QRDude Build

The time has come to build some QRD diffusers ...
The location of these is described on the Home Theatre tips page. It was decided to build three N13+4 diffusers, each being 1200mm high, 500mm wide and 150mm deep, made of 3mm luan ply over a core of styrofoam, weighing in at at around 7kg. Here is the design...



Drawing of advanced QRD diffuser
N13+4 diffuser

Build style

Most builds are done using a backboard, which is the easiest option, but I was keen to see what the challenges would be in building without one. This requires the maximum depth wells to use the same width floor plates as the other wells, as shown in the third option below. For the fins to hold this plate, they need to be 150mm deep. This method relies on the top and bottom endplates to provide the strength for the maximum depth wells.

Drawing of Diffuser backboard options


The diffusers will be cut from sheets 2400mm * 1200mm, and be 1200mm high overall.
Subtracting the thickness of the endplates gives a length of 1194mm for both the fins and the floor plates.
Since the plywood supplier has a much better saw than mine, I'm getting them to cut the strips.
An extra fin will be ordered for each diffuser to be cut in half for the top and bottom plates.
Two extra floor plates will be ordered for each diffuser to be cut up to make stiffening braces if needed.
The guys at Mr Plywood rewarded me for sticking to only two sizes, doing a very good deal on the cutting charges.


Parts list

4 sheets of 3mm luan ply 1200 * 2400
- trim sheets down from 1200 to 1194
- cut 45 strips 150mm wide
- cut 45 strips 35mm wide
- any leftover wider than 700, cut lengthwise down the middle (to fit in my little grey volkswagen...)
- I should have checked out which way the grain ran - cutting the sheets in half first and turning 90 degrees before cutting the strips would have looked better.


Adding styrofoam to the mix

After talking to Terrry Jones, it seems that the ply may need more support, so there will be some polystyrene foam used under the floor plates to keep everything square. In fact, building without a backboard would be a nightmare if not for the foam!
It turned out to be easiest to pre-build smaller sets of wells first, and then join these together later on.


The build...

Here is the anticipated sequence...


The 150mm and 35mm strips of 3mm ply
Photo of the 150mm and 35mm strips of 3mm ply


The back face of the ply is not consistent in color and quality like the front face. Luckilly, there aren't too many sheets with the darker timber or stained areas. These faces will be hidden by using them either side of the shallowest wells.

Variations in color on the rear face of the ply
Photo of variations in color of luan plywood

Ply was not exactly 3mm thick - closer to 3.6mm - Will not matter for well depth as this can be compensated for when cutting the styrofoam packing. The overall with of the diffusers will be slightly larger than the planned 500mm, but in the intended location, this will be OK.


Working with the styrofoam

Polystyrene sheets in the exact thickness to suit the well width were not available, but combining two thicknesses gives the correct size.
These were found at Insulation Industries for not a lot of money...

Polystyrene (styrofoam) sheets in 25mm and 10mm thicknesses
Photo of Polystyrene sheets in 25mm and 10mm thicknesses


Glue selection for joining the two sheets together is tricky. Normal PVA glue needs access to timber grain and / or air to cure. Contact cement can contain solvents that dissolve the polystyrene. A Google search recommends Aquadhere Durabond (Gorilla glue), which doesn't contain solvents, and cures using moisture. The glue was spread onto one sheet with a plaster scraper, whilst the other sheet was wiped over with a damp cloth to activate the glue. The two sheets are clamped overnight...

Gluing polystyrene sheets together
Photo of polystyrene sheets glued together


Once the glue proved to be OK, the rest of the sheets were done the next day. Took nearly the whole 460ml bottle of glue. (Coverage 3.6m^2)

All five pairs of the polystyrene sheets
Photo of all the sheets clamped for the glue to dry


Cutting the 35mm thick combined polystyrene needs a long sharp blade, so a snap-off knife was purchased. To get an even coverage with stain, an all-in-one stain+poly product was also bought...

Photo of glue, tools and finishes for the diffuser


The new knife tended to tear the styrofoam a bit, and was hard work, so a fine saw blade was liberated from a mitre saw. This gave a smoother cut, and could shave quite a fine amount, as was needed to trim the 1200mm sheets by 6mm.

Cutting the styrofoam
Photo of cutting styrofoam using a hand saw


Unfortunately this approach made it impossible to cut at 90 degrees, as can be seen by the different directions of leaning in this sneak peek at the relative heights of the wells

hmm... a bit of a lean on some of 'em
Photo of styrofoam cores to support the plywood used on the sound diffusers


The saw table was wheeled out, and the edge that will support the well bottoms was trimmed square. For each set of same sized pieces, the saw was set to skim the smallest piece. The remaining pieces were then fed through, giving the same height. Some shims will be needed under the styro during assembly, to compensate for the loss in block height.

Getting a square edge with the table saw
Trimming the foam to get a square edge using a table saw


Here are all the pieces ready for gluing to the ply

All the styrofoam
Photo of all the styrofoam pieces ready for the project


Building smaller sections first

Whilst the original plan was to first glue all the floor plates to the foam, it was going to be a pain to keep the styro in line, as it had inherited a slight bend from the earlier gluing being done on a surface that was bowed. D'oh!

The solution was to build a jig that would hold six equal-sized wells at once, keeping them square and aligned. The fins and floor plates for each well would be glued onto the foam block for that well. The wells would not be glued to each other, and after drying, would be separated for later assembly into their correct positions in the three diffusers. The jig holds six blocks of foam and twelve fins.

Here is the test-fit of all the zero-depth wells, for the three diffusers. Angle brackets hold them upright, whilst an end-stop keeps all the components in line. Sitting on top is the set square that was used to position the end-stop when it was clamped.

