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On covering bass traps

post #1 of 46
2/27/13 at 2:27pm
Thread Starter 
My eyeballs hurt form wading through the 10k messages in the master acoustic treatment thread smile.gif

I'm going the DIY superchunk OC703 bass trap route. The question I'm trying to find an answer to is this: I have an AT screen.. I was going to line the wall behind the screen with duct liner. Can I liner right over the bass traps (assuming I wanted to make it difficult to access them later)? Or do I need to put the traps on "this side' of the liner?

Thanks for any help!
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post #2 of 46
2/27/13 at 3:44pm
What's behind your AT screen? A proper baffle wall or just stud framing?
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post #3 of 46
2/27/13 at 6:02pm
Thread Starter 
Dark grey primed drywall. smile.gif

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post #4 of 46
2/28/13 at 5:25am
Quote:
Originally Posted by Spall View Post

Dark grey primed drywall. smile.gif


If you want to absorb bass, another layer of drywall separated by ribbons of green glue can do lots.

Most of the fiberglass absorbers that people build as bass traps really don't do a lot below 100 Hz. This does not detract from their benefits, but the point is what are you really trying to do?

Check this:

http://www.avsforum.com/t/1136101/my-bass-story-ii#post_16201573
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post #5 of 46
2/28/13 at 7:48am
Thread Starter 
I guess the best answer I have for what am I really trying to do is: get as far into a good room as I can on a DIY budget. I ask questions at the points before I do something where my brains says "are you hurting or helping"

As for the wall, it is actually 6" thick staggered stud double 5/8" drywall on both sides with green glue in between. I thank you for the suggestion, though. smile.gif
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post #6 of 46
2/28/13 at 9:51am
Quote:
Originally Posted by arnyk View Post

Most of the fiberglass absorbers that people build as bass traps really don't do a lot below 100 Hz.

Arny, I know you know this too, .. but that's operator error, if they were trying to affect below 100hz.



Spall,
How much room can you utilize for bass trapping?

I inquire, because the most effective broadband bass trapping is fortunately, just about the cheapest. If you could fill as much of the area behind the screen as you can with loose/fluffy batts, it's the simplest, and most effective you can implement.

I'm talking 12", 18", 24", this would make for a superb bass trapping approach. And it's most likely a great deal less than a lot of rigid 703.



I just posted this a day ago;
"Porous absorption can be effective quite deep, I've seen measured results down into the 30hz range. The thicker the better, if you go very thick, you use the less dense material. If you opt for less thickness, utilizing an air gap multiplies the effectiveness greatly.

As a rule of thumb, based on how much space off the surface you can use;
up to 4", use 3 lb fiberglass equivalent (4lb) mineral wool
4-6", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap
6"-12", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap off the surface, or fill the space with loose/fluffy
12" and more, fill the space with pink fluffy

For corners;
One can either straddle a corner with a 4"-6" thick panel, or employ stacked fluffy, stacked rigid 703, superchunk style.
But be mindful that filling the corner is better than using a 4"-6" thick panel, however only somewhat better. Instead of filling the entire area, one may be better off utilizing the same material elsewhere, covering more corners.

Best case, fill all the corners as deep as you can spare. Remember, the thicker the porous absorber, the less dense (gas flow resistivity) the material is optimum. For example, if one could spare 18" across the entire back wall of a studio, pink fluffy will out perform 703 rigid fiberglass."


Good luck
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post #7 of 46
2/28/13 at 10:19am
Quote:
Originally Posted by Spall View Post

My eyeballs hurt form wading through the 10k messages in the master acoustic treatment thread smile.gif

I'm going the DIY superchunk OC703 bass trap route. The question I'm trying to find an answer to is this: I have an AT screen.. I was going to line the wall behind the screen with duct liner. Can I liner right over the bass traps (assuming I wanted to make it difficult to access them later)? Or do I need to put the traps on "this side' of the liner?

Thanks for any help!

