My mention of vapour barriers was not intended for acoustic effect, only as basic home construction. f0zz describes his basement as concrete walls with 1" of spray foam on them, then as much as 16" of insulation over that. Because of the thick insulation over the spray foam, the foam will become very cold during the Ontario winter. Moist air from the basement will migrate through the insulation and condense and even freeze on the foam. I am suggesting leaving sufficient gaps at the top an bottom of the insulation to allow warm air to circulate to the spray foam to prevent this. Alternatively, it is recommended to have a vapour barrier at no more that 1/3 the way into the R value of the wall to ensure that the barrier stays warm enough to prevent condensation. 

If none of this makes sense I would recommend that f0zz checks the spray foam behind this wall during the winter to make sure it is staying dry. Sorry for the confusion, just trying to save someone from a problem in the future!


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## f0zz (Dec 1, 2011)

When the foam was sprayed, I was told I could go over it with the bat insulation, and no vapor barrier would be needed. The spray foam is R14, and I was assured that this was the "right" way to do it. It's been two years, and no moisture build up yet. Its not even cold to the touch! I just checked to make sure it wasn't getting moist. Still seems good.

Fingers crossed.


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