Building a broadband absorber (on the cheap)
When I talk about room treatment I only discuss the simple broadband absorber. I’ve read discussions of tuned membrane bass traps, tube traps, and diffusors, but my approach so far has been guided by the slogan, “You can never have too many broadband absorbers in a small room.”
As I mentioned in a previous post, my construction method for broadband absorber panels is cheap and simple. I use no frame or other hardware, but simply wrap two sheets of OC703 in burlap, like wrapping a package, and fasten the fabric with glue.
Bill of Materials
So the material list for a single panel goes like this:
2 sheets of OC703 or equivalent.
3 yards of 30″ wide burlap
glue
Fiberglass is not the only material that will steal a little energy from passing sound waves, but compressed fiberglass sheets like OC703 are very handy because they’re self-supporting. If you choose some less rigid material like acoustic cotton for your absorber core you’ll have to frame it in some way.
When I built my first batch of panels the only material I could find locally was a John Mansville equivalent to Owens Corning OC703, and it came with a foil face attached, which I removed. I now know that the facing improves bass attenuation and should be used for panels that are not at first reflection points. Another advantage of the facing is some reflectivity of high frequency sound. If the room starts to sound too dead, panels with foil facing can add some high end sparkle.
Faced vs Unfaced
For this panel project I found a local supplier with both faced (FSK/FRK in Owens Corning terminology) and unfaced panels. The faced panels cost a little more, and come in cartons of six panels. The unfaced come in cartons of 12. I bought one carton of each, to make a total of 9 panels. Each panel is made of two 2″ thick sheets, for a total thickness of 4″.
Panels made with FSK facing should only have one sheet with facing, and the facing should be on the room side rather than the wall side.
Many different fabrics can be used to cover a broadband absorber panel. The only requirement is that the fabric “breathe” or allow an easy passage of air. If you can blow through the material, it’s suitable.
When I built my first set of panels I stumbled over a bolt of burlap at our local fabric outlet, so burlap has become my cover of choice. Since we’re only building 9 panels I once again bought the fabric locally. If I were building 20 panels or so, I would order a roll of garden burlap in a 30″ width for a lot less than the local fabric store charges.
Frameless Construction
With no frame I have nothing to staple, pin, screw, or otherwise attach to. I’m sure the really correct way to build one of these panels is to sew a sack out the burlap, then sew the fiberglass panels inside. But I don’t sew. And I don’t plan to learn.
Instead I have used various glues. For my first efforts I tried fabric glue from a crafts store. This was both expensive and not very effective, with the seams giving way after a short while. My next choice was 3M Super 77 Multipurpose Spray Adhesive. In the two panel projects that preceded this one, I coated my little corner of the world with 3M Super 77, let me tell you.
I expected to use 3M Super 77 along with hot glue, but when I checked the glues available at my fabric outlet I found LocTite Heavy Duty Spray Adhesive. I bought a can and I’m glad I did. This spray adhesive has better pattern control for the spray, a stronger initial tack, and a stronger bond than 3M Super 77. After a little research I’ve learned that 3M also makes a similar product called 3M 90, but I’ve never seen it in the outlets I frequent.
Before I started this recent project I decided to give hot melt glue a try. I was surprised at the low cost of a glue gun at the craft store, and I’m willing to keep using this little gun for the current set of panels. But if I were firing up to construct 20 panels or so I think I’d be looking for a gun with a somewhat higher wattage. Glue guns come in high, low, or switchable temperatures. For our project high temp is the way to go.
Video Demo
Here’s a YouTube video showing the process of building one broadband absorber panel.
I shot the video about midway through the project, and things changed by the time I finished. I switched to using hot glue exclusively for a few reasons. It’s cheaper, easier to control, and doesn’t generate a cloud of chemical fumes. On the down side, I found myself squeezing the trigger so hard that I tweaked my hand and couldn’t play guitar for the rest of the day … be patient.
Here’s the bill of materials for 9 broadband absorber panels:
OC 703 Rigid Fiberglass
48 sq ft FSK faced @ $2.00 per sq ft
96 sq ft unfaced @ $1.66 per sq ft
Total: $254.40
Source: MacArthur Co., Oakland, CA
Burlap
48″ width
108″ (3 yards) per panel
27 yards @ $2.99 per yard
Total: $80.72
Source: Jo-Ann Fabric and Crafts
Glue
LocTite Heavy Duty Spray Adhesive
$14.99
Hot Glue Gun
$8.00
Glue Sticks
$5.99
Total: $28.98
Source: Jo-Ann Fabric and Crafts
Grand Total for 9 panels: $364.10
Per Panel Cost: $40.46
Hmmmm, now that I look at that final figure I realize why people are so reluctant to invest in room treatment to the degree needed. When I see posts about absorbers I usually see installations with two up to about six panels. When I get these installed I’ll have 22 panels in my room, and I’d bet that more would be better. But I would have had a hard time making myself spring for $900 for panels back at the beginning of my recording experiments.