To answer your direct question, yes. You can use liner directly over the superchunks without any loss. As it is stated in the post above, if you fill some of the empty space from wall to wall, between the front wall and the liner (I'm assuming a false wall) with pink fluffy fiberglass, you can get a lot of great bass absorption in the room.
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post #8 of 46
2/28/13 at 12:57pm
Quote:
Originally Posted by Spall View Post

I have an AT screen.. I was going to line the wall behind the screen with duct liner. Can I liner right over the bass traps (assuming I wanted to make it difficult to access them later)? Or do I need to put the traps on "this side' of the liner?

Thanks for moving this to the forum. You're already getting good advice, but I can add a little. To answer your direct question, adding more rigid fiberglass in front of rigid fiberglass already used as a corner chunk is fine. But most rooms do not need absorption all over the front wall. At least not if the rest of the room is treated properly. This is such a common misconception that I wrote this article to explain the issues:

Front Wall Absorption

Also, rigid fiberglass bass traps can indeed target problems well below 100 Hz. But it has to be thick, and also cover enough total surface area.

--Ethan
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post #9 of 46
2/28/13 at 1:39pm
Quote:
Originally Posted by arnyk View Post

If you want to absorb bass, another layer of drywall separated by ribbons of green glue can do lots....
Got any data on this?

I'm not seeing anything about double-layer drywall that could possibly improve in-room bass response; am I missing something? The more likely result of soundproofing the room (normal application for double-layer and GG) is to decrease room losses, exacerbating room modes signficantly due to the higher level of reflected energy.

What are you thinking happens?

HAve fun,
Frank

PS Putting plastic over a resistive-type absorber only reduces its high frequency absorption; no effect in the bass range. Now, cover the 703 with thin hardboard and make the resulting cavity air tight and you have a really good bass trap.
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post #10 of 46
2/28/13 at 1:43pm
Quote:
Originally Posted by fbov View Post

Got any data on this?

I'm not seeing anything about double-layer drywall that could possibly improve in-room bass response; am I missing something? The more likely result of soundproofing the room (normal application for double-layer and GG) is to decrease room losses, exacerbating room modes signficantly due to the higher level of reflected energy.

Well, if you absorb bass frequencies with the green glue, doesn't that naturally mean that the amount of reflected sound would go down, thus decreasing the intensity of the room modes?
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post #11 of 46
2/28/13 at 2:42pm
Thread Starter 
Quote:
Originally Posted by FOH View Post

Arny, I know you know this too, .. but that's operator error, if they were trying to affect below 100hz.



Spall,
How much room can you utilize for bass trapping?

I inquire, because the most effective broadband bass trapping is fortunately, just about the cheapest. If you could fill as much of the area behind the screen as you can with loose/fluffy batts, it's the simplest, and most effective you can implement.

I'm talking 12", 18", 24", this would make for a superb bass trapping approach. And it's most likely a great deal less than a lot of rigid 703.

***snip***

Good luck

I believe I have about 34" from the front wall to the back of the screen. I need room for 3 LCRs that will be on stands to fit inside the screen height and 2 subs. I also possibly need room for some 8" flex duct as I have a dead vent in that space emptying into the room (not my first choice, but space outside the room dictated where it had to go). I had considered just leaving the 8" vent open and letting the air flow through the AT screen and the fabric that will be on the screen wall, but if you're suggesting I fill a lot of that space in I can actually direct it to an exit grill on the front wall somewhere.

I should note: the intended 703 superchunk material is already purchased and cut. I was planning on assembling them tomorrow. However, I can add more behind the wall if that's the general consensus.

oh.. and thank you smile.gif

Quote:
Originally Posted by GIK Acoustics View Post

To answer your direct question, yes. You can use liner directly over the superchunks without any loss. As it is stated in the post above, if you fill some of the empty space from wall to wall, between the front wall and the liner (I'm assuming a false wall) with pink fluffy fiberglass, you can get a lot of great bass absorption in the room.

Thanks! My main driving force for asking was because I want everything behind the screen to be black as well as prevent any fiberglass fibers from collecting on the screen material. I thought I could save a bit on wrapping the superchunks in black fabric by just covering them on the duct liner I was already planning on using. If I add some pink fluffy in that space, can I just shove that in some thick pockets of duct liner? Stuff it in bags of black fabric? Is this a case of more is better or is there a too much point?

