how to design construct broadband absorbers

7/29/2019 How to Design Construct Broadband Absorbers

1/14

How to design and construct low-cost Broadband absorbers / Bass traps

WARNING: Do not attempt this project unless you are comfortable with power tools. I do not want anybudding musicians sawing off useful appendages or stapling themselves to anything gnarly (unless, of

course, that is part of their shtick) ;-)

Time: Estimated time to assemble and install will depend on your familiarity with tools and constructiontechniques, and how finicky you are. As a general frame of reference, I am competent with tools andquite persnickety, and I built and installed 2 of 16 x 48 x 4 corner traps in approximately 6.5 hours(not including time to buy materials, or wood finishing time). Add at least 30 minutes for having to readthese verbose instructions.

Costs: Cost will depend on several factors, including choice of lumber and choice of fabric. If youdesign intelligently and spend wisely, you can build excellent traps for an average price of less than $40each. For example, I built 2 large wall traps, 3 standard corner traps and a 3 x 5 cloud (including allmounting and hanging hardware) for well under $250:

lumber $140 insulation $34 cloth $23 hanging anchors, chain, screw eyes, glue, etc. $30

Recommended tools and hardware: Table saw (or pre-ripped lumber; most hardware stores will do this for you for a small charge) Miter saw / chop saw Brad nailer or stapler (air or electric) Wood glue or multi-purpose glue (I use WeldBond or TiteBond) Tape measure & pencil

Sanding block and sandpaper Square or T-square Ceiling anchors and eye hooks and chain or wire (if you are hanging a cloud)

Instructions & Commentary as follows:


2/14

1. Choose the size & style of the broadband absorbers (traps) you want to build.Comments: Select your sizes based on the materials you are going to use. I decided on 48 long traps(actually 47 & 7/8 finished interior height after cutting) because you can easily buy 4 and 8 boards,and that is the general length of a bat of sound insulation. For the same reasons and because a thickerpanel will better absorb lower frequencies I chose 2 depths: 4 and 6. And also for the same reasons,

I chose 2 widths: 16 16.5 and 24 - 25.5. The cloud I made roughly 3 by 5 as those dimensions cuteasily from a 8 boards. For aesthetic reasons I made my cloud with octagonal corners, but that wasstrictly personal preference and to match my room. Note that:

a. Most fabric is available in 54 width (length sold by yards or meters). If you cut carefully, youcan cut fabric for 3 of ~16 traps or 2 of ~24 traps from a single 54 width.

b. A ~16 trap is 1 insulation bat wide, and a ~24 trap is 1.5 bats wide, so cutting and waste areminimized

c. Summary recommended sizes [overall outside dimensions]~16.5 x ~49.5 x ~4 (standard trap)~25 x ~49.5 x ~6 (large trap)~36 x ~60 x ~4 (cloud)

Note: I chose to not put fabric on the back on my traps as that was entirely unnecessary. It was notrequired to hold the insulation in place (held by friction fit), all fabrics are potentially reflective (whichis counter the whole point of this absorption exercise), you do not see the backs, it saved money andtime, the Roxul insulation was not noxious, and so on. But suit yourself if you feel the need to cover theback sides.

2. Choose and purchase your materialsComments: I selected Roxul SafenSound insulation (rock wool) as it was reasonably priced, sonicallysuperior to regular thermal insulation (wool or fiberglass), incredibly easy to work with, and readilyaccessible at my local Home Depot store. 12 of 3 bats per bag, so 1 bag will make 2 large traps, 3medium traps, and a 3 x 5 cloud with no waste and no overage.

For information on acoustic properties of various materials, you can check out:http://www.bobgolds.com/AbsorptionCoefficients.htm

Comments: I selected laminated knotty pine panels (Crafters Panels), because there are light, easy towork with, inexpensive, look good unfinished or finished, and are readily accessible at my local HomeDepot and Windsor Plywood stores. As well, the Crafters Panels come pre-cut in various widths (8,12, 16, 24) and lengths (3, 4, 6, 8) so design and construction are a breeze.

Comments: I selected an acoustically transparent tweed upholstery fabric from a local fabric store(Fabricland), on clearance for $4 per meter (compared to some speaker cloths I have purchased for $30 -$50 per meter). As per Gavin Haverstick (www.haverstickdesigns.com), you can easily check fortransparency by blowing through the fabric and by listening to a voice spoken through it. The lessinterference, the better for these purposes. If you feel breath restriction or hear significant muffling, youmay well have reflection getting in the way of absorption.


3/14

3. Assemble the frame outline of the trapComments: I chose to put the 4 lengths inside the top and bottom to best accommodate the insulationbats. Glue and nail the corners. To add stability, I put corner blocks in the back half of each corner (toleave room in the front half for the grille cloth frame). Confirm square, and set aside to dry.

Comments: First, this is the perfect time to give the frame a quick sanding with any medium or fine gritsandpaper. I also suggest a once over in the work area with your Shop Vac to prevent your cloth fromgetting contaminated with sawdust and debris.