Of course, by now I’ve spent a lot more than that on gear – a/d converters, preamps, mics – and I honestly think the broadband absorbers have done more to improve the quality of my recordings than any gear I’ve bought.
This entry was posted on Friday, March 6th, 2009 at 4:10 pm and is filed under Acoustics, Tutorials. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
Anton Emery said in post # 1,
on April 26th, 2009 at 9:56 pm
Thanks alot for this post. Very informative. I plan to give these a try someday.
Anton
James Voos said in post # 2,
on October 23rd, 2009 at 8:45 pm
If you are concerned about the look of the panels, you might try
http://www.guilfordofmaine.com
This is the stuff lots of people use for walls in high end home theaters. Lots of color choices, textures, etc. I looked into it when I was considering building out a home theater. Also, do any of the big box home centers (HD, Lowes) carry the OC703?
Thanks for the great, lucid explainations for those of us setting up our own home studios!
Dominick said in post # 3,
on May 22nd, 2010 at 12:46 pm
I was looking at MacArthur website for the OC 703 you were using. Not really seeing the OC 703..Maybe its under a different name in their catalog, or somewhere on the site???
Great video also…
Fran Guidry said in post # 4,
on May 22nd, 2010 at 6:46 pm
Dominick, I didn’t locate inventory on their website, but I called and spoke with a salesman. He had quite a variety (FSK, unfaced, 703, 705, 2″, 4″) It was a bonanza!
Fran
Beverage Dispenser said in post # 5,
on October 19th, 2010 at 2:05 am
home theater systems made by Panasonic are the best in my opinion, they sounded great~~.
Mike said in post # 6,
on January 27th, 2011 at 1:30 pm
Thanks for putting this up! My OC703 just arrived today and I’m going to hang it (wrapped in red burlap) from the ceiling over the drums and over my microphone position. Nice video job!
Fran Guidry said in post # 7,
on January 27th, 2011 at 2:16 pm
Glad you found the video useful. I thought adding some broadband absorbers made much more difference than any equipment upgrade.
Fran
Mike said in post # 8,
on January 28th, 2011 at 12:23 pm
Fran-
Do you think 703, wrapped in fabric like yours and suspended from the ceiling without a frame will SAG? I plan on running butterfly “nuts” and 6″ bolts with huge washers on the end through the 703 up into the butterfly “nuts” in the ceiling.
Fran Guidry said in post # 9,
on January 28th, 2011 at 12:59 pm
My 4″ panels do not sag. I suspend them from the ceiling using hooks and twine crossed under the panels, but your solution should work fine.
Fran
Mike said in post # 10,
on February 4th, 2011 at 8:19 am
I just built 6 of these, using your directions, especially in folding and glueing the corners. They came out great! And I only used LocTite Spray Adhesive, which sells for half the price of the 3M spray. No hotglue gun used. BUT I didn’t use the burlap. Instead I opted for a black speaker grill cover fabric my local fabric shop had that cost less than the red burlap! It looks very professional.
The 703 is a lot like heavy styrofoam in that it can deform if you squeeze it or lean on it. So try to avoid that. I only wore a t-shirt on my upper body, as it was an unusual 70 degrees the day I covered them. I also used cheap, disposable latex gloves, three pair for making the 6 absorbers. Smoothing the seams and patting the corners flat transfered the glue to the gloves, so replacing them every couple of absorbers was needed. I did get some itching/irritation just above where the gloves were on my wrists.
You used twine to hang yours, but I opted for a black ribbon with thin wire on each side sold by the same fabric shop. This MAY prove to be a mistake, as it does break we found out as we tightened the square knots holding the loop at each end of the panel. I ended up not drilling through the panels and wrapped the ribbon around each end, hanging each end from a simple ceiling plant hanger, then wrapping the ribbon around the ceiling hook enough times to level it. It worked! IF I have to replace that ribbon, I may try black fiberglass mesh gutter guard material I found at Lowe’s home supply. It will have to be cut lengthwise, but I think it’s pliable enough to tie a knot in, and it’s strong. I’d go with it if I had it to do over.
