Inexpensive but Effective Broadband Absorbers


Overview

Broadband absorbers (sometimes called Bass Traps, though strictly they're not...) available commercially are quite expensive, but much cheaper versions can be built that perform just as well acoustically.  This paper shows how to build them out of common materials for a total before-hanging cost of about $32.00 apiece.  The hangers we chose were fairly expensive, adding about $9.00 to the price of each absorber.  If you're hanging from the ceiling, or choose a cheaper method, you'll be even more money ahead.  Included is a breakdown of all costs involved, and instructions on how to make them and where to get the materials.  Figure about an hour for construction and installation of each one.

Each absorber is 4 inches thick, and stands off from the wall another 4 inches, giving a total effective depth of 8 inches.  For more information on why this airspace adds to the effective thickness, please see Ethan Winer's wonderful acoustics FAQ on his website.  This article will not go into great detail about the theory behind broadband absorption, so if you aren't familiar with the theory, please read up on Ethan's products, particularly the MiniTrap, which this project vaguely emulates.  They're extremely good, and very well built.  Certainly more impervious to destruction than our panels, but ours won't be shipped around the country. Ethan's also have a more versatile mounting facility. We actually have eleven of his traps, and they're wonderful. But the approach outlined here is much cheaper, since you're putting in all the labor yourself.

Also, it is advisable to set up an assembly line approach if you're building more than a couple.  Cut all the wood first, then build all the frames, then stuff them all, and then hang them all.  That will go faster than doing them one at a time.

Materials

Basically, you need rockwool insulation, a frame of some sort, a covering, and a method of hanging.

Rockwool
We used 2-inch thick, 8 lb/cu.ft rockwool, stacked 2 per absorber.  It would have been possible to use 4-inch thick rockwool, but our supplier (Southwestern Insulation and Sheet Metal in Aurora, Colorado, 303-371-2000) didn't normally stock it, so there would have been a shipping charge associated with ordering it.  There is absolutely no difference between two pieces two inches thick, and one piece four inches thick. Don't bother calling Home Depot or Lowe's for rockwool.  They won't have any way of getting it for you. 

It's also possible to use Owens-Corning 703 instead of rockwool, but it's more expensive, and has not been shown to be meaningfully more effective.  703 is more rigid though, and may or may not require a frame.  We didn't feel comfortable hanging unframed pieces of insulation where people could bend them.  The rockwool comes either faced or unfaced for the same price.  We chose unfaced, but in retrospect perhaps faced would have been better, as we have so far not covered the rear of the panels. 

We advise you to cover the rear of your panels in some way.  The faced rockwool will also absorb a little lower in frequency than unfaced.  Our cost was 65 cents per square foot, and since we needed two layers, that made our effective cost $1.30 per square foot.  This price included delivery to our door. 

By the way - delivery people typically don't get paid much, and you can imagine that they work their rear ends off hauling and lifting all day long.  So do yourself a favor, and give them an extra ten or twenty bucks to put the giant bags in your garage or inside your door for you.  You'll make their day, because contractors never tip them.  (The tip was not included in our cost sheet.)

Frame
We used moulding from Home Depot (stock number 746-426) for our frame because we didn't want the extra weight associated with 1X4 wood.  You can save even more money if you go with the solid 1X4, but get ready for extra hassle, because they'll be heavy.  We also decided against framing out of angled steel, as it would have been about three times as expensive.  The steel's advantage would be that it would leave the sides open for more surface area, but the cost was simply too great, so we just made a few extra panels rather than worry about the extra surface area.  The moulding was $1.24 per linear foot, and each panel requires twelve feet.  There are two 4-foot pieces and two 2-foot pieces for each frame.

To attach the frame sides together, sticks of 1X1 square dowel (Home Depot stock number 477-709) can be cut into 4-inch pieces.  Cut the moulding into four and two-foot sections, and make a rectangle by screwing them together with four screws per corner. Figure 1 shows a corner attaching detail, and Figure 2 shows a number of the frames that have just been built.
Figure 1 Figure 2
Figure 1 Figure 2


We experimented with angling two of the frame pieces to help hold the rockwool in one place, but the advantage wasn't worth it.  The rockwool stays in one place just fine anyway, thank you, and angling the frame meant manually cutting the entire length of rockwool away, a messy and imprecise process.

