Acoustic panels and bass traps

[gates70]

Hi guys, I will be building my own acoustic panels and bass traps but I have a question. First of all I'll be using Roxul Safe N Sound insulation since I have a full bag left over from work I was doing in my HT. For the bass traps I will be using 1X3 wood for the frames and 1X2 for the acoustic panels since I'll make those "skinnier" for the walls. Now for my question...for the backing on these, would it matter if I close it off with vapor barrier so none of the insulation comes out ? Or do these have to "breathe" for the sound to go through in the back ? What I mean is, is it ok if the back is closed off ?

[jomari]

you can go both ways, personally, it really depends on how you will be placing your acoustic panels,

will they be mounted on the walls? or stand alone? acoustic panels seem to perform better on stand alone mounts (albeit ugly as hell), using the suspended technique.

in regards to bass traps, wood sounds good, but is it solid enough so to speak to handle the corners? in essence, ive seen most basstraps to have some weight to eliminate the lower frequencies. others use 'foam' (as an understatement).

sorry, im at work, and clearly havent had my coffee.

i have a compilation on a stick here...

[gates70]

Originally Posted by jomari

you can go both ways, personally, it really depends on how you will be placing your acoustic panels,

will they be mounted on the walls? or stand alone? acoustic panels seem to perform better on stand alone mounts (albeit ugly as hell), using the suspended technique.

in regards to bass traps, wood sounds good, but is it solid enough so to speak to handle the corners? in essence, ive seen most basstraps to have some weight to eliminate the lower frequencies. others use 'foam' (as an understatement).

sorry, im at work, and clearly havent had my coffee.

i have a compilation on a stick here...

Thanks jomari I've read through a lot of that already and most of what I've read use wood frames with 1X2 or 1X3. These will be hanging on the wall. I just didn't know if the backing mattered for performance.

[jomari]

personally, it doesnt really depend,

you wouldnt care much if a picture frames back is ugly would you?

unless of course, it is exposed...

all in all, id focus more on how effective your panel would be, aesthetics aside.

[mattym]

if you mount the panel on the wall it will make no difference, but if you have the panels freestanding then you would lose the benefit of the distance from boundary layer, unless you use a thin barrier that is tight to the fiberglass, this could actually give slightly better bass absorption. Experimentation is your friend!

[jomari]

glad to see someone around here aside me appreciates good acoustic treatments.

[FreddieFerric]

I'm LMAO! I just found out that Audio Advisor is selling the identical acoustic panels that I intalled in my audio room a year ago for around 400% more than I paid for them. Beware guys! Good treatments don't need to cost an arm and a leg.

[kareface]

If you use some acoustically transparent material for the back of the foam it would be ideal. I would use the method used in this video:

http://www.youtube.com/watch?v=iyYUpkpL0gw

If you are concerned about appearance you don't need to make the panel as thick, 2" will suffice, but leaving the gap at the back will increase the effectiveness of the panel between 80-125hz region. If you are looking for proper bass traps you can't use a material like Roxul, as it has almost no effect below 100hz. Going with a dense foam in the corners and mounting it 1-2 inches off of the wall is the best idea for bass trapping. You should calculate your rt60 to see how many panels you'll need to hit .5 reverb time (the ideal time for home theaters w/o killing the warmth to the room), then test of out of phase bounces, if you are going to dampen you might as well kill 2 birds with 1 stone. Also, you should work up your FR and decay from the listening positions as it's easier to even out harsh dips and peaks with panels in smaller rooms.

[mattym]

Originally Posted by BluRayFred

I'm LMAO! I just found out that Audio Advisor is selling the identical acoustic panels that I intalled in my audio room a year ago for around 400% more than I paid for them. Beware guys! Good treatments don't need to cost an arm and a leg.

Depends on the materials in them, most look the same on the outside, but internally they may have different materials...foam is a classic example of this, properly engineered acoustic foam has other things in it to make it keep its colour and improve its performance.

Originally Posted by sentinel41

If you are looking for proper bass traps you can't use a material like Roxul, as it has almost no effect below 100hz. Going with a dense foam in the corners and mounting it 1-2 inches off of the wall is the best idea for bass trapping.

sorry, I disagree. Foam is not any use below 100hz, rockwool type materials are for more suitable and much cheaper.

