DIY Broadband Absorber Panels and Corner Bass Traps
Note: Click to enlarge images.
After reading Ethan Winer’s articles and information from Auralex Acoustics, RPG , Jon Risch’s great DIY site and the enlightening Ethan Winer: Acoustics Forum at Music Player Network, I decided to build a modified version of the 703 2’x4’x4” panel bass traps and then two "Super Chunk" Corner Bass Traps. Initially for the panel traps I planned to build a wooden frame as others have done.
But after reading Ethan’s “Real Traps - Measuring Absorption and the Numbers Game” article I decided to attempt a design more resembling the Real Traps frame. My goals were to build a light yet strong frame and to simultaineously maximize the amount of absorption on the 4” sides of the trap by not using a solid wood frame. Of course at a DIY project I also wanted to to build the traps as cheaply as possible.
Building the Panels:
After reviewing my design options, I decided to attempt a frame made from metal corner bead and aluminum rivets. For those of you unfamiliar with construction materials (as I was) this is the angle that is used in drywall corners. Typically the corner bead is “mudded” over to create a smooth and strong wall corner. I checked out all the corner bead options from my hardware stores (Lowes, Home Depot, and a local hardware store). They had several choices of 1” or 1 ½” plastic and metal corner bead. Since I wanted the frame to be sturdy enough to support the weight of 4” of rock-wool/mineral fiber I opted for a 1 1/2 “ metal corner bead. The one I chose also had the added benefit of having triangular cut-outs further adding to the amount of side absorption. – 8’ Kal-Corner Bead F/Blue BOA – this is made by Dietrich Metal Framing. (www.dietrichindustries.com) ** Note: Unfortunately their PDF catalog states that Kal-Corner is no longer made by Dietrich Metal Framing, but their mini bead is their recommended alternative.
To save expense, I decided to use Mineral–Wool (rock wool) instead of Owens-Corning 703 since the absorption properties are fairly similar. Available from http://www.insulationworld.com. Here is a picture of the Mineral–Wool bundles (six 2 inch panels per pack).
Mineral Wool Specs:
The mineral wool was 8 lbs/sqft. and manufactured by IIG (Industrial Insulation Group) Min-Wool LLC. This company is a joint venture of the Calsilite Group and Johns Manville. They bought an Alabama Owens Corning plant in 2003. The mineral wool sheets I purchased were Type 1280. These were 2x24x48. I used two panels in each broadband trap so they were actually 4x24x48 each. The "Super Chunk" corner traps were just cut triangles stacked floor to ceiling of the same material.
Here are the acoustical specs from the manufacturer:
And their density specs:
I was able to buy each 8 ft. section of corner bead for $1.99. Measuring 24” from each end I made a cut through one side of the corner bead. Bending this into a 90 degree corner, I then drilled 2 holes in the overlapped corner secured it with two 3mm aluminum pop rivets. It is very important to check setting the right angles so all the frame's corners are perfectly square.
This gave me three sides of a face of a frame. I then cut an 8’ section into two 4’ sections and completed a frame face by drilling and riveting the corners. So three 8’ sections are needed to build the top and bottom frame. An additional 16” is needed to be cut into four 4” pieces for the vertical part of the frame.
So once I had the top and bottom of the frame. I drilled and riveted the four 4" corner pieces to the bottom of the frame. Again I used two rivets on each face of each corner
Then place two pieces of 2’x4’x2” rock-wool inside the bottom frame. Set the top frame in place and drilled and riveted all the corners. To make the frame strong I always riveted at least two or three rivets on each side of the corners.
Around the metal frame I wrapped a thin polyester 4 oz. batting to 1) help keep mineral-wool fibers inside the trap and out of the air and 2) to keep the metal frame from reflecting high frequencies.
Next cut thin polyester batting to cover the bottom and sides of the trap. Secure the poly batting with a light adhesive spray. Then cover the bottom and sides with black burlap securing the fabric to the metal frame (and itself) with hot glue. All gluing was done on the back of the trap . Burlap was chosen over the very expensive Guilford fabrics to keep the cost down. (available from Jo-Ann Fabric Stores ) Also burlap doesn't reflect high frequencies as many tighter weave fabrics do.
Next I screwed in two eye-screws near each corner using the holes that were manufactured in the corner bead. One eye-screw might have been strong enough but I wanted to be sure the traps would never fall. I needed to screw these eye-screws in at a slight angle to get it through the corner bead holes. Another option would be to use eye-bolts with a nut and washer. But using bolts would require that they be attached prior to riveting on the top of the frame.
