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Corner bass traps: leave an air gap or fill it up?
Old 26th November 2015
  #1
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Corner bass traps: leave an air gap or fill it up?

Hello!

I'm thinking about building my own corner bass traps. The front side would be roughly 24 inches in width and have maximal depth of 12 inches all the way in the corner.

What would be the most efficient method to tame the lower frequencies:

1) Four inches of OC 703 or Rockboard 60 followed by an air gap, or

2) The whole triangle filled with layers of the same material, or something less expensive…

3) None of the above...

Thanks!
Old 27th November 2015
  #2
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Quote:
Originally Posted by dramis View Post
Hello!

I'm thinking about building my own corner bass traps. The front side would be roughly 24 inches in width and have maximal depth of 12 inches all the way in the corner.

What would be the most efficient method to tame the lower frequencies:

1) Four inches of OC 703 or Rockboard 60 followed by an air gap, or

2) The whole triangle filled with layers of the same material, or something less expensive…

3) None of the above...

Thanks!
Hi Dramis,

Filling the gap is always preferable to not filling thew gap in a corner. For corner bass trapping you can use something of less density to get very good results
Old 27th November 2015
  #3
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Quote:
Originally Posted by sheggs View Post
Hi Dramis,

For corner bass trapping you can use something of less density to get very good results
Thank you sheggs! But if I did use denser material would be even more efficient on those lower frequencies or that's just not the way it works?
Old 27th November 2015
  #4
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Quote:
Originally Posted by dramis View Post
Thank you sheggs! But if I did use denser material would be even more efficient on those lower frequencies or that's just not the way it works?
Its all to do with gas flow resistivity. If it is too dense it will not really get to hose lows. You def don't want something more dense than 60kg/m3

Regards
Old 27th November 2015
  #5
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Great! Thanks a lot four your help!
Old 27th November 2015
  #6
A 2015 DTU Engineering Acoustics paper - as well as my own experience - show that a density of around 120 kg/m3 or slightly lower is where you get the most linear development in the material regarding both entry angle and frequency.

At lower densities (e.g. 35-70 kg/m3) you have higher alpha value, but a decidedly less linear response. I think there's a lot of focus on the alpha value alone.

At higher densities than 120 kg/m3 there is no increase in low end absorption and high frequency rejection increases.

I personally opted for 110 kg/m3 since that sounded the best in the low end compared to the more dense 127 kg/m3 material. It was costly affair to test this in practice ($6,000 down the drain), but sometimes you need to make mistakes. Lower densities did not absorb anywhere nearly as well for the corner traps, so that was out.

I can't post the paper yet, but I've asked for permission, or perhaps by PM only.
Old 27th November 2015
  #7
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Quote:
Originally Posted by Lagerfeldt View Post
A 2015 DTU Engineering Acoustics paper - as well as my own experience - show that a density of around 120 kg/m3 or slightly lower is where you get the most linear development in the material regarding both entry angle and frequency.

At lower densities (e.g. 35-70 kg/m3) you have higher alpha value, but a decidedly less linear response. I think there's a lot of focus on the alpha value alone.

At higher densities than 120 kg/m3 there is no increase in low end absorption and high frequency rejection increases.

I personally opted for 110 kg/m3 since that sounded the best in the low end compared to the more dense 127 kg/m3 material. It was costly affair to test this in practice ($6,000 down the drain), but sometimes you need to make mistakes. Lower densities did not absorb anywhere nearly as well for the corner traps, so that was out.

I can't post the paper yet, but I've asked for permission, or perhaps by PM only.
I am looking forward to reading the paper. Thank you.

Andre
Old 27th November 2015
  #8
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Quote:
Originally Posted by Lagerfeldt View Post

At lower densities (e.g. 35-70 kg/m3) you have higher alpha value, but a decidedly less linear response.
Ok... so if I understand this correctly the alpha value is the number we always see associated to a certain frequency. For example, a 4-inch panel of OC 703 wool has an alpha value of 0.84 at 125 Hz, right?

And what you are saying is that although 0.84 seems like an interesting alpha value, the linear development is not as good as it would be in a higher density material?

But what exactly do you mean by linear development in relation to the angle and frequency of the sound waves absorbed? Is it the material's capacity to absorb lower frequencies coming from a wider range of angles?

Not sure if I'm getting this right...

Thanks Lagerfeldt!
Old 28th November 2015
  #9
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My impression from the modeling in Soundflow is that the higher density can sometimes absorb in a more linear fashion but that really depends on just what the density it is as well as the thickness and well as the GFR.