Note: If you are going to finish the wood with stains or lacquers, this is the opportune time. As I dontyet know the colour palette of my new home, I left my wood unfinished You can certainly finish thetraps later, but it means more masking and more careful work (and time) involved.


4/14

4. Assemble the mounting blocks for the trapComments: I chose a hidden interior mounting style, so that the mounting block fits inside the short sideof the trap. If the trap is to be mounted parallel to a flat wall, the support block can be a simple L-shapewith supports. If the trap is mounted in a corner (i.e. wall to wall. or wall to ceiling) you need to make 2of 45 cuts from a square end to make a pointed board. Use a square to decide on the depth (eg. For a

16 corner trap, you need ~8.5 from corner to back of the trap to allow for clearance, so I made theblock 11.5 from point to end 9.5 for corner clearance and 2 to recess into the trap). Nail and gluesome rectangular pieces onto the pointed end, and use a 45 cutting to add support to the structure.


5/14

5. Install the grille cloth frame (and supports if necessary)Comments: Cut nailing strips to fit the sides, then the ends to go inside the sides (for a medium trap, 2each of ~48 and ~13). Lay the cut fabric good side down inside the facedown frame. Try to stretch abit end-to-end as you nail one side of the grille frame, and keep and eye on the fabric pattern (if any) tokeep it consistent. Stretch and nail the opposite side, then the ends. Try to keep constant pressure of the

strip and the trap against the floor to ensure that the grille cloth is flush with the front of the trap.


6/14

6. (For clouds only) Install cross bracesComments: As the cloud hang from the ceiling, the weight of the insulation will cause the fabric to sagor droop. To avoid that, install a few nailing strips as cross braces to support the insulation.


7/14

7. Install mounting stops on the ends of the trapComments: Use the mounting block and tape measure or square to centre the mounting block in the trapend, and mark it. Nail and glue some strips to act as stops for the block. I set the stops at the midpoint ofeach trap (3 deep for 6 traps, 2 deep for 4 traps).


8/14

8. Install the insulationComments: Assuming you have chosen rational sizes, this is a quick and easy step. Cutting Roxulinsulation is easy with any long-blade serrated saw or knife (a bread knife works great, but please washit before putting it back in the drawer). For a 4 trap you can either leave a 1 air space or cut 1 layersto fill the framed grille area. In either case, fill the remainder with a 3 bat. For 6 traps fill with 2 bats

deep.


9/14

9. (optional) Pre-mount the mounting blocks into the trapsComments: If your install location is difficult (eg. a high wall to ceiling location) you may want to pre-mount your blocks, then remove them. Holding the trap in place while you try to pre-drill pilot holes andthen change driver bits can be awkward.


10/14

10. Install the mounting blocks in the desire locationComments: Based on the venerable maxim measure twice, cut once, make accurate measurements ofwhere you want the trap to go. Give yourself a bit of leeway to allow for inaccuracies (eg. drywallcorners and never square because of taping and compound buildup). Install the brackets the desired

distance apart less ~1/8 - 1/16. It is much easier to install on brackets a tiny bit too narrow than ones atiny bit too wide. Most 90 corners have 2x4 backing in both directions. Use longer screws (eg. 2.5+) tomake sure you reach the backing studs. IF you are installing on a flat wall, search for studs or use highquality drywall anchors.


11/14

11. Install the trapsComments: It is best to have help for this step, to hold up the end that you are not screwing into place. Ifyou did not pre-mount the mounting block in Step #9, consider starting the screws into the trap endsbefore climbing the ladder. Make sure you put the screws in a location that will solidly bite into themounting block.


12/14

12. (For clouds only) Install ceiling hooksComments: unless you are screwing directly into joists, you will want to use special ceiling anchors. Ichose VersaHook by Cobra ($7 per pair), as they will support 18 pounds each in ceiling drywall only.Regular drywall anchors may pull out under the load, so if you value your gear and your skull, spend thebit extra on special anchors. Nuff said.

13. (For clouds only) Install cloudComments: Cut equal lengths of chain or wire (I used 6 lengths) and install on the ceiling hooks. Installhook eyes in the edges of the cloud and hang them from the chains. Note that with the airspace abovethe cloud and thanks to gravity, you can always add additional layers or bats of insulation on top of thecloud if additional absorption is required.


13/14

14. AdmireComments: Just in case any of you suffer from Triskaidekaphobia, I have added this gratuitous step #14.


14/14

And if you find this article to be useful, I recommend that you check out Understanding your Room aseminar by Gavin Haverstick of Haverstick designs (www.haverstickdesigns.com) presented by JoeGilder of Home Studio Corner (www.HomeStudioCorner.com).

Now go and do what you were supposed to have been doing all along; make music!

Copyright 2010 AuraTone Productions & Gary W. CableYou may use or adapt these instructions without permission,but please seek permission before reprinting or publishing

how to design construct broadband absorbers

Documents