Next I covered the back of the trap with a layer of poly batting to encase the mineral wool fibers and keep them from becoming airborn.
The final finished trap 4’x2’x4” weighs in at 18lbs.
To hang the traps I over-engineered a bit. I used 75lb. military grade 14” plastic ties through the trap’s corner eye-screws to connect through ceiling and wall-mounted eye-screws which were threaded into shelf drilling nylon drywall anchors. I linked multiple 14” plastic ties together to add length when more than 7 ” was needed. This method of mounting I found extremely easy and very secure.
Building SuperChunk Bass Traps was fairly simple. But as other have said, it is very important to wear protective gear (long sleeves, gloves, dust mask) to protect oneself from the mineral fibers. I used a large buck knife, a metal straight edge, and cardboard underneath to protect the knife tip.
First cut each 2”x4’x2’ mineral-wool panel in half (into two 2’x2’). Then cut these diagonally so you have a 24”x24”x34” see the 703 wedge image at the bottom of: http://forum.studiotips.com/viewtopic.php?t=535
The cut triangle mineral wool panels were then stacked into the corner from floor to ceiling.
I covered the front with a burlap-covered frame made from 2”x2” wood. The frame's three horizontal pieces were cut at 45 degree angles to allow for the vertical pieces to be flush with the walls. Screws attach the horizontal and vertical framing pieces.
I used heavy duty staples and hot glue to secure the burlap to the wooden frame. Since the burlap I had was only 36” wide, It couldn't’t cover the width of the frame plus have enough extra to wrap around to the back of the frame for stapling and gluing. So I used three pieces of burlap width-wise to cover this frame. I used a light amount of hot glue to secure the pieces together. I also glued the fabric to the cross piece of wood to help keep the burlap taught. The frame fit tightly corner without using anything to anchor it to the wall. The ductwork above and carpeting below held the burlap covered frame very snugly in place. One might add a layer of light poly-batting under the burlap to keep the mineral -wool fibers in the trap as I did on the panels above.
I need to preface this section with the oddities of my room. The11'x18' room has 8'ceilings, but ductwork (painted blue) is framed around all four ceiling corners. I decided to place the panel bass traps below the framed and wall-boarded ductwork. Perhaps my results would have varied or improved if I hung all the panels from the ceiling and down over the ductwork. The same issue affected placement of the SuperChunk bass traps in the corners. They could only be 7 ft. high -- up to the ductwork. The other oddity is that there are two doorways currently without doors. The left doorway goes to my "live room" for recording, the right to the stairway. Installing heavy doors are planned as a future project.
I built seven 4’x2’x4” bass traps (~$25 each), 2 mid-high frequency traps (2” thick) for above the mix position (~$20 each), 1 2’x2’x2” mid-high trap (~$10) on the back wall, and two Studio Tips “Super Chunk” bass traps (7’ high) (~$70 each). With the 4 remaining mineral wool panels left over after this project I built two movable gobos (4’x4’x2”) each with a "live" side and a "dead" side.
In a word ASTONISHING! The once muddy room now sounds fantastic! I can hear pristine mixes, much sharper sound stage, clear imaging, and much tighter bass lines. My studio has never sounded better! I feel like I'm finally getting to hear what my studio equipment and monitors can do. Installing acoustical treatments as described above will help absorb bass frequencies, reduce standing waves and low frequency reverb time, help control flutter echoes and ringing, and a controlled lesser amount at mid and high frequencies. This is where all home/project studio owners should begin in their quest for better mixes. If one can't afford commercial products then one should DIY.
The calculated standing waves and modes as shown in the four images below are derived from various room mode calculators. The calculations are estimations for a rectanglar room. So they are only best guesses as to the modes in my control room. Given that I have framed ductwork around all four ceilings, and have two open doorways, the calculations are bound to be somewhat off. That said, problematic modes have been greatly reduced through the installation of the acoustical panel and "SuperChunk" traps. Click to enlarge images.
Below is a graph showing before and after installing the acoustical treatment. The severe dip at 105Hz. and peak at 138 Hz. have both been tamed. The peak at 90Hz. has been completely annihilated by -12db! Overall bass response has been greatly flattened.
An added benefit is that sound transmission out of the studio control room has been greatly reduced as well -- although there are better methods for achieving transmission loss by placing Sheetbloc or mineral fiber in the wall framing, sealing doorways, windows, etc.
Bottom line: it sounds awesome!
Using Ethan Winer's RealTraps Test Tone CD test frequencies from 20Hz to 300 Hz.