July 22, 2013

Bass traps are expensive.  Actually, scratch that -- they are very expensive, just like every other piece of sound treatment for sale in today's market.  But because bass traps are so important, I knew I had to get at least 8 of them and fill up the corners of my new control room if nothing else.  My control room is 17.5' long by 9' wide by just under 8' high.  Especially when working in such a small, rectangular control room, well... there are listless problems relating to bass monitoring, room modes, and decay.  Corner traps were an absolute must as a first step towards addressing these sub-Shroeder frequency issues.

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Finished traps in the corner of my Control Room.
Before setting out to do my own DIY traps, I strongly considered buying them new or used, but it proved to be too difficult:

1) Check prices online for new, pre-fabricated bass traps... uh oh.  Not affordable!
2) Check eBay for used deals... nothing.
3) Check local Craigslist (maybe someone is giving them away cheap!)... also nothing.
4) Call my father-in-law to see if I can crash his shop for the weekend to build these things together for the price of materials... success!

Luckily my partner in crime is a master cabinet builder, so I knew it would be an easy task for us to come up with a design and implement it in a reasonable amount of time.  A few hundred bucks and an entire weekend later, we had made our traps.  This is how we did it:

Step 1: Acquire Insulation Material
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This was only about half of the cellulose that I scored for free.
Part of what shotgunned this whole DIY bass trap effort was a deal I found on Craigslist for a free truckload of cellulose insulation in La Costa.  During the time I was hoping to find completed traps for sale at a discount on CL, I ran across the posting for what ultimately amounted to an entire truck bed full of cellulose.  I'm not exactly sure how much this much cellulose would have cost to purchase new, but I was happy to take it off of the guy, who was in the process of remodeling and re-insulating a room and needed to just get rid of the stuff.  Cool!  (FYI, my backup plan was to order bulk scraps of AFB from ATS acoustics at about $12/20 lb)

Step 2: Draw a Blueprint
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Before determining how much wood we needed, it was necessary to draw a very simple blueprint with overall dimensions and build strategy.  We ended up with a design that had each edge jutting out 13", while the face of the trap is 22" across.  We designed 6 of our traps to be 46" high and 2 of them to be 47" high based on Control Room ceiling height of 93".

Hence, our cuts needed to be as follows (per 1 bass trap):

Diagonal cut for the top/bottom pieces.
  • Side Panel A: 13" x 44.5" (1/2" ply)
  • Side Panel B: 12.5" x 44.5" (1/2" ply)
  • Top/Bottoms (2x): 15" x 15" (1/2" ply) cut diagonally in half.  (After which we cut off 2" at each edge (to equate down to 13") and belt sanded the resulting corner for a rounded front edge that came out slightly over the front edge of the top and bottom, allowing more stuffing to go in the teddy bear!)
  • Top/Bottom Upholstered pieces (2x): same as above, but with 1/4" plywood.  These duplicative pieces were cut in order to make upholstery easy later on.
We also needed to wrap the front of our wooden frames with plastic mesh to hold in the otherwise out-of-control cellulose insulation.  Hence, we cut 2' strips of plastic mesh to about 45.5" long and trimmed them to shape around the rounded front edge during the stuffing process, FYI.  More on this later.

Step 3: Calculate Material Supply Need (and Tools!)

After generating our blueprint, it only took a few more minutes to figure out how many sheets of 1/2" plywood we would need to complete this job -- about 3.  We also needed some 1/4" plywood for the tops and bottoms that we designed, although this is optional.  

Since we already had some scrap on hand, we were able to save a sheet or two, which saved another few bucks.  Don't underestimate the power of scrap wood for projects like this!  Buying everything new starts to add up, so keep your eyes out for people getting rid of wood -- sometimes the pieces are large enough to work.  Once we had our calculations made, it was a simple two-block round trip to the Home Depot to collect our supplies.  Please keep in mind that we already had several tools on hand that were necessary for this job -- here's a complete list of tools you'll need if you are tackling this job yourself:
  • Table Saw
  • Air Stapler
  • Regular Staple Gun
  • Gorilla Glue
  • Belt Sander
  • 3M Super 77 Adhesive (for upholstery)
  • Fabric Scissors
  • Rolling Fabric Cutter
  • Drill and Screws
The rest of the needed materials:
  • 3 sheets of 1/2" plywood
  • 1 sheet of 1/4" plywood
  • Approx. 14 yards of 5' wide fabric
  • About 16-18 cubic feet of insulation material
  • 12 yards of 2' wide plastic mesh (only necessary to hold in cellulose insulation, may not be necessary for other types of insulation)

Step 4: Fab Fabric and More
We ended up using gray for the traps but got this maroon for
additional acoustic panels going up in the Control Room.
In addition to figuring out how much wood we needed, and after making sure we had all the tools we needed, the next step was to choose fabric.  There are a lot of different types of fabric out there, some being stretchier than others, more visually appealing, easier to work with, and so on.  Technically there is no right answer since this is all about personal preference, but I'd highly recommend using bulk sweatpants fabric.  It proved to be very easy to work with, stretched tight around the boundaries of our completed traps, and was relatively cheap from the local Joann's.  It didn't hurt that it was on sale at the time, too.  I kept getting lucky when it came to the budget for this project!

