Although a variety of treatments are available for commercial use, they tend to be quite expensive. After some research both online and in print, we came across several sources for DIY acoustic treatments using rigid fiberglass panels and simple frames. These are often referred to as "bass traps," although the ones that we're focusing on have a fairly wide rage of absorption. While commercial versions are available for almost $100, we were able to make these panels for about $24 each.
We can not take credit for this design, but have combined several people's ideas into a step-by-step guide. Most importantly, we give credit to Ethan Winer's excellent article, Build a Better Bass Trap.
For more information, check out the good folks in the acoustics forum at recording.org
Step 1: Plan your setup
For our room, we decided to build 12 panels. Since it's a room for music recording, we can take down some of the panels when we want a more lively sound, or we can put them all up when we want a more controlled sound. IMPORTANT: a "dead" sounding room is not natural, and causes the ears to fatigue much faster than in a typical space. Since the point of this project is to enjoy music sound MORE, you want to avoid too much absorption.
Step 2: Gather Your Materials
4" x 3/4" Board - 14 feet, cut as follows: 2 - 4' long boards, 2 - 22 1/2" boards, 4 - 6" long corner brackets (see picture below)
3/4" x 1" moulding - about 8 feet, cut into two 46 1/2" lengths
2 inch thick rigid fiberglass (Owens Corning 703 or similar) - One 2' x 4' sheet
Acoustically transparent fabric - about 2 yards @ 38" wide
16 - Drywall Screws
4 - 4" x 1/4" carriage bolts
8 - 1/4" nuts
3-5 feet - Picture hanging wire (at least 30 lb. capacity)
2 - Heavy duty frame tabs
1 or 2 - Picture Hooks (at least 30 lb. capacity)
2" strip of clear tape
50 - Upholstery staples
16 - Brads
4 - Light duty felt furniture glides
Drill with 1/8" bit and phillips head driver
Phillips head screwdriver
2 Bar Clamps
Lumber - We decided to build the panels to 2' x 4' outside dimension because it was cheaper to trim a bit off the fiberglass than to purchase and waste all the extra lumber that we would have needed to cut the slightly longer frame pieces. For our frames, we decided to use MDF because it is inexpensive and fairly strong. We used 2 4' x 8' sheets for all 12 frames. First we ripped a sheet and a half into 4" wide boards (18 boards), and then we cut the remaining board into 1" strips for our moulding. You could use an alternative such as pine, but it's more expensive, and its not going to show in this design anyway.
Fiberglass - Rigid fiberglass looks just like the roll of fluffy stuff you stick on your walls, but it is compressed and is considerably more dense. It is not available at most general home stores or construction supply houses, but if there is an insulation supplier in your area, they usually have it in stock or can order it rather quickly. It is also sold as "fiber board" and "mineral fiber." It is not necessary to use Owens Corning branded material, but check with the manufacturer to ensure a density of about 3 pounds per cubic foot.
It is available unfaced or with a backing called FRK (foil reenforced kraftpaper) which looks similar to the material used to package pre-made garlic bread. While this option does help increase the panels effectiveness in the low frequency range, it is considerably more expensive (about 33% where we bought ours) and also increases high frequency reflections, so for general use, it's better to get the unfaced material. For our project, we made 8 unfaced panels and 4 faced panels, placing them in strategic locations (basically closer to the corners) to help with specific problems in our room. If you are more familiar with room acoustics, a combination of panels might be a better option.
When working with fiberglass, please be sure to wear a dust mask, eye protection, gloves, and long sleeves to prevent injury and irritation from the material.
Fabric - IMPORTANT: Spend the extra money to buy flame retardant fabric or purchase a fireproofing spray for fabrics and use it on your panels! If the fabric is not flame retardant a warm light, and open flame from a candle, etc. could cause a major fire as you will have flammable materials hanging on your walls! This is not only dangerous, but against the fire code in most areas.
Although you can buy acoustically transparent fabric made for this purpose, it tends to be rather expensive and provides no real benefit in this application. Muslin, burlap and other such materials are great choices. Just find something that will allow air to pass through it freely. If you can hold it up to your mouth and blow through it easily, it will do fine for this project. Just DON'T FORGET TO MAKE IT FLAME RETARDANT.
We used $ .99/yard muslin and treated it with a flameproof spray. It's not very durable, so you may want something a little better if you're planning on moving the panels or tend to find your walls getting bumped and dented a lot. The Cadillac of fabrics for this type of panel is a product made by Gilford's of Maine. It's expensive, but it is fire rated, durable, and comes in a nice selection of colors.
Picture hanging materials - Although picture wire, hooks, and eyelet tabs are readily available at hardware stores and big box home stores, the markup on these items is very large and the quality tends to be sub par. We recommend going to a local picture framing shop and asking to purchase these items out of the framer's inventory. A framing shop may also be willing to help you select the right products and even make sure that you're using them properly. We paid about ten dollars for all of the materials needed to hang 12 panels. Equivalent materials from a national retailer would have cost us over 60 dollars!
Due to the weight of these frames, we recommend overestimating their weight requirements. We used 50 pound wire and hooks, even though they should have worked fine with the 30 pound versions.
Step 3: Build the Frame
Stand a long and short board upright and form a corner with the long board on the outside. Use a corner bracket to check that it's square.