And thank you, too smile.gif
Quote:
Originally Posted by Ethan Winer View Post

Thanks for moving this to the forum. You're already getting good advice, but I can add a little. To answer your direct question, adding more rigid fiberglass in front of rigid fiberglass already used as a corner chunk is fine. But most rooms do not need absorption all over the front wall. At least not if the rest of the room is treated properly. This is such a common misconception that I wrote this article to explain the issues:

Front Wall Absorption

Also, rigid fiberglass bass traps can indeed target problems well below 100 Hz. But it has to be thick, and also cover enough total surface area.

--Ethan

Ethan, when you mention the rest of the room you bring up a secondary question. I'm not sure which theory to follow: 1) Treating all the way around the room up to ear level or 2) treating the early reflection points.

However, going back to my above statement of being a too much point: Should I choose to line the entire front wall, am I hurting the room (potentially) or just risking wasting materials? I'm trying to get a better understanding of what is actually happening inside the room. smile.gif

And a third thanks is in order, sir.


I suspect I'm going to have a question about rear wall diffusion here pretty soon, also wink.gif
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post #12 of 46
3/1/13 at 10:26am
Couple of things you need to do back there:
- absorb the back wave from the LCR speakers which will otherwise cause speaker boundary interference related suckout at the listening area
- damp the front to back axial room modes which are some of the strongest and most damaging

The simplest would be some superchunk or fluffy pink fiberglass traps. You could include a mass loaded vinyl element to improve low bass absorption. Finally rather than filling the whole space behind the screen you could also hang 703 sheets from the ceiling with air gaps between them.

My current screen wall designs use a baffle wall with two or three layers of sheet material. The speakers are flush mounted like you would see in a studio context. Behind the screen is a thin layer of foam. Only purpose of that is to absorb the ultra-high frequency chatter between the screen and the baffle wall. Around the screen use 4-5" thick diffusers to provide some support for the surrounds. The baffle extends maybe 3ft above, below and to the sides of the speakers. The rest of the wall is semi-open, saving for the diffusers, and bass trapping is used behind the baffle wall, typically by hanging 703 sheets from the ceiling if there is space.
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post #13 of 46
3/1/13 at 10:29am
Quote:
Originally Posted by fbov View Post

Got any data on this?

I'm not seeing anything about double-layer drywall that could possibly improve in-room bass response; am I missing something? The more likely result of soundproofing the room (normal application for double-layer and GG) is to decrease room losses, exacerbating room modes signficantly due to the higher level of reflected energy.

What are you thinking happens?

HAve fun,
Frank

PS Putting plastic over a resistive-type absorber only reduces its high frequency absorption; no effect in the bass range. Now, cover the 703 with thin hardboard and make the resulting cavity air tight and you have a really good bass trap.

There's better damping of the resonant modes of a single layer of drywall when you use two layers with green glue. The big benefit comes from using isolation clips, these make quite a big difference in absorbing bass below 60Hz compared to either a single or double layer of drywall attached straight to studs. The data I have is unpublished from Kinetics.
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post #14 of 46
3/1/13 at 12:43pm
Quote:
Originally Posted by Nyal Mellor View Post

There's better damping of the resonant modes of a single layer of drywall when you use two layers with green glue. The big benefit comes from using isolation clips, these make quite a big difference in absorbing bass below 60Hz compared to either a single or double layer of drywall attached straight to studs. The data I have is unpublished from Kinetics.

I was going to comment on the Green Glue suggestion as well,

Isn't this in reference to noise transmission as opposed to sound absorption inside a room? I've never seen evidence that constrained layer dampening will change the room response too much at all. And adding mass to the wall partitions can actually increase modal resonance, but perhaps this doesn't apply to CLD.
Edited by GIK Acoustics - 3/1/13 at 12:48pm
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post #15 of 46
3/1/13 at 12:45pm
Quote:
Originally Posted by Nyal Mellor View Post

damp the front to back axial room modes which are some of the strongest and most damaging.

Why do you suggest the front to back dimension provides the strongest room modes? Because placement of the mains drives that mode harder? If that's the case, subs would dictate room mode intensity more than the mains, yes (at least for the low octaves)?
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post #16 of 46
3/1/13 at 12:54pm
Quote:
Originally Posted by Spall View Post

Thanks! My main driving force for asking was because I want everything behind the screen to be black as well as prevent any fiberglass fibers from collecting on the screen material. I thought I could save a bit on wrapping the superchunks in black fabric by just covering them on the duct liner I was already planning on using. If I add some pink fluffy in that space, can I just shove that in some thick pockets of duct liner? Stuff it in bags of black fabric? Is this a case of more is better or is there a too much point?