Step 5: Ripping Sheets of Plywood
After cutting a strip down to 13", here we are cutting to 44.5"
Now that we'd formulated our design concept, figured out what materials we needed, and gone out to collect all the goods, it was time to fire up the table saw and start hitting up our ply sheets with cross cuts.  The real key to success here is to be patient and go slow enough that your measurements are correct.  Also, make sure you cut all your wood of a same length/size at the same time, with the guide in the same place, to ensure all-around accuracy later on.  For example, even if you're off by a 1/16 of an inch, at least they'll all be off the same amount and everything will still assemble seamlessly in the end.

Step 6: Cutting Corners -- Making Rounded Front Edges
Tracing around the edge of a paint can to make
a curved edge at the corners.
Putting a rounded corner on the front edge (top and bottom) of these traps yielded a very professional looking and feeling product in the end.  Instead of a plain triangle, the front edge curves around and bows outward, allowing for more insulation to be packed inside while simultaneously providing a lip for the front panel fabric to stretch around evenly.

In order to achieve this, we had to cut a square 15" x 15", then slice off 2" at each end to bring the edges in, and then belt sand around the resulting front corners to create the curvature.  If this doesn't make sense from my explanation, look at the picture to the left with the paint can.  You can see where we cut off the 2" at the tip of the triangle at each corner that met the hypotenuse of the triangle.

Then, we drew the curve by using an old paint can.  You simply line it up at the corner and draw yourself a sanding line by tracing the edge of the paint can.  Finally, you get out the belt sander to finish the job, which goes quick with 1/4" plywood.

Sanding off the curved corners with the disc on the belt sander.

Step 7: Bass Trap Frame Assembly
This is where things always start getting fun, because you start seeing your yet-to-be-finished product taking shape.  To those of you out there who might be following along with this build model, this part is easy.  First let's review the basics of putting the wood together:

  • For all situations where wood is going to contact other wood, first get the soon-to-be-touching edges wet with a brush dipped in water, then add a THIN line of Gorilla Glue to one of the surfaces prior to pressing together.
  • Then hold the surfaces together and use the air stapler to staple the wood together starting at one end and then every 3-6" until reaching the other end.
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Wetting the edges with water before applying glue and staples.  This allows
the Gorilla Glue to expand, foam, and fill the pores of the wood for strength.

With this in mind, this is how you can expect to put together the frames:
  1. In putting the sides together, make sure to put Side B (12.5") is inside of Side A (13.0").  This ensures that the overall measurement from the back corner to either front corner is 13" since we're working with 1/2" plywood.  (In other words, this is your basic warning to make sure that you factor in the width of the wood you're working with when making your calculations, making your cuts, and performing assembly)
  2. Once Side A and Side B are glued and stapled together, you can move onto the Top and Bottom.  Grab one of your curved top/bottom pieces of 1/2" wood (NOT 1/4" -- these will be for later upholstery.  Make sure you're using 1/2" wood for the top and bottom of the frame), line it up to make sure it's flush, then glue and staple it down accordingly.  Repeat for the other end.
Placing the tops and bottoms on the bass trap frame.

Step 8: Applying Plastic Mesh and Stuffing the Frame with Insulation
Completed frames waiting for insulation.
Cellulose is great for soundproofing design concepts, and in fact it's superior to most AFB and mineral wool substitutes on the market.  The trade-off is that it's a pain in the ass to work with because it flies all over the place while you're working, and it's not really the most healthy thing to be breathing in (nor is any type of insulation).  Make sure to get out your face-mask for safety purposes!

In order to keep the cellulose relatively under control inside our bass traps, we bought plastic mesh to staple down to the front edges of our frame.  As mentioned earlier, we cut it to match the length of the trap so it would perfectly go over just the front surface of the frame, holding in the insulation.  By the way, we found it to be easiest to cut this stuff with an exacto knife.

Rolling out the sheets of plastic mesh to proper length.

The method applied was to use a regular staple gun (the air gun chewed through the plastic, defeating the purpose -- so a less hi-powered device was necessary to get a staple around the edge to hold it down without going through the plastic) to staple down the edges of the plastic mesh at the edges of the frame.  And we only stapled down HALF of the plastic mesh to one side (length) of the frame, then stuffed the entire trap with insulation, and only then stapled down the other half of the mesh.  The end result looked like this after stapling, stuffing, and stapling some more for all of our traps:

Stuffed traps waiting for the final step - upholstery.

Step 9: Preparing the 1/4" Tops and Bottoms for Upholstery
The purpose of making another set of top and bottom pieces was to ensure a professional finish in the end after the upholstery was done.  Even though the frame already had a top and bottom, the plan (which worked out really well) was to add a pre-upholstered 1/4" top and bottom to each unit, finished with a few screws to hold it in place (screwed into the frame).