You get a resonant bump in the low frequencies with thicker densities due to the membrane effect, but whether or not you want this bump depends on what frequency the resonant bump is at and what frequencies you're having modal issues with. Usually you see a falling off of absorption immediately above the resonant bump and then a rise again as it gets to higher frequencies.

Low density, low GFR materials tend to have low absorption coefficients (their membrane effects are generally smaller and higher up in the frequency range) until you get above a certain minimum and then the absorption coefficient rises dramatically and pretty linearly (according to the dictionary, yes, that is in fact a word).

Last edited by mpos; 30th November 2015 at 04:07 AM..
Old 28th November 2015
  #10
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Quote:
Originally Posted by themightymikep View Post

You get a resonant bump in the low frequencies with thicker densities due to the membrane effect, but whether or not you want this bump depends on what frequency the resonant bump is at and what frequencies you're having modal issues with.
Thanks for your answer. So I shouldn't try to build my own bass traps with without having first analyzed the modal issues in my room? From what I gather, there is no "average" absorbing material that will take care of most common low frequency issues? It should ideally be chosen to fit the acoustic properties of my room?

Darn... this is a complex issue!
Old 28th November 2015
  #11
I know that Thomas Jouanjean uses two different types of absorption materials for this reason. What I use is one material with a 80/20 layer combo (with the 80% being around 92 kg/m3). There are so many variables to acoustics that theory alone will get you nowhere, in my opinion. I have some theoretical understanding of the subject and a lot of practical experience. I rely on a combination of theory, paid consultants and expensive DIY trial and error experience over the last 20 years.

This Trevor Cox graph shows the correlation between the attenuation, angle and frequencies at different densities.
Attached Thumbnails
Corner bass traps: leave an air gap or fill it up?-screen-shot-2015-11-28-11.09.15.jpg  
Old 28th November 2015
  #12
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Quote:
Originally Posted by Lagerfeldt View Post
What I use is one material with a 80/20 layer combo (with the 80% being around 92 kg/m3).
Interesting! And what is the density of the remaining 20%? Is it a commercial product (what model if so...) or one you designed after trial and error?

Thank you for the charts, all is clearer now!
Old 28th November 2015
  #13
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Quote:
Originally Posted by dramis View Post

What would be the most efficient method to tame the lower frequencies

3) None of the above...

!
Welcome to my nightmare. Yes, room treatment is probably the most important thing in listening to music. While you will learn a lot here, be prepared for conflicting advice, commercial interests, highly technical and at times, argumentative, conversations.

Fiberglass really is not efficient below 125hz. You end up placing so much absorption in an average room, that you end up with a dead room. Higher than 125hz is relatively easy to deal with, but the real sound improvement comes from the bass zone, which is hard to treat.

Some will say put the speakers right up against the wall, others will say not near any walls etc. It will depend on your room and purpose. There was progress for some using Tim's limp mass absorbers to target low end bass cheaply. Some lambasted those plans, yet there are now plenty of commercial products using the same plans.

Start with the following sites to get up to speed fast. GIK, Real Traps and Arden. You can learn a lot there and they seem to somewhat agree. If you want to go deeper, watch Acoustic Fields on YouTube, but realize he will conflict some of what others say, again it all depends on room usage.

Calculate your room modes and then measure your room to see if you verify those modes are of incidence. I believe treating the first reflection points including the ceiling will be the most important. 4" preferably deeper with airspace. Next bass in corners preferably targeting room modes. I would guess that will get you 75% of the way there. Further improvement may be had with the law of diminishing returns applying. Finally after treating the room, some subtractive eq might assist you in dialing things in further.
Old 28th November 2015
  #14
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Quote:
Originally Posted by midwestm View Post
Welcome to my nightmare.
Haha! At least I'm not alone...

Thanks for the sound advice midwestm. I already have 8 acoustic panels (2' x 4', 3.5" thick, made of Roxul) placed in the first reflection spots. So I think everything above 125Hz is pretty much covered. Here's a lovely and elaborate plan of the room. Ahem...

http://mondevivant.webs.com/photos/studio-plan.jpg

I will be recording acoustic guitars and vocals in the rear part of the room. Two of the side panels can easily be taken down from the wall an serve as a shield behind the mic. So I'll have two panels in front of me, two behind, one above and bookshelves on my left and right for a little readind and diffusion...

Bass traps are the missing link. But as you said, I need to test the room before moving any further.

Thanks again for the advice!
Old 30th November 2015
  #15
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Quote:
Originally Posted by dramis View Post
Haha! At least I'm not alone...

