Kiwishred's 3" Absorber

originally from: AVS Forum

In the spirit of giving back something to this great thread, here are construction details for my absorbers (ideas borrowed from here and Bob Golds).

I chose to go with an open frame construction for reasons of (a) light weight, (b) open sides for greater total area of fiberglass exposed, & (c) ease of custom-tailoring thickness. For room aesthetics reasons, total thickness was limited to 3”. I used a stack of three 1” sheets of 3 pfc rigid fiberglass in each frame. However, the construction method would also easily allow 2” of fiberglass with a 1” gap to wall (or any other desired combination). While the third sheet might not increase acoustic absorption much, it apparently does no harm and only adds a relatively small incremental cost ($3.52).

Top and bottom frames constructed from 3/4”*1/2” hemlock and clamped square. Top frame used “shoe” moulding and mitred corners to provide a rounded front edge. Bottom frame used rectangular molding and butted corner joints.

Nailing and gluing the top and bottom frames together. Pillars are 2 1/8” for 3 1/8” total thickness.

Drilled guide holes for the #17 * 1” wire nails used to tack the mitred top frame pieces onto 3/4" square pillars using a cut-off #18 * 1” nail (an old trick my dad taught me :) ).

Front views of finished raw frame (left) and completed, covered, frame (right).

Speaker cloth stretched and stapled onto frame following instructions here. Cloth is actually a lot darker than it appears in this photograph. Yellow stuff is the "rigid fiberglass" acoustic absorber.

BOM (per absorber):
- 4’ * 2’ * 1” Johns Manville 3 pfc rigid fiberglass sheets. Source: E J Bartells, 700 Powell Ave SW, Renton, WA. 425-228-8807. Cost: 3 sheets @ $3.52 = $10.56.
- 3/4” * 1/2” hemlock 1/4 rounded shoe for front frame. Source: Lowes. Cost: 2 8’ lengths at $3.36 each = $6.72.
- 3/4” * 1/2” hemlock “rectangle” (for back frame and 6 mid posts). Source: Lowes. Cost: 2 8’ lengths at $3.36 each = $6.72.
- 3/4” * 3/4” hemlock square (for pillars). Source Dunlumber. Cost 2’ @ 85 c/foot = $1.70.
- 60” wide black speaker cloth. Source Jo-Ann. Cost: 1 yd @ $6.80/yd = $6.80.
- miscellaneous (nails, glue, sandpaper, etc) $0.50.

Total cost per absorber = $33. This includes some waste as could only buy the molding in 8’ lengths.

Total time per absorber (once in production mode & not counting time for glue to dry !) approx 2 – 3 hrs. So, I probably spent less time making them than reading about how to make them plus the multiple trips to various stores buy materials :eek:.

Observations & tips:

- Top and bottom frames were laid out on a sheet of 3/4” MDF to provide a nice flat surface as the glue set. Didn’t have any significant problems with warping.

- I mitred the top frames and made them out of “shoe” moulding to provide a nice “rounded” appearance. The rounding was hard to actually notice once the black grill cloth was on the frame. Despite this, and the fact that this approach requires twice as many cuts, I still think it was worth the extra effort.

- All cuts were made by hand using a 16 point dovetail saw and mitre box. All lengths were marked using pre-cut wood pieces rather than a tape measure. This really helped speed & consistency.

- Added 1/4” clearance for fiberglass so that total outside dimensions were 49 3/4” * 25 3/4” * 3 1/8”.

- Didn’t bother painting the wood. The bare frames and the (yellow) fiberglass are not visible through the speaker cloth.

- I was pleased with the way the finished absorbers turns out. Not too obvious that they are DIY. The 3/4" * 1/2" hemlock provided a nice balance between rigidity and weight (although something a bit beefier would be required for frames any larger than these).

Hope the above is useful to other would-be DIY’ers and thanks to other contributors to this thread

I guess the edges could also be rounded with a router. Problem is, I don't own one.

This is a works in progress so mounting has not yet been determined . First thought is to just hang them over a couple of nails that are 5/8" proud of the wall. Nails would fit in gap between upper horizontal cross piece and fiberglass. The heads of the nails would provide a lip to help stop them falling off. However, these things are fairly light (or more accurately, have a high surface area to weight ratio) so I am not all that concerned about what would happen if they fell of the wall. Would be more of a "thud" than a "crash". The alterative would be to make little flat plate hangers, or perhaps use a short length of chain (anything that is an inch or so long and has a couple of holes) that could be screwed into both the frame and wall.

I am yet undecided if I am going to put some sort of cloth backing on the these. Once on the wall, the fiberglass cannot fall out. And the fibers seem fairly well bonded......

The rigid fiberglass I used was "unfaced". 703 is for sure available unfaced as well.