You should calculate your rt60 to see how many panels you'll need to hit .5 reverb time (the ideal time for home theaters w/o killing the warmth to the room), then test of out of phase bounces, if you are going to dampen you might as well kill 2 birds with 1 stone. Also, you should work up your FR and decay from the listening positions as it's easier to even out harsh dips and peaks with panels in smaller rooms.

sorry to disagree again, but its not easy to calculate the room RT without knowing what the absorption coefficients of all the materials in the room are, nor specify the amount of panels without knowing how the panels perform. For me, all this information is in a folder on my PC, and you may know how to do it, but not everyone will have the info or the knowledge to do this themselves.
Ideally, you want the reverberation time to be between 0.3 and 0.5, and introducing diffusion is far more beneficial than people realise to the reduction process, and it will make the room feel bigger.

Small rooms are evil. treating them require much more low frequency control and much more efficient panels to get to the 30/40/50hz range.

[FreddieFerric]

Originally Posted by mattym

Depends on the materials in them, most look the same on the outside, but internally they may have different materials...foam is a classic example of this, properly engineered acoustic foam has other things in it to make it keep its colour and improve its performance.

I certainly do appreciate that observation and in certain cases it is probably an accurate observation. Just not in this case. Identical in every respect, both inside and out.

[mattym]

Originally Posted by BluRayFred

I certainly do appreciate that observation and in certain cases it is probably an accurate observation. Just not in this case. Identical in every respect, both inside and out.

you have a link? Im not familiar with the product(being a brit and all that..I like to learn..)

[kareface]

sorry to disagree again, but its not easy to calculate the room RT without knowing what the absorption coefficients of all the materials in the room are, nor specify the amount of panels without knowing how the panels perform. For me, all this information is in a folder on my PC, and you may know how to do it, but not everyone will have the info or the knowledge to do this themselves.

What are you even disagreeing to? You are saying that it's not easy to calculate rt60, but you aren't suggesting that you shouldn't? You aren't really disagreeing as much as you're pointing out he'll need additional information. Most applications that calculate rt60 include the coefficients of the materials among a selectable list, you can also look up the information on the materials and enter the values manually. All you really need to know are the materials of the room surfaces, or for softer surfaces like carpeting what is beneath as well, and the dimensions of the room. As for the panels, he said it was safe 'n sound which has all the absorption coefficients information available on bobgolds or one of 100 other sites.

Ideally, you want the reverberation time to be between 0.3 and 0.5, and introducing diffusion is far more beneficial than people realise to the reduction process, and it will make the room feel bigger.

In a room that small .3 will be fairly abrupt. The sound stage will sound smaller, .4-.6 is recommended for home theaters.

Small rooms are evil. treating them require much more low frequency control and much more efficient panels to get to the 30/40/50hz range.

The thickness of the panels to effectively dampen that low would be a joke. I'd like to see any lab data the suggests that wall mounted dampening panels made of rockwool will have any dramatic effect on the 30-50hz region. Once you get below 100hz, most of the changes needed to dampen have to be made to the wall itself. There is a reason the STC curve doesn't go below this point.

I have to go so I can't address some of the other problems in your post. Rigid foams will outperform any insulation based dampening on the low ends. Anyways, off to home depot!

[mattym]

Originally Posted by sentinel41

What are you even disagreeing to? You are saying that it's not easy to calculate rt60, but you aren't suggesting that you shouldn't?

Sorry I wasnt clear, anyone can do it with the right information, which most folk dont have, as you say here:

You aren't really disagreeing as much as you're pointing out he'll need additional information. Most applications that calculate rt60 include the coefficients of the materials among a selectable list, you can also look up the information on the materials and enter the values manually.

It would have been more useful to post a link to an on-line calculator for people to find, I know what you are refering to, but how many other people would know?

All you really need to know are the materials of the room surfaces, or for softer surfaces like carpeting what is beneath as well, and the dimensions of the room. As for the panels, he said it was safe 'n sound which has all the absorption coefficients information available on bobgolds or one of 100 other sites.

when you say what is beneath, you mean if its a rubber backed carpet or different?
Im not familiar with safe n' sound, I missed that reference

In a room that small .3 will be fairly abrupt. The sound stage will sound smaller, .4-.6 is recommended for home theaters.

dont use diffusion at all then? 0.3is fairly tight, but its not unpleasant when done correctly. Absorption is too sterile so combining diffusion with it gives much better results, and really opens the soundstage.