Thanks for the sound advice midwestm. I already have 8 acoustic panels (2' x 4', 3.5" thick, made of Roxul) placed in the first reflection spots. So I think everything above 125Hz is pretty much covered. Here's a lovely and elaborate plan of the room. Ahem...

http://mondevivant.webs.com/photos/studio-plan.jpg

I will be recording acoustic guitars and vocals in the rear part of the room. Two of the side panels can easily be taken down from the wall an serve as a shield behind the mic. So I'll have two panels in front of me, two behind, one above and bookshelves on my left and right for a little readind and diffusion...

Bass traps are the missing link. But as you said, I need to test the room before moving any further.

Thanks again for the advice!
REW is good for testing - Room EQ Wizard Tutorial - GIK Acoustics
Old 30th November 2015
  #16
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Quote:
Originally Posted by Lagerfeldt View Post
A 2015 DTU Engineering Acoustics paper - as well as my own experience - show that a density of around 120 kg/m3 or slightly lower is where you get the most linear development in the material regarding both entry angle and frequency.

At lower densities (e.g. 35-70 kg/m3) you have higher alpha value, but a decidedly less linear response. I think there's a lot of focus on the alpha value alone.

At higher densities than 120 kg/m3 there is no increase in low end absorption and high frequency rejection increases.

I personally opted for 110 kg/m3 since that sounded the best in the low end compared to the more dense 127 kg/m3 material. It was costly affair to test this in practice ($6,000 down the drain), but sometimes you need to make mistakes. Lower densities did not absorb anywhere nearly as well for the corner traps, so that was out.

I can't post the paper yet, but I've asked for permission, or perhaps by PM only.
Did you do a before and after testing in your room. Be interesting to see
Old 30th November 2015
  #17
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How accurate is this calculator?
Porous Absorber Calculator

It shows quite well difference on leaving air gap or not. Also it shows that 8000 Pa.s/m2 is way better than 16000 Pa.s/m2 on deeper traps. But how does density affect on gas flow?
Old 30th November 2015
  #18
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just my 2p/2c: an air gap will allow a little ventilation of the wall itself, which can definitely be an issue if it's an outside wall. Suddenly stacking a foot or so deep of very effective insulation hard up against it will bring the dew point inwards and may well result in some unpleasant mould... ymmv

from my experience of this I will *always* leave an air gap...
Old 30th November 2015
  #19
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check out this post and related links in this post. Helped me a lot understanding the basics about air flow resistance, density and air gaps behind absorbers.

https://www.gearslutz.com/board/6328646-post88.html
Old 30th November 2015
  #20
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Quote:
Originally Posted by mutetourettes View Post
just my 2p/2c: an air gap will allow a little ventilation of the wall itself, which can definitely be an issue if it's an outside wall. Suddenly stacking a foot or so deep of very effective insulation hard up against it will bring the dew point inwards and may well result in some unpleasant mould... ymmv

from my experience of this I will *always* leave an air gap...
This is something that people can't be reminded too often. I had air gap on floor to ceiling corner trap, but the low part touched the baseboard. After few months the wooden baseboard and the corner of the trap started to go black. Not a good thing.
Old 30th November 2015
  #21
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Quote:
Originally Posted by sheggs View Post
REW is good for testing - Room EQ Wizard Tutorial - GIK Acoustics
Thank you sheggs, I will try it before going any further!

Quote:
Originally Posted by mutetourettes View Post
just my 2p/2c: an air gap will allow a little ventilation of the wall itself, which can definitely be an issue if it's an outside wall.
Yep... two corners are against an outside wall and it gets pretty cold in the winter here in Montreal... Good point mutetourettes.

Quote:
Originally Posted by decoder23 View Post
check out this post and related links in this post. Helped me a lot understanding the basics about air flow resistance, density and air gaps behind absorbers.

https://www.gearslutz.com/board/6328646-post88.html
Great stuff in there. Easy to understand for a newbie. Thanks decoder23!

Quote:
Originally Posted by El-Burrito View Post
This is something that people can't be reminded too often. I had air gap on floor to ceiling corner trap, but the low part touched the baseboard. After few months the wooden baseboard and the corner of the trap started to go black. Not a good thing.
As I said, the wall facing the exterior and the windows get pretty cold in the winter here (-10 celsius this morning). Bad for condensation... I'll have to be careful with the corners. Thanks to you and mutetourettes for bringing it up!
Old 30th November 2015
  #22
So the bass trap should not touch the floor in the corners?
Old 30th November 2015
  #23
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Quote:
Originally Posted by Limit54 View Post
So the bass trap should not touch the floor in the corners?
That's what I understand. Maybe leave half an inch or so between the trap and the floor/ceiling so that the air can circulate?
Old 30th November 2015
  #24
Quote:
Originally Posted by dramis View Post
That's what I understand. Maybe leave half an inch or so between the trap and the floor/ceiling so that the air can circulate?
Won't the bass get behind there then?
Old 30th November 2015
  #25
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Quote:
Originally Posted by Limit54 View Post
Won't the bass get behind there then?
Let it go behind and stay there!!!