Covering
You can pick any fabric that's fairly breathable (meaning you can breathe through it and not be too restricted).  We picked craft felt from JoAnn Fabric stores.  We managed to get a bunch on sale, so our prices ranged from $1.49/yd to $5.00/yd.  There are two sizes - the 36-inch wide bolt is cheaper, but each absorber will use 1 - ½ yards.  The wider 72-inch bolt is more expensive, but will only require 1 yard per absorber, so the prices end up being almost identical. 

The moulding we picked has a nice wide edge on what will be the back side, so glue the felt to that wide edge with a hot glue gun.  Then pull the felt pretty tight to the opposite edge, and glue that.  Not too tight, or you'll tear the felt.  Now do the same with the two remaining edges.  Fold the corners around and glue them wherever they reveal any space.  You can see that this was done in Figure 3. 
Figure 3
Figure 3

Trimming your fabric edges down to the moulding will make the folding easier.  Note that a glue gun was used - we would advise against using spray glue, as the spray will have difficulty permeating the fibers of the wool.  You'll end up with a tiny amount of glue on the fabric face, and the felt will pull off.  A fabric store's glue stick price will be about a quarter of a home improvement store.

Stuffing
Wear gloves anytime you're near the rockwool, and most would recommend breathing protection as well.  It feels soft and fluffy while you're working with it, but without gloves you'll regret it later.  A lot.

Cut 1-inch square sections out of each corner of every piece of rockwool.  This will be much easier if you do it while a bunch of them are stacked, as you can just slice down the height of the stack with a pocket razor knife - the kind that you can break off a little when it gets dull.  You're cutting the corners out so the rockwool will fit in the frame inside those 1X1 corner pieces.  Otherwise, get ready for a big mess. 

It's not required to put both pieces of rockwool into the frame at the same time, but there's no reason not to, and the whole rockwool panel should slide with a little force into the frame you just built.  If you got faced rockwool, expose the face at the back of the panel.  This will help with fiber retention, and you don't want the face at the front, or you'll get high-frequency reflection.

That's the end of the construction process for what we did, but if you have fibers exposed, you'll want to glue another piece of fabric to the back.

Mounting
Obviously, you can mount any way that's convenient for your application.  Our preferred method was with shelf supports (Home Depot sku 094416502500 and 094416502661), which allowed us to hang the panels away from the wall exactly four inches.  Another advantage to these supports is that the panels can be removed in about five seconds to change the room response. Figure 4 shows the hanger detail, and Figure 5 shows a portion of the room with several absorbers installed.

Mount with toggle bolts.  Perhaps you've heard of those little tubes that drill right into the wall, where the arm folds back when you screw into it.  If you have yet to try them, have an extra computer monitor on hand, because you will surely throw the tubes in frustration, and they'll go right through your CRT display.  Trust me.  Use the toggle bolts instead..
Figure 4 Figure 5
Figure 4 Figure 5


Although the absorbers do hang four inches away, using this method requires that you screw two hooks into the top corners (and into the 1X1 corner supports).  Try to screw them in a little in back of the center line, so the absorber tends to hang a little backwards into the wall.  That way, you can set some 4-inch blocks behind them to push them back into position, and they won't have to swing when someone touches them.  We used those green foam blocks florists use.  They're not included in the cost because you don't have to use them, and I don't recall the Ebay price anyway.

Cost Sheet
Item Cost Cost per Absorber
Rockwool (8 lb/cu. Ft., 2 inch thick) .65/sq ft $10.40
Framing (746-426) $1.24/ft $ 14.88
1X1 corner pieces (477-709) $2.69/piece $ 1.25
Felt $3.00/yd $ 4.50
Glue $3.00/50 $   .18
Hooks (727-348) $7.37/50 $   .29
Toggle Bolts (261-262) $3.47/15 $   .46
Hanger Bracket (094416502500) $2.48 $5.96
Hanger Stand (094416502661) $1.60 $3.20
Total Cost per Absorber $41.12