You can just add it to the pockets yes. Try not to compress it too much otherwise you might lose some of the low frequency absorption capabilities. You don't necessarily need to fill the space, but with mains and a sub up front, filling a lot of space would be beneficial.
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post #17 of 46
3/1/13 at 1:50pm
Quote:
Originally Posted by Spall View Post

Ethan, when you mention the rest of the room you bring up a secondary question. I'm not sure which theory to follow: 1) Treating all the way around the room up to ear level or 2) treating the early reflection points.

Option 1) is bad advice. It wastes absorption material, and doesn't properly treat where you really need it. What I had in mind was the rear wall behind you. How far away that wall is from your ears is a factor, but the basic premise remains for any home-sized room. Sound from the speakers comes toward you, goes past your ears, and eventually hits the rear wall. If the rear wall is treated, sound will not reflect back toward the front of the room. So you don't need absorption on the front wall. But you do need to treat the rear wall because that's a source of strong reflections.
Quote:
Should I choose to line the entire front wall, am I hurting the room (potentially) or just risking wasting materials? I'm trying to get a better understanding of what is actually happening inside the room. smile.gif

Mostly that wastes material, but it also risks skewing the decay times in your room unless you use very thick absorption. One goal in treating rooms of any size is for the decays times to be uniform at all frequencies. Many people put one-inch thick foam or fiberglass all over the front wall, and often on the other walls up to ear level, and they also have carpet. So the room sounds dead at mid and high frequencies, but bass frequencies bounce around uncontrolled. A proper treatment strategy addresses this so all frequencies decay more or less at the same rate.

--Ethan
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post #18 of 46
3/1/13 at 2:56pm
Quote:
Originally Posted by Ethan Winer View Post

Mostly that wastes material, but it also risks skewing the decay times in your room unless you use very thick absorption. One goal in treating rooms of any size is for the decays times to be uniform at all frequencies. Many people put one-inch thick foam or fiberglass all over the front wall, and often on the other walls up to ear level, and they also have carpet. So the room sounds dead at mid and high frequencies, but bass frequencies bounce around uncontrolled. A proper treatment strategy addresses this so all frequencies decay more or less at the same rate.

--Ethan

Perfect points from Ethan. Evening out decay times is usually a primary goal for treatment IMO.
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post #19 of 46
3/2/13 at 6:39am
So what is being said is if you absorb, absorb it all.

I maintain, in this situation, a huge amount of fluffy batts behind the screen wall is likely the least expensive, and most effective, aesthetically acceptable (can't see it), and it takes up no room space,..as behind the screen wall is dead anyway.

Nyal, Ethan, and Alex, what would be a more appropriate option to pursue?
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post #20 of 46
3/2/13 at 10:16am
Quote:
Originally Posted by FOH View Post

I maintain, in this situation, a huge amount of fluffy batts behind the screen wall is likely the least expensive, and most effective, aesthetically acceptable (can't see it), and it takes up no room space,..as behind the screen wall is dead anyway.

Yes, the front wall is a fine place for bass trapping! But that's not what most people do. The much more common treatment is 1-inch thick foam or 703.

--Ethan
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post #21 of 46
3/2/13 at 1:36pm
Thread Starter 
Quote:
Originally Posted by Ethan Winer View Post

Option 1) is bad advice. It wastes absorption material, and doesn't properly treat where you really need it. What I had in mind was the rear wall behind you. How far away that wall is from your ears is a factor, but the basic premise remains for any home-sized room. Sound from the speakers comes toward you, goes past your ears, and eventually hits the rear wall. If the rear wall is treated, sound will not reflect back toward the front of the room. So you don't need absorption on the front wall. But you do need to treat the rear wall because that's a source of strong reflections.
Mostly that wastes material, but it also risks skewing the decay times in your room unless you use very thick absorption. One goal in treating rooms of any size is for the decays times to be uniform at all frequencies. Many people put one-inch thick foam or fiberglass all over the front wall, and often on the other walls up to ear level, and they also have carpet. So the room sounds dead at mid and high frequencies, but bass frequencies bounce around uncontrolled. A proper treatment strategy addresses this so all frequencies decay more or less at the same rate.