Seriously, I don't know if it matters that much. I've read that corner bass traps don't need to be air tight. So let's say you leave a little gap above the floor and below the ceiling so that a column of air can circulate behind the trap, I guess some waves will slide in but will they get out? And it's certainly better than having molds in there...
Old 1st December 2015
  #26
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Quote:
Originally Posted by dramis View Post
Thanks for your answer. So I shouldn't try to build my own bass traps with without having first analyzed the modal issues in my room? From what I gather, there is no "average" absorbing material that will take care of most common low frequency issues? It should ideally be chosen to fit the acoustic properties of my room?

Darn... this is a complex issue!
Well you should test your room and figure out the modal issues no matter what you're planning on doing.

IMO most of the modal absorption taking place with porous absorbers is due to the membrane effect of the material itself (unless extremely thick). The porous absorber calculator is accurate as far as it goes but it does not take this effect into account and therefore you usually see higher low frequency absorption in tests than the calculator predicts. This membrane effect is "tuned" to a specific frequency depending on the density and thickness of the material.

All else equal, a higher density causes the frequency of max absorption to occur at a lower frequency but with less peak absorption and a higher Q. (A lower density does the opposite). A thicker panel causes the frequency of max absorption to be at a lower frequency but with higher peak absorption and a higher Q. (A thinner panel does the opposite.)

Generally the Q of the absorption is broad enough to give a noticeable effect over a decent range of frequencies, but of course you'll get maximum effect if the frequency of peak absorption coincides with the room resonance/s you're trying to attenuate.

I've attached a couple of examples from Soundflow (which takes both flow resistivity and density into account) to illustrate this. The order of both flow resistivity and density going from lowest to highest is Johns Manville R-21, OC703, and OC705. The gas flow resistivity also rises in the same direction as density (but not proportionally). I'm not sure exactly what would happen if keeping the GFR constant and varying the density, but I may try to model this soon if I have some free time.
Attached Thumbnails
Corner bass traps: leave an air gap or fill it up?-6-inch-absorbers.jpg   Corner bass traps: leave an air gap or fill it up?-12-inch-absorbers.jpg  
Old 1st December 2015
  #27
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Quote:
Originally Posted by Limit54 View Post
So the bass trap should not touch the floor in the corners?

I understand 'baseboard' to mean 'skirting board'... is that wrong? so he's still talking about wall, just the wooden bit that goes along the bottom bit of the wall.

Floor may be an issue if it's stone directly on mud (like mine) but it's more likely to fare better, same with ceiling.

In the end though, it's going to be better having the air circulating all around these things, just to be sure, and the effect on sound-absorbtion performance is very very minimal.

Let the Bass get round there and blow the cobwebs out...
Old 1st December 2015
  #28
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Quote:
Originally Posted by themightymikep View Post
All else equal, a higher density causes the frequency of max absorption to occur at a lower frequency but with less peak absorption and a higher Q. (A lower density does the opposite). A thicker panel causes the frequency of max absorption to be at a lower frequency but with higher peak absorption and a higher Q. (A thinner panel does the opposite.)

Generally the Q of the absorption is broad enough to give a noticeable effect over a decent range of frequencies, but of course you'll get maximum effect if the frequency of peak absorption coincides with the room resonance/s you're trying to attenuate.
Interesting charts themightymikep,

Looking at the chart for the 12 inches thick panels I couldn't help but wonder how we could take advantage of the differing absorption properties of each material.

What would happen if we were to juxtapose the materials from higher to lowest density, i.e. 4 inches of OC 705, followed by 4 inches of OC 703 and then 4 inches of JM R-21? Or even reverse that order?

Couldn't we get some acceptable average that would absorb a maximum of low frequencies between 20 and 125Hz?

Might not make sense since the original data on the chart is based on 12 inches of the same material...
Old 1st December 2015
  #29
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Quote:
Originally Posted by mutetourettes View Post
Let the Bass get round there and blow the cobwebs out...
And maybe get trapped in the cobwebs!
Old 2nd December 2015
  #30
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Quote:
Originally Posted by dramis View Post
What would happen if we were to juxtapose the materials from higher to lowest density, i.e. 4 inches of OC 705, followed by 4 inches of JM R-21 and then 4 inches of OC 703? Or even reverse that order
I did something like this in this post.
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