The thickness of the panels to effectively dampen that low would be a joke. I'd like to see any lab data the suggests that wall mounted dampening panels made of rockwool will have any dramatic effect on the 30-50hz region. Once you get below 100hz, most of the changes needed to dampen have to be made to the wall itself. There is a reason the STC curve doesn't go below this point.

fiberglass and foams wont do 30-50hz, Ive never said they will, though a 4" thick panel over a corner would get you down to 80hz, to get lower, you have to use specialist panels. I can produce links for them if you havent heard of any. 4" deep panels that nail 50hz..

STC is a different animal(sound transmission class), we are not talking about transmission loss here. Reverberation time measurement curves dont go below 100hz for ASTM or BS ISO as its not possible to get accurate measurements due to modal interferance and instabilities, a room needs to be huge to avoid standing waves that would interfer with the measurements. Drywall could produce resonance that may suck out bass below 100hz, but stimply stiffen the wall by adding another layer of drywall.. the British Gypsum White Book spec gives 2 layers per side as a basic wall construction standard for rooms, it stiffens up the wall enough to lower its resonance point so as not to interfer.
Id love to see data for rigid foam insulations below 100hz, and some brand names so i can get some to try out

I have to go so I can't address some of the other problems in your post. Rigid foams will outperform any insulation based dampening on the low ends. Anyways, off to home depot!

Feel free to point out any of the problems in my post, and please do qualify them with some links, Im genuinely interested in seeing data on any materials or products, im always keen to learn new things.

[kareface]

Hah, I wrote out this huge response and firefox crashed and reopening the tabs only brought the window back, none of the text. I'm going to quickly rewrite it but I might be a bit sloppy with my words, lol.

It would have been more useful to post a link to an on-line calculator for people to find, I know what you are refering to, but how many other people would know?

Anyone with google, lol. I don't like most of the online calculators, I prefer plug-ins that work with 3d models.

when you say what is beneath, you mean if its a rubber backed carpet or different?

Carpet is great for mids and highs, but lows to lower mids will pass through fairly easily. There is a dramatic difference between reflections off of a cement floor with carpet pad and carpet on top vs. a sub flooring with carpeting and pad on top. You'll see a lot more reflections off the cement floor, which won't matter unless your room is poorly setup to dampen in those ranges, which most standard rooms will not be. If you use some super thick or dampened carpet pads it will help. If you use thin carpeting on cement and have a normal ceiling you can get standing waves, which are generally lower because the carpet will dampen the higher range pretty well.

dont use diffusion at all then?

It depends on the source of the sound, some drivers will have a greater listening axis then others. If you are getting an over localized effect then diffusers are great. I agree that simply throwing panels into a room (especially with small rooms) isn't a great idea. You need to use them to not only reduce the reverb time, but also even out the frequency response by addressing the more audible room modes and reducing out of phase reflections while not hurting the sound stage. You can be a lot more precise with the acoustics of smaller rooms. Each room is different, I can't say what I would or wouldn't use in this instance w/o more info & feedback.

STC is a different animal(sound transmission class), we are not talking about transmission loss here.

It's material dependent. If the material is too stiff it will reflect mids and highs more then dissipate them. If it was a solid surface I would agree. This won't be an issue with rockwool, so there will be a coloration between STC and reflection reduction assuming the thickness is the same between both samples. Second is if the materials dissipative hysteresis region is easily achieved, which can be tricky with lower energy displacement. You'll only find beneficial results going that route with epoxy and other flexible materials which can be impractical on larger scales.

Drywall could produce resonance that may suck out bass below 100hz, but stimply stiffen the wall by adding another layer of drywall.. the British Gypsum White Book spec gives 2 layers per side as a basic wall construction standard for rooms, it stiffens up the wall enough to lower its resonance point so as not to interfer.