In order to do this, first we had to cut triangular pieces of fabric that could wrap around the edges of our 1/4" top and bottom finishing pieces.  Then we used 3M spray adhesive around the edges and glued down the fabric after pulling it tight to ensure a nice clean, taught fabric look on the visible side.  Note, the glue is enough to hold the fabric down alone, and this side where the fabric is tucked around will eventually be hidden when it is placed on top of the frame.  Another note -- having a rolling blade to cut the fabric here is recommended.

Our stack of pre-upholstered toppers/bottomers to screw flush into the frame.

Step 10: Cutting and Wrapping Fabric
Fabric starting at the back edge is wrapped around the front
and makes its way back to the back again, with the excess
at the top and bottom folded over and stapled down.
Almost there -- we just needed to cut out some fabric and wrap it around the traps so we could get to the next and final step.  Measuring and cutting the fabric to just the right size to wrap around our bass traps proved to be a little more difficult than we'd planned, so here's a quick word to the wise -- cut a sheet that's a little too wide and a little too long at first.  If you're working with 5' wide fabric like I was, you should have approximately 6" of slack spilling over onto the top and bottom at each end.  As for length, plan for 50" -- this accounts for 13" along both sides, about 22.5" around the front, and a little slack for overlap at the back corner.  You can trim the slack in both dimensions down once you get the majority of the fabric wrapped beginning at the back corner all the way around the front and back again.  (Lengthwise you COULD cut exactly to 48", and if you stretch it just right, it should be a perfect fit, assuming you have stretchy fabric.  Otherwise you should aim for 49.5"... either way, try one first to see how it goes before cutting all of your fabric to size in case you want to make adjustments.)

  1. First, along the back corner of the frame, bring the fabric all the way to the edge, and go up and over the edge about an inch (wrapping around to the adjacent side) and use 3M spray adhesive to glue it down.  This will have your back edge down in place.
  2. Then you'll want to liberally apply spray adhesive to both sides and wrap the fabric all the way around the first side, the front, and then the other side until the fabric reaches the back corner again.  This is much easier with two people since one person can keep the fabric pulled taught while the other places it down onto the glue.
  3. When you get to the back edge again, if you've measured right and you made good cuts, you can put some more spray glue down and glue the final edge of the fabric to the original edge where you brought it over the back corner by 1".
  4. Now you'll want to put in some staples every few inches at the corner along the back edge to hold the fabric down and in place, even though a good spray adhesive should be reliable.  If you staple evenly and apply proper pressure, the staples should barely be visible when you're done, giving a clean finished look.
  5. At this point, you'll have excess fabric flapping around the top and bottom, which you can trim with your fabric scissors and either glue or staple down to the top/bottom frame pieces.  See the picture above for a clear visual.

Step 11: Screwing on the Pre-Upholstered Tops/Bottoms
Traps waiting for their tops/bottoms to be screwed down.
This was our final step.  The frames had been constructed, stuffed, and upholstered, and now we just needed to add the pre-upholstered top/bottom pieces to clean the whole thing up and provide a professional finish.  We simply placed these pieces on the top and bottom and drilled 6 screws through the top and into the frame -- one at each corner, and one in between each corner at the edge.  The screws at the corners were enough to provide stability, but we added the screws along the middle of each edge to ensure that nothing could ever get caught in between the edges of these pieces and the frame pieces.

Step 12: Reflection
Finished traps in the corners.  Yes!
Upon completing all this work, we stacked a couple of the traps up in the corners to see how they fit and how they looked.  Perfect!  Our labor had paid off -- our measurements were accurate and they fit vertically in the space more or less perfectly.  Furthermore, it was clear just by putting a handful of these in place that the room's bass was already tightening up.  Sweet!

Having added up all my bills, I realized that I saved over $1k from what I would have paid retail for 8 pre-fabricated bass traps.  Even though it required the work of two men for the better part of a weekend, it was time well spent and it was fun to do, too.  Given the high price of acoustic treatment materials and the pride that comes with DIY work, not to mention exact custom shapes, sizes, colors, etc., yes I would recommend this project to a friend.  Please feel free to let me know if you have any questions!


  1. Awesome My bass Traps will be Built, should be able to build all kinds of absorpers I am also going to do some with holes and mesh in the sides as well. I am thinking I can make them flat as well like a 4x4 wall panel.by putting baffles in them and screen material.

  2. Hi,
    Great post, very helpful and thanks for documenting your process. I'm looling into building my own and I too am trying to do it on the cheap. Can I ask is the cellulose insulation the same stuff that goes into cheap duvets and cushions? Would that work? If I can get it for free then might be worth a try. Cheers.

    1. Hey Ross, you're welcome. Pretty sure it's not the same stuff, no -- the cellulose insulation you're looking for is the type home builders put into attics and walls. Sometimes it really pays to keep an eye on Craigslist, because people doing remodels, companies getting rid of things from construction sites, etc., may be dumping anything from cellulose to nicer roxul insulation for free, or close to it. Good luck!