Now I have 2 panels above my vocal mic position, 3 above my drums, and 1 placed lengthwise in the corner by the drums. Many, many thanks again for your inspiration and video work here!!
Fran Guidry said in post # 11,
on February 4th, 2011 at 10:10 am
Thanks very much for stopping by and giving a description of your project. Sounds like you came up with some excellent alternatives.
I’m sure someone can come up with a better way to fold the material, but my simple technique works well enough. I wish I’d known to ask about speaker grill cloth at my fabric shop. It never occurred to me to ask for what I wanted (grin).
Good luck with your recording.
Fran
Kevin said in post # 12,
on March 30th, 2011 at 8:06 am
What about matress pads? Would this not work as well? You know the pads you put atop a mattress, looks like an egg container?
Fran Guidry said in post # 13,
on March 30th, 2011 at 9:03 am
Hi, Kevin, thanks for stopping by.
If the mattress pad is foam, which most of the ones I’ve seen are, it will not absorb the low frequencies. The cliche “eggcrate foam on the walls” is a recipe for boomy recordings with muted highs, because the foam only absorbs high and mid- high frequencies but lets the lows and mid-lows build up and make your recordings muddy and boomy.
As far as I know at this time, the materials that work for absorbers are compressed fiberglass, rockwool, and dense cotton insulation material. Compressed fiberglass has the huge advantage of being stiff enough that no frame is required, so it’s possible to build panels much more quickly and cheaply. Unframed panels also weigh less and are easier to place. They just don’t look as professional.
Fran
Wess said in post # 14,
on April 23rd, 2011 at 1:57 pm
Is there Any Specific Reason For using Burlap and not another Fabric?
Fran Guidry said in post # 15,
on April 23rd, 2011 at 7:00 pm
Wess, no reason at all. It’s just a matter of a “breathable” fabric – if you can easily blow air through it the fabric will work well. I used burlap because it was inexpensive.
Fran
Mike said in post # 16,
on May 3rd, 2011 at 5:05 am
I have to add that the LockTite spray glue did NOT hold the corners of my panels wrapped in speaker grill cloth. They slowly started popping open, so I hot-glued each corner. It took me 15 minutes. The LockTite seems to be holding the long seams fine- little stress on them there I guess. So now I highly reccommend LockTite on the long seams and hot-glueing the folded corners.
I also have to say I LOVE my absorbers and how I can easily discern their efficiency at stopping reflections just by speaking softly while standing under them. You will NOT regret making a few of these!! Hang two or three from the ceiling abutting each other and record instruments and vocals under them. What a difference!
Robert said in post # 17,
on May 13th, 2011 at 8:46 am
I’m a relatively new acoustic music home brewer. I’ve been looking for info on what to do for my recording room. I have done studio work in the past, and I know I need to do something about the space before I invest in any more gear. This blog post is a huge help in that direction. I don’t live in the US these days so the brands you mention are not available and I’ll have to punt on the materials. I know I’ve seen compressed fiberglass panels in the home stores around here. I don’t recall any that were foil-faced on one side… plastic of some kind, like drop-ceiling tiles. I hope they will work in this application because it’s all there is. I saw your other post on how you deploy these things in your space… was wondering about that.
Thanks for making this available. I’ll be nosing around some more.
Fran Guidry said in post # 18,
on May 13th, 2011 at 10:15 am
Hi, Robert, thanks for your comment. Try the Sound on Sound acoustics forum http://www.soundonsound.com/forum/postlist.php?Cat=&Board=DESIGN Gearslutz acoustics: http://www.gearslutz.com/board/studio-building-acoustics/ and the Acoustics Forum: http://forum.studiotips.com/index.php for more brands and types of material. I often read of Rockwool being used by folks outside the US.
Fran
nick said in post # 19,
on September 29th, 2011 at 9:56 pm
hi thanks. would love to see how u hang em on the walls
Fran Guidry said in post # 20,
on September 30th, 2011 at 4:52 am
I don’t hang them. My ceiling is 8 feet high, the panels use 4 foot fiberglass, so with the covering they squeeze perfectly into 8 foot space. I put a 4″ spacer behind the panels to obtain the air gap advantage.
I also lean several of my panels against the wall instead of hanging them. This lets me rearrange them easily, use them as gobos and such when I need to.
Fran
L’acoustique du home-studio (7ème et dernière partie) : disposition du studio et des traitements « Le Chant du Signe said in post # 21,
on November 1st, 2011 at 6:02 am
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