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.

or if your prefer, download the clip

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

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.

A Look at the Room

Since we’re treating a room it makes sense to start by examining the room a bit further. After a little playing in Sketchup I came up with this:

Digital Duck Recording Studio in Sketchup

Believe me I feel lucky to have so much room to devote to recording. My space has no sound proofing features, and sits on a fairly busy street, with a playing field across the street, so serious recording has to wait until the late evening. Soundproofing is a difficult and expensive business, so I just live with the noise.

The triangle represents the listening position, with a pair of Dynaudio BM6p speakers a little over 5 feet in front of the window. Here’s a pic that shows the layout:

Digital Duck Studio with Bobby and Chuck providing inspiration

Gear

The computer sits in the next room, our guests get to listen to the noise so I don’t have to. My audio interface is a LynxTwo-C, with 6 inputs and 2 outs. The recording rack holds a John Hardy M-1 preamp along with a pair of M-Audio DMP-3 preamps.

I have a half-broken mic boom stand that I use for a headphone stand. Since I’ve embarked on room my room measurement endeavors I’ve pressed it into service to position the SPL meter and the measurement mic. Here’s a shot of the boom – the phones are Audio Technica ATH-M50s and they’re terrific.

Boom stand holding SPL meter and measurement mic

Just to give you an idea of the level of engineering that goes on around here, a closeup of the DPA 4061 and the Radio Shack SPL meter. Note the NASA grade paper clip and masking tape:

Measurement mic and SPL meter up close

Measuring with Room EQ Wizard

I was fooling around with Room EQ Wizard and decided to try moving the measurement mic around a bit to see how the results changed. One of the usual problems with an untreated room is a wide variation in the sound in different locations in the room. I set up REW and took three measurements, starting at the listening position then moving back a foot each time.

At the listening position

This first shot shows the full spectrum from 40 hz to 20 khz, but in an untreated room the reflections swamp the signal in the higher frequencies. The problems at the low frequencies seem pretty fierce, with big swings between 100 and 200 hz. These are the bass notes on the guitar, extending from 73.42 hz for the low D I usually use, up to the third string G at 196 hz. The huge swings in response will make some of these notes boom and some disappear.

Here’s the graph for the same measurement, but 1 foot further away from the speakers.

1 foot behind the listening position

This graph looks a bit better, but we still see over a 20 dB swing in the frequency response between 100 and 200 hz, and we’re seeing that different notes will be emphasized and muted.

The third graph shows the measurement from 2 feet behind the listening position.

2 feet behind the listening position

Once again the specific notes impacted by the frequency response variations has changed. The effect is easier to see when all the plots are displayed on a single chart:

REW graph 40 to 1000 hz, 3 measurements combined

Each of these measurements has at least one steep dip between 100 and 200 hz, and each one will mute a different musical note. And these effects were found by moving only 1 foot each time.

The Sound of the Room

Since my goal is to improve my recording space it seems reasonable to do some before and after comparisons of recordings, so I set up a Jecklin disk mic array with my Shure KSM141 mics switched to their omni setting. This arrangement of omni mics with a baffle between them is usually used for concert hall recording and other classical music sessions:

Jecklin disk mic array with a Shure KSM141 pair

This mic arrangement uses omnidirectional mics, so we’re getting the great extended bass of a fine omni mic but we’re also getting a lot of the room sound. I positioned the mic about 2 feet from the guitar which gives a wonderful stereo image and very natural tonality, but once again brings in a lot of the room sound. Here’s a picture of the recording arrangement:

Vamping for the microphones

Most of us recording at home usually use directional mics and place them much closer to the guitar, in an effort to reduce the impact of the room. But directional mics exhibit a rising bass response called proximity effect which increases as the mic gets closer to the sound source. If we can help our room sound better, we gain a lot of flexibility in the mic techniques we can use.

Here’s the brief clip I recorded:

or if your prefer, download clip 1

Then as an experiment I moved the mics and chair both about 2 feet, keeping the distance between them the same, and recorded again:

or if your prefer, download clip 2

I don’t put much stock in listening tests where we hear different performances, because it’s impossible to tell whether the sonic differences are due to the recording technique or the playing technique. In this case I can tell you that the difference in the two recorded clips is much greater than the difference in the two performances, and I think it’s due to the wildly varying frequency response in this untreated room.

Even though it’s not strictly necessary, it’s interesting to use some kind of acoustic measuring tool to evaluate the room and gauge the results of treatment. I recently learned of a free program that works with a PC or Mac and their audio systems to measure and display room response. It’s called Room EQ Wizard and it’s available for download at the Home Theater Shack. The Shack is a discussion forum focusing on home theater (well, duh), but Room EQ Wizard, or REW as they call it, is just as useful for recording spaces as it is for home theater.

The home theater users seem to have systems that are a bit more complex than my simple recording rig. I don’t have a subwoofer and accompanying crossover, nor do I have an equalizer in my playback chain. So my connections were simpler than those illustrated in the REW help pages.

I bought one extra piece of equipment for this project, a Sound Pressure Level (SPL) meter. These are available from Radio Shack and other sources. The Radio Shack unit I bought is accurate enough to measure a baseline volume setting for all my readings. It’s possible to use the SPL meter as the measurement mic, but it is not accurate above about 3Khz, fine for tuning subwoofers but not much help for a full range monitor system. The Behringer ECM8000 is widely recommended for an inexpensive measurement mic, but I have a couple of DPA 4061 miniature omni mics in my collection, so I chose that for my measurement mic.

The first step in using REW is a soundcard calibration procedure. This procedure compensates for nonlinearities in the soundcard and also ensures that the basic system is working, with REW finding the soundcard inputs and outputs as needed.

I use a LynxTwo-C soundcard and REW found it easily. Here’s the Settings page where the soundcard is configured and measured.

The soundcard calibration starts by creating a loopback connection, that is, the output of the audio interface is connected to the input. This is easy with a recording oriented system, where balanced line in and line out connectors should be available. On the Lynx breakout cable the XLRs just clicked into place. On other systems a male TRS to male TRS may be needed. Don’t forget to turn off your power amp or mute your speakers – now how would I know to remind you about that??

I’ve created a YouTube account for videos related to the blog. You might stop by http://www.youtube.com/user/homebrewedmusic if you’re in the neighborhood. This video goes through the steps to run the calibration measurement.

After saving the soundcard calibration file we need to adjust the levels so we’re getting a good signal to noise ratio but avoiding clipping. First we restore the loopback connection to our normal hookup and turn on our power amp or unmute our speakers.

This video demonstrates the steps:

Since we’re moving these input and output volumes all the time we’ll probably need to run this level adjustment routine before taking measurements.

With the levels set we’re ready to measure our room/speaker response. Well, actually we’re also including the mic preamp and power amp, but those are probably quite linear, especially compared to our room and speakers.

Here’s a screen video of the measurement process.

And here’s the resulting graph. Looks pretty ragged to me, with huge swings between 40 hz and 200 hz, and lots of comb filtering in the higher frequencies. I suspect that this is the normal response of a medium small room. In our next entry we’ll see what we can accomplish with our current batch of broadband absorbers.