--Ethan

So is that to say that #2 above and treating the back wall is the way to go?

Maybe that brings about the question of diffusion. Do I need to add any? Is this where I should throw in some room dimensions including where my seats are in relation to the back wall?

Quote:
Originally Posted by GIK Acoustics View Post

Perfect points from Ethan. Evening out decay times is usually a primary goal for treatment IMO.

This is starting to get a bit over my head.. well.. maybe not just starting smile.gif


---

I greatly appreciate all the advice thrown around in this thread so far.

Thanks!
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post #22 of 46
3/2/13 at 5:52pm
Quote:
Originally Posted by GIK Acoustics View Post

Why do you suggest the front to back dimension provides the strongest room modes? Because placement of the mains drives that mode harder? If that's the case, subs would dictate room mode intensity more than the mains, yes (at least for the low octaves)?

Placement of mains and subs at one end of the room in / behind the false wall drives all of the length modes. Across the room width ways sub and mains placement normally creates some 'natural' modal cancellation.
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post #23 of 46
3/2/13 at 5:53pm
Quote:
Originally Posted by GIK Acoustics View Post

I was going to comment on the Green Glue suggestion as well,

Isn't this in reference to noise transmission as opposed to sound absorption inside a room? I've never seen evidence that constrained layer dampening will change the room response too much at all. And adding mass to the wall partitions can actually increase modal resonance, but perhaps this doesn't apply to CLD.

No definitely sound absorption. It's unpublished data.

ASC also use constrained layer damping, it's called 'WallDamp'.
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post #24 of 46
3/3/13 at 11:55am
Quote:
Originally Posted by Spall View Post

So is that to say that #2 above and treating the back wall is the way to go?

Yes, early reflections are the most damaging, especially when they're strong.
Quote:
Maybe that brings about the question of diffusion. Do I need to add any? Is this where I should throw in some room dimensions including where my seats are in relation to the back wall?

Absorption and diffusion both solve the basic problems at mid and high frequencies, though diffusion is considered to sound a little better. Which you choose depends more on your budget than anything else. Diffusers are more complex to build, so they cost more than absorbers whether you buy commercial products or DIY your own. These video should help:

All About Diffusion
Hearing is Believing

--Ethan
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post #25 of 46
3/3/13 at 2:02pm
Quote:
Originally Posted by FOH View Post

I maintain, in this situation, a huge amount of fluffy batts behind the screen wall is likely the least expensive, and most effective, aesthetically acceptable (can't see it), and it takes up no room space,..as behind the screen wall is dead anyway.

Precisely.
Quote:
Originally Posted by Nyal Mellor View Post

No definitely sound absorption. It's unpublished data.

Thanks, I'll try and contact Kinetics about it. I've sent a mail to a friend over at Green Glue as well to see if they have any supporting data. I know you've noted the iso-clips being an important part of that absorption so I'm not sure if GG will have similar data (but who knows - they've got more than a 'few' tests of their products).

I'm not in construction so I don't mean to refute you, I only find your suggestions counter intuitive (as acoustics usually is).
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post #26 of 46
3/6/13 at 1:49pm
Quote:
Originally Posted by GIK Acoustics View Post

...Isn't this in reference to noise transmission as opposed to sound absorption inside a room? I've never seen evidence that constrained layer dampening will change the room response too much at all. And adding mass to the wall partitions can actually increase modal resonance, but perhaps this doesn't apply to CLD.
Exactly my thought process.
Quote:
Originally Posted by Ethan Winer View Post

... One goal in treating rooms of any size is for the decays times to be uniform at all frequencies. Many people put one-inch thick foam or fiberglass all over the front wall, and often on the other walls up to ear level, and they also have carpet. So the room sounds dead at mid and high frequencies, but bass frequencies bounce around uncontrolled. A proper treatment strategy addresses this so all frequencies decay more or less at the same rate.

