Homebrewed Music

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.