(photo 1212, 1213)
Drill two pilot holes as shown. Repeat for the remaining three corners. NOTE: if using MDF, you MUST countersink all of the holes or the board will split and the holes will strip if you try to use the screw to "suck" the boards together. Use your judgement with other materials.
Glue the edge of the small board, and dip a screw about 1/2" into some wood glue. This will coat the threads and help the screw to stick. Again, do not skip this step when working with MDF or the joint will not hold.
When you finish both sides, use bar clamps to hold the frame while the glue sets.
Place a corner bracket in each corner and mark which way is "up." We realized we should do this after we picked one up to put glue on it and then couldn't remember which side was which. If you get it spun around, it will be difficult to line up the pilot holes.
Line up the bracket with the top edge of the frame and drill two holes into each edge where the bracket attaches to the frame. Countersink, glue, and screw as before.
Drill a 1/4" hole through the center of each bracket. You do not need to measure the exact placement. This is where you will install the spacer bolts later on.
Step 4: Add the fiberglass retaining moulding
Unfortunately we did not take a picture of this step, but you can see the strips installed in the following photo.
Step 5: Trim the fiberglass
Use caution when cutting fiberglass. Wear long sleeves, gloves, a breathing mask, and proper eye protection to prevent the fibers from causing irritation. When you are done cutting, thoroughly clean your work area with a broom, vacuum cleaner, and damp cloth.
NOTE: A perfectly straight edge isn't too important since the board will be covered by fabric anyway. We found it better to aim for cutting slightly less off than to cutting more, because the extra friction helped keep the board in place while covering it with fabric.
Step 6: Install the fiberglass
NOTE: If you are using FRK faced insulation, you do not need to cover the back. Follow the this step as written, but begin by placing the board with the FRK side facing up and do not use the fabric covering.
Begin by placing the panel onto a sturdy, flat work surface. The panel must not hang over the edges of the table.
Lay the fabric to cover the panel. Trim the fabric lengthwise so that it drapes over the edge of the board by about 1 1/2 inches. Don't worry about trimming the long edges of the fabric at this time.
(1225, 1226, 1227)
Take the frame face down (bracket side up) and place it over the board. Fit one side of the frame over the board. Make sure the fabric stays in place as you push down from one side to the other. The board should now be seated in the frame.
Flip the whole unit over and push the board fully into place. It should be pressed securely against the moulding strips.
Step 7: Wrap the unit in fabric
We are currently working on a more detailed Instructable just for canvas stretching. Check back soon and hopefully this paragraph will be replaced with a link.
The point of this step is to cover the fiberglass and the wooden frame in fabric. If you have another method you'd like to use, feel free to do so.
Step 8: Install the spacer bolts
If you can not afford to lose that much space your room, consider adding half inch rubber feet at the corners to help give the panel a little "breathing room."
Step 9: Install the picture wire
First, measure down from the top of the frame (10 inches if you're hanging horizontally, 18 if you're hanging vertically) and mark this point along the inside edge. Typically, you would measure down 1/3 the height of the frame, but we need to add a couple extra inches to compensate for the spacers. If you don't do this, the wire will pull "out" from the wall instead of pulling down on the hook.
Drill a pilot hole into the inside edges, about halfway between the moulding and the back edge of the frame. Be careful not to drill too deeply or you'll drill through the outside edge and snag the fabric.
Using a manual screwdriver, screw the tabs into the frame.
Take the picture wire and feed about five inches through the tab.
Fold the wire over on itself, and crimp it at the fold
Make a loop with the free end on top and then slide the free end through the loop.
Tighten the loop to leave about a 3/8" opening, and then feed the free end of the cable through the tab.
Bring the free end around and feed it through the loop created by the first knot in the cable.
Grab both ends of the cable and tighten the knot. It doesn't have to be super tight to be secure. It just needs to be tight enough to get the slack out for accurate measurement.
Twist the free end of the cable around the long end to keep it from hanging loose.
Pull a length of cable across the frame to the other tab. Pull enough slack so that if it's pulled into a "V," the point is about 2 1/2" from the top edge of the frame.
Pull an additional five inches of slack and cut the wire.
Measure and crimp the wire as before, and repeat all steps to fasten the cable to the second tab.
The panel is now ready to be hung.
Step 10: Hang the panels
For each panel, measure the distance you'd like to have from the top of the panel to the ceiling. Add this distance to the distance from the peak of the picture wire to the top of the frame (approx. 2 or 3 inches). Measure that total distance down from the ceiling and mark that point with a pencil.
Affix a 2" strip of clear tape vertically in the area immediately above the pencil mark. This will help keep the drywall/plaster from crumbling when you install the picture hook.
Nail the picture hook into the wall so that the bottom of the hook is in line with the pencil mark.
Hang the completed panel on the hook. Use a level straighten the panel.
Enjoy your improved room acoustics!
Step 11: Conclusion
Now that we have gone through this project once, we've talked about making another batch if need be. This first batch of twelve took two of us about 20 hours of work, but we could probably cut that time down by 5-10 hours now that we know what we're doing. Don't be intimidated by this project. We designed these steps to avoid most of the mistakes and time wasters we encountered along the way, and the remaining tricky spots are clearly noted.
Finally, if you have any questions about this project, feel free to ask us and we'd be glad to help! If you have questions/curiosities about recording studio acoustics, check out the acoustics forum at www.recording.org . Good luck!