--Ethan
+1 Got to love consensus!
Quote:
Originally Posted by Nyal Mellor View Post

No definitely sound absorption. It's unpublished data.

ASC also use constrained layer damping, it's called 'WallDamp'.
Certainly, CLD is an absorption mechanism, and GG between sheetrock panels forms an appropriate system. Properly isolating the panels with isolation clips prevents transmission to the support structure, but I'm bothered by the added mass driving absorbance lower in frequency, below the bass/near-infrasonic range. Whatever's not transmitted or absorbed is reflected back into the room.

That's why I asked about data; empirical evidence is the basis for theory, not the other way around.

Have fun,
Frank
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post #27 of 46
3/7/13 at 10:06am
I would guess that the isolation clips lower the resonance frequency of the whole system because there are less points of support, and that's what improves the low bass absorption. The constrained layer damps the resonances.

I've seen the effects enough times in real listening rooms and theaters that I've measured to know this is a usable system, and the unpublished data supports this.
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post #28 of 46
3/7/13 at 1:01pm
So you're thinking that boundary conditions affect panel resonance frequencies? I can see them affecting absorption coefficient or effective absorber area, both for the better, but not frequency as thats dependent on mass density and resonant air space behind. Specifically, I would expect a double drywall with GG would drive peak resonance into the single digits, below the range of small room resonances. Of course, diaphragamitc absorbers aren't high Q, so perhaps it's a net plus, regardless, due to increased alpha and area.

Or are you thinking this follows a different model?

Have fun,
Frank
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post #29 of 46
3/7/13 at 1:09pm
Quote:
Originally Posted by fbov View Post

So you're thinking that boundary conditions affect panel resonance frequencies?

When absorbers are added to a room, the modal resonance frequencies shift downward a little bit. You can see that in the Before / After graphs below for the lowest peak at 40 Hz. The higher frequency peaks shift even more, but that's more difficult to see because some of them are removed completely. Not sure if this addresses the issue at hand, but I though it was worth mentioning.

--Ethan

dlr_before.gif

dlr_after.gif
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post #30 of 46
3/7/13 at 11:09pm
Quote:
Originally Posted by GIK Acoustics View Post

Thanks, I'll try and contact Kinetics about it. I've sent a mail to a friend over at Green Glue as well to see if they have any supporting data. I know you've noted the iso-clips being an important part of that absorption so I'm not sure if GG will have similar data (but who knows - they've got more than a 'few' tests of their products).

I'm not in construction so I don't mean to refute you, I only find your suggestions counter intuitive (as acoustics usually is).

I've received mail from GG. They don't have any supporting data for modal absorption, though they did send me data they had on absorption of in-wall resonance which had some low frequency TL benefit at around 80 Hz.
Quote:
Originally Posted by Nyal Mellor View Post

I would guess that the isolation clips lower the resonance frequency of the whole system because there are less points of support

Yeah, that would be my impression as well.
Quote:
Originally Posted by Nyal Mellor View Post

and that's what improves the low bass absorption. The constrained layer damps the resonances.

I've seen the effects enough times in real listening rooms and theaters that I've measured to know this is a usable system, and the unpublished data supports this.

This is the part I don't understand. Are we talking about lowering the wall cavity resonance or room resonance at listening position? It really does make sense that CLD could attribute to a little absorption in the room - I mean the mechanism upon which it works is literally vibrational losses which could, in principle, lower modal resonance at LP though I don't really see how it could make a worthwhile difference (nor have I seen supporting evidence). Again, not refuting the data you guys have, just stating my thoughts about it. Considering the results are unpublished I can't really go around recommending it. I do still need to send an e-mail to Kinetics about it. Not like it matters too much as I'm in design, I just find it intriguing.

Quote:
Originally Posted by Ethan Winer View Post

When absorbers are added to a room, the modal resonance frequencies shift downward a little bit. You can see that in the Before / After graphs below for the lowest peak at 40 Hz. The higher frequency peaks shift even more, but that's more difficult to see because some of them are removed completely. Not sure if this addresses the issue at hand, but I though it was worth mentioning.

--Ethan

dlr_before.gif

dlr_after.gif

I believe this phenomenon is due to the fact that sound travels slower in porous absorbers, shifting the audible frequencies down a slight bit.
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