Even double layers of 5/8th drywall will have an resonance in the 100hz range (assuming a double leaf wall construction with no mechanical decoupling) which is very audible. You can correct this using layers laminated together with something that remains flexible which will create a dramatic impedance difference across the boundary to reduce transmission. As a byproduct the resonate point of double leaf walls will be reduced because less energy will be cross the boundary to be amplified by the double leaf effect. The only other way to avoid that problem is the addition of mass or increasing the space between the sides drywall. Mechanical decoupling while greatly improving transmission reduction has the negative side effect of increasing the resonance frequency of any leafed walls, greatly increasing the amount of mass required in the wall or space required to compensate. I tell anyone planing to go this route to leave a good foot of space between the boundary and the next wall over, way cheaper then the cost of adding excessive amounts of mass to the wall.

Id love to see data for rigid foam insulations below 100hz, and some brand names so i can get some to try out

Any high density rigid foam or mass loaded vinyl will be effective down to the 40hz region. I found the foam to be a better solution for dampening floors, just make sure you get a high pound per sq foot. I think we went with Owens Corning on the floors, and I just ordered this so I should know, but I'll have to check with him later in the week to be sure.

fiberglass and foams wont do 30-50hz, Ive never said they will, though a 4" thick panel over a corner would get you down to 80hz, to get lower, you have to use specialist panels. I can produce links for them if you havent heard of any. 4" deep panels that nail 50hz

Sure, but the point I'm trying to make is rockwool, what he said he plans to use, won't dampen worth anything below 100hz unless you decide to make 6" thick panels, which would look silly. Isolating the sub, adding a layer of vinyl or HD foam to the floor and trying to reduce standing waves is the best you can below 100hz w/o rebuilding the walls or using something that is grossly over priced.

Edits: My thoughts are a little sloppy, trying to clean them up a little.

[gates70]

Thanks for all the replies guys. This is getting very technical and somewhat confusing ! For the people that don't know, this is the stuff I'm talking about http://www.roxul.com/residential/pro...sound%e2%84%a2

[kareface]

I'll keep it simple for now, lol. You should construct your panels like what was instructed in the youtube video:

http://www.youtube.com/watch?v=iyYUpkpL0gw

Use sound transparent fabrics for the back at least, ideally for everything. Microsuede tends to reflect slightly, making it not ideal. Also, you should pop onto this site and figure out how many you'll need to deal with reflections in your room:

http://www.trinitysoundcompany.com/rt60.html

Just is just a general number, you'll likely need a few more then this if the room is bedroom size to correct the sound a little. Don't worry about placement yet. You can cross that bridge when you get there.

[gates70]

Originally Posted by sentinel41

I'll keep it simple for now, lol. You should construct your panels like what was instructed in the youtube video:

http://www.youtube.com/watch?v=iyYUpkpL0gw

Use sound transparent fabrics for the back at least, ideally for everything. Microsuede tends to reflect slightly, making it not ideal. Also, you should pop onto this site and figure out how many you'll need to deal with reflections in your room:

http://www.trinitysoundcompany.com/rt60.html

Just is just a general number, you'll likely need a few more then this if the room is bedroom size to correct the sound a little. Don't worry about placement yet. You can cross that bridge when you get there.

Thanks Sentinel, you can see my room in the gallery.

[kareface]

What do you have on the ceiling in the later images?

[mattym]

Originally Posted by sentinel41

*snip

I agree with most of what you have written, and the link to Joel's video is fine. His suede fabric is far far better than some of the others on the market.

the link for the calculator is also a good one, not everyone is so happy to google for stuff.

There are a few differences in application from what you would do and what we do, for example, stud walls, in the UK we dont have large enough rooms to give away a foot on each side, so a smaller gap and better materials do the trick, Green glue (http://www.thegreengluecompany.com) used between 2 layers of soundbloc or similar material, we've found that this is far more effective, and a damned site quicker, than using MLV in walls.

You mention using MLV for panels, we do have a limp mass trap but they can be improved greatly by moving that mlv layer into the panel and using a thinner layer on the front, if you frame the unit you can fit the mlv in after 1" of foam and back with rockwool. I use 6lbcuft materials in all our wall panels, its not quite as good in the mids as the lighter materials but it keeps it shape better when handled, so visually its a little better and performs well enough.

FOr the Roxul traps, if you can find a paper faced version, you can get bettermid and low frequency absorption from that

[kareface]

I luv me some green glue, used it in my theater build and it's good stuff. Awesome for getting the bass region.

For those that aren't google savvy, here's detailed instructions on how to find what you're looking for:

http://lmgtfy.com/?q=rt60