I found that installing these panels made a lot more difference in the quality of my recordings than upgrading a preamp or a/d converter, or even buying a new microphone. By improving the sound in the room, the acoustic treatment made the whole recording process much easier and more enjoyable. So when people ask me how to improve their recordings, one of the first things I suggest is room treatment.

Minimum Impact, Maximum Result

Unfortunately many people don’t have a dedicated recording space. They can’t stuff a dozen panels into corners or lean them against walls, and they often ask how few panels they can use and gain any benefit. My standard answer is two, just two panels can reduce reflection in one small area, an area just right for recording acoustic guitar or vocal or many other acoustic sources.

There are a number of commercial products born out of this idea of a zone of controlled reflections. Some of these are rather small and thin, so I’m doubtful that they’re strongly effective. Some are fitted with frames and/or attachment hardware. This adds flexibility and a pro look, but also adds weight and cost. My preference is a pair of simple unframed panels, 2 feet by 4 feet by 4 inches of OC703 compressed fiberglass. I made mine with burlap and hot glue, but premade panels are available as well. Here’s the blog entry on building these panels: Building a broadband absorber (on the cheap)

I deploy the panels in a V surrounding the microphone(s), often by simply leaning them against the mic stand if it’s sturdy enough. If needed I lock the panels in place using a bent coathanger poked into the fiberglass to add a little stability. I’ve experimented with different positions for the panels and this arrangement has seemed the most effective to me. The sound leaving the guitar, or at least a lot of it, passes through the panels as it leaves, so it hits the microphone full strength, then gets attenuated by the panel. Then after the sound hits the wall it returns through the panels for further reduction before hitting the back and sides of the mics.

Let’s Listen

In order to evaluate this two panel approach I set up in our guest bedroom, a 12 x 14 x 8 foot space with no room treatment and only a futon for furniture. This is a fairly reflective room and seemed like a good place to test the two panel solution. I used the Zoom Q3HD as the “mic” and placed it about 24″ from the guitar, with the stick-on wide angle lens attached Zoom Q3HD with a Stick-on Wide Angle Lens then I setup the Xacti HD2000 for a wider shot.

Here’s the resulting video:

Conclusion

I hope you’ll listen and make your own judgment, but my opinion is that the two panels do a remarkable job of cleaning up the early reflections and reducing the “small room” sound. The direct to reflected ratio is improved with a couple of benefits. Since the reflected sound includes a lot of high mids, the sound with the panels is “warmer” with those reflections reduced. The other effect is to make the recording sound like it was made with a closer mic position or in a much larger room.

I’m pleasantly surprised by the effectiveness of this two panel approach. They’re small enough to be stored under a bed or behind a door, they’re light enough that they can be moved into position quickly and easily, and even purchased commercially the cost is under $200. It’s possible that one or two more panels might improve things further, but in my preliminary comparisons it seemed that the big improvement came from two panels, and one or two more panels did not provide much additional benefit. Your situation could certainly be different, of course.

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.

My panel construction is crude compared to a lot of examples I see. I use no frame, relying on the rigidity of the compressed fiberglass to hold itself up. I simply glue together two 2″ panels and fold the burlap around them, then shoot the burlap with glue and hope they hold. They’re not very rugged, not terribly pretty, but they hold together well enough for my needs, they’re easy to maneuver, and do a nice job of improving the sound of the room.

Broadband absorber, quick and dirty style

Hardware Free Mounting

I’m lucky to have 8′ ceilings in my recording space, so the 4′x2′ dimension of the OC703 works great. I can easily shove two panels into a corner and they’ll hold themselves in place. I can place two of them on a wall with a 4″ spacer (like a small cardboard box) behind them and they’ll wedge into the space and stay there.

Here’s a model of Digital Duck Studio with 13 panels installed.

Digital Duck Studio with 13 broadband absorbers installed

and a picture of the business end of the listening setup.

Digital Duck listening position with broadband absorbers

My layout simply follows the advice given so generously by Ethan Winer, concentrating on the corners. The two forward corners are the easy ones, every other vertical corner in the room has some obstruction. The floor to wall corner is the next easy one to cover. The forward traps are out of the way, so I angle them out a bit, nearly 45 degrees straddling the right angle. On the side walls, where space is more valuable and also where we need mid and high damping, I stand the panels up and lean them against the wall at a small angle. They’re tall enough to cover the first reflection points on the side walls, and since they’re not fastened, they’re easy to move if needed.

On the left I have a pair of panels spaced 4″ from the wall, once again following Mr. Winer’s advice.

Mounting of two broadband absorbers using a spacer

These two panels are self-supporting by being wedged against a spacer, a conveniently sized piece of styrofoam. Once again, this arrangement can be altered in minutes.

Room Testing with Room EQ Wizard

Now lets fire up Room EQ Wizard and see if we can measure any improvement with 13 panels. Here are three measurements taken at three different locations, just as we did in the previous post. The first measurement is light blue, taken at the listening position. The second, green, has the mic moved back 1 foot. The third purple graph was taken another foot back.

Three frequency response plots - 40 to 1000 hz

and for comparison here’s the graph from the untreated room:

REW graph 40 to 1000 hz, 3 measurements combined

I’m pleased with the this graph in the area between 100 and 200 hz. In the untreated room measured in our last blog entry each measurement showed a distinctly different dip, right in the low strings of the guitar. The new measurements with the treatment in place may not be flat, but they’re much more consistent between positions which makes getting a good recording a lot less of a headache.

And speaking of recording, here’s a clip with the same mic layout we used in the untreated room.

post2clip1.mp3

or if your prefer, download the clip

Here’s the clip from the last post, in the untreated room.

post1clip1.mp3

or if your prefer, download the clip

To my ear this new clip in the treated room has a tighter stereo image and sounds much more like a studio recording. This recording shows how much the broadband absorbers do to control the early reflections that make a small room sound “cheap” and tinny.

Well, if a little is good, then more is better, and too much is just enough. In the next stage of the panel project I’ll buy some more compressed fiberglass and some more burlap and make a few more panels to try to improve the room a bit more.