Photo of a jig to hold pieces square while they dry
Setting up the jig


Deciding how to break up the build

The colors in the next drawing show four different sets, each of six wells. (a pair from each of the three diffusers)
The blue set will require temporary packing whilst in the jig, using an additional nine fins, so it needs to be done before all the fins are used up. Its floor plates will be added at final assembly time.

Drawing showing order of construction
Four sets of equal block heights


Because there is no backboard, and no rebated endplates to hold the fins in place, it will be much easier to assemble the diffuser from these pre-built blocks than to try and glue everything from scratch in one session.

Before starting, it is worth grading all the fins so that defects on the back side of the ply can be hidden by facing that side into the foam. Put the worst fins against the shallowest wells (ie the one with the most foam). The best floor plates also go with the shallowest wells. Here is one of the sets ready for gluing, (the 55mm ones in the drawing above)

Photo of all the foam and luan plywood for one gluing session
The pieces for one set


With the deeper wells, it's not possible to get the glue nozzle all the way down to where the floor plate will sit, so the right order of assembly had to be thought out...
Firstly, some sandwich plastic is laid down on the work surface to stop the glue from sticking to it.
Next a zig-zag bead of glue is put onto one side then spread with a brush to give full coverage...

Photo of spreading PVA glue onto strofoam
Full coverage with the glue


The fin is added, then the glue applied for the floor plate. An extra bead of glue is laid along the seam where the floor plate will touch the fin...

Photo of plywood floor plate about to be glued onto one well of the diffuser
Floor plate about to go on


With the floor plate on, the glue can be applied for the second fin. Again an extra bead of glue is run next to the exposed edge of the floor plate, and then brushed onto the edge. The second fin can then be added, and the assembly dropped into the jig for alignment.

Photo of glue applied for the second side of the QRD well
Glue applied for second fin


When the entire set is glued up, clamps are used to hold everything parallel. (not so tight as to crush the foam). Leave for 24 hrs.

Photo of one full set of identical diffuser wells clamped for drying
One of the sets drying


Once the four sets are finished, the same jig can be used to glue the LHS end wells for the three diffusers onto three of the wells from the 55mm set. This step means that all fins are held upright by at least one block before the final assembly begins. The final three fin-sized pieces, that are yet to be cut in half for the endplates, will first do duty as packers for this step.

Photo of the remaining fins being glued into place
Supporting the lone end fins


Gluing the sections together

The sets arranged for gluing to make one of the diffusers

Photo of all the subunits for one diffuser arranged in order
Now we're getting there


Sets glued together. The smaller clamps in the foreground were used to pull a warped floorplate into line.

Photo of one full diffuser clamped for glue drying
Those new clamps were definitely needed


Some spare floor plates were broken up and used as packers during clamping.

Photo of packers used to prevent warping of the fins as the glue dries
Packers in place for gluing and clamping


The packers hadn't been thought of when building the sets, or assembling the first diffuser. Here is the first one sitting on top of the second one. You can see that the fins on the right end aren't quite at the proper angle. Not a huge problem here, but definitely something to watch out for if you opt for this build method.

Photo showing problem if packers are not used
First diffuser wasn't packed


Finally, a look at how they turn out...(just the endplates to go)

Photo of diffuser without the endplates
First look


Carefully stood up...and then turned around. The plastic will come off now to give the residual glue a chance to dry.

Photo of front view of diffuser     Photo of rear view of diffuser showing plastic sheet used to stop glue sticking to the workbench
Another look


Tie-down straps used for gluing all the endplates
A plastic drinking straw taped to the glue nozzle allows the deep wells to be sealed.

Photo showing endplates held on with tie-down strap while glue dries


Photo of first look at QRD diffusers in position on the wall
Where they will go...


Photo of three QRD diffusers placed side by side to form an array
How three periods look if they are mounted together...


Photo of diffusers being stained


Pho of finished diffuser
Stained and finished with low-sheen poly



The original intention was to use brackets mounted to the wall using expanding plug screws. The diffusers were to push over the brackets, deforming the styrofoam, giving a firm flush mount.

Photo of recess in back of panel for mounting bracket
Foam was carved out from the back of the zero-depth wells to accept trimmed-down brackets


Unfortunately, the lath-plaster walls of the house weren't strong enough to take mounting plugs for the brackets, so a new approach was needed. Flat steel plates were mounted across the IB manifolds, behind the grilles. Wooden blocks glued into the back of the zero-depth wells allow the diffusers to be mounted using screws through the plates.

Diagram of mounting hardware
Mounting plates


Photo of wooden mounting blocks
Mounting block about to be glued in flush with back of diffuser panel


Photo of mounting blucks glued into the back of the panels
Blocks in place


Photo of steel mounting straps
Steel straps


Photo of QRD diffusers mounted next to subwoofer manifolds
Finally mounted


Cost in Aussie dollars (2010)

Plywood, cut into strips, including $22 cutting charge for 90 strips110
Polystyrene sheets 1200mm*600mm - 5 @ 10mm, 5 @ 25mm36
Glue to join styro sheets and to bond mounting blocks - Aquadhere Durabond (US equivalent is Gorilla glue), two 460ml bottles48
Glue for ply - Aquadhere Exterior (US equivalent is Titebond II), nine 500ml bottles135
Polyurethane with Stain - Cabots all in one New Teak gloss, two 1litre tins88
Polyurethane - Cabots low sheen, one 1litre tin26
Plastic plaster scraper (to spread glue on styro)3
Flat steel plate for mounting 50mm * 3mm * 6m25
Total for 3 diffusers$481

Extra glue had to be bought several times during the build - Buying the 4 litre size for $56 at the start would have saved $64


Lessons learned


Additional QRDude pages:


/qrdude-sample-build.htm last modified: 31 August 2019
Privacy         Contact