The Beginning...

On July 10th 2007 I began making a personal addition to my home theatre's sound system - 80 days later I completed it: the world's first Rubik's Cube subwoofer...

In case you are unfamiliar with subwoofer construction, Brian Steele runs a great site - the DIY Subwoofer Page. On it, you'll find advice, calculations, templates, and examples of those who have dared before you. Since there are literally hundreds of sites, software programs, and books on the subject matter I’ll summarize my final specs below:

Sealed Enclosure: 3 Cu. Feet
Driver Diameter: 15", Aluminum Cone
Outer Enclosure: 3/4" MDF Shell with 3/4" MDF 'tiles' overlaying it, forming the Rubik's Squares
Reinforcement: 1 Y/Z-Axis piece of 3/4" MDF, cut to allow driver. 1 X/Y-Axis piece of 3/4" MDF, cut to allow driver and Plate Amp. 1 Z-Axis piece of 3/4" MDF, cut to allow Driver. 1 full piece of 1/2" MDF at top. 1 Cut piece of 3/4" MDF on bottom.
Total Box measurements: 20.75" Wide, 20.75" Tall, 20.75" Long
Tile size: Face: 7.5" X 7.5", Cube: 7.5" X 7.5" X 7.5"
Total Measurements: 22.5" Wide, 25.5" Tall (w/ 3" feet), 22.5" Long
Total Weight w/o Speaker, wiring, Poly-fil, Amplifier: ~98 lbs.
Total Weight: ~144 lbs.

It's a beast, I must say, but before we begin how about some background information? My current system utilizes 2 Paradigm v3.0 Studio 100s (fronts), 1 CC-590 center, and 2 Studio 20s for 5.1 surrounds. Driving all that is an NAD T773, biamped with 2 NAD C272s for the fronts and discrete 7.1 inputs biamping the 20s - Long live the Canadian sound! Likewise, when I finally moved into my new apartment with space for a television (that's a luxury in Boston) I wanted the extra kick for DVDs - what HT is complete without a sub? I'll admit I’m a bit of a purest and can’t stomach the thought of attaching an external sub to my stereo so my search began for something that could rock the 60hz range in movies but neither force me to rewire every time I want to play a CD...

Being a huge fan of my Paradigms, I naturally went to the accompanying model for my Studio 100s - the Direct Servo 15. Despite making a great sub, I also played around with the option I eventually went with - making one. Reasons for this were two-fold - I hadn't taken on any art pieces in a while and just having dropped first, last, security, etc for the new place I didn't want to make a large purchase (insert your Alanis Joke here). Long story short, I chose to go with a Rythmik Audio Servo-15. One visit to Rythmik Audio's website and you can see that these guys aren’t messing around.

If you're completely new to building (Hi Mom!), here's what you need to know in order to make sense of the pics below: A sealed Sub system (this one) is comprised of air-tight box, a large (usually 8" or more) driver and an amplifier to power it. The driver vibrates the air inside the box which causes low-end sound waves to form. Since the box is being shaken so violently it is extremely important that box be very well braced - as in you should be able to stand on it and jump - it should also be heavy enough that it doesn't move. The interior size of the box is a carefully calculated volume of air that must be present along with the interior support.

So, after 2 weeks of drawing up plans, I finalized my measurements and got started...
The ceremonial first picture. I can't emphasize this enough: work everything out on paper first - the yellow pad there is full of calculations that I referenced every step of the way. Also I’ll take this time to mention I built this in my old (rented) apartment, the space you see here is my landlord’s basement, haha.

My design calls for the outer 3/4" MDF to overlap in certain places and underlap in others. I'm using all butt-joints and reinforcing the bottom with a second layer of 3/4" MDF and the top with an additional 1/2" sheet of MDF. The pieces have been marked for easy reference. Notice on the first piece you'll see '.5' corresponding to my 1/2" MDF and next to that 19.25" X 19.25". This corresponds to the outer 3/4" MDF in which the .5" sits - my original calculations called for a total size of (19.25x19.25+.75+.75) 20.75" X/Y/Z for .25" thickness overhang (from the Rubik’s tiles) surrounding the shell. This design was changed in tiling stage and I'll explain why below!

The interior braces of the box, I’m reinforcing on all 3 axis, the cuts and holes you see drawn are for amp/driver displacement, but more importantly air displacement.

The Glue-Up

The first Glue-Up!! Representing the sides and top, accordingly. Note: I attached the clamps to hold for a picture, then attached 4 more, you can never have too many clamps!

Nor can you ever have too much glue...

Glue seeping out the edges is a good indicator that you've used enough in speaker building - your box needs to be air tight.

Cutting the Shell

Cutting the Interior Braces

Two of the braces Cut

Legs v1.0. These were the first round of woofer-legs I bought, purchased them from Home Depot and trimmed them for about 3 inches of clearance between the cone and floor. We'll come back to these...

The final trimmed legs

All of the MDF cut to size - Note: the corner bracing is marked where the center brace overlaps, this amount of overlapping must be taken into account when calculating your box volume

According to the Q equations from Rythmik, I need exactly 3 cu. ft of air surrounding the driver. Here's the math:
Total Interior Box Volume:6762.766"^3
Total Volume of Driver: 871.873"^3
Total Volume of Amp: 406.0625"^3
Total Disposable Air: 5484.8305"^3
V of Tall Brace: 263.484"^3
V of Side Brace: 277.922"^3
V of Tall Brace: 263.484"^3
V of overlap on Side Braces: 2.8125"^3
V of overlap on Tall Braces: 2.25"^3
(-) Necessary 3 Cu. Ft of air: 5184"^3
= -498.9965"^3

So, 498.9965"^3 is the net volume of air that must be displaced, save for the 3 calculations below (the base MDF to be removed from each Brace):
V from Tall Brace 1: 102.75"^3
V of Tall Brace 2: 102.75"^3
V of Side Brace: 190.8517"^3

This brings the total volume of MDF that must be removed (after the initial removal) from the braces to: 102.6448"^3

A close up of a corner brace

The interior braces and their air displacement - the goal is to keep them sturdy while still removing wood for air, while keeping the box the target size.

The cut corner braces

The main vertical brace assembled

The main vertical brace with amp cutout

Constructing the Box

Trim away all excess glue when assembling

Now, that's a box...

The Amp Side

As you can see here, I've attached drywall screws for extra reinforcement. Two things of note: all the screws have been countersunk with the outer edges and I am using these as secondary reinforcement only! Screws cannot be swapped for wood glue.

Reinforcing the Box

The supports are inserted and glued/screwed to the shell

All Y/Z/X supports in place and clamped

At this point my back is starting to hurt...

The reverse bird's eye view. The downward firing driver sits centered over the supports with 1" clearance on all sides - 3" clearance between the driver and amplifier.

The sanding begins - no matter how exact your cuts are the sides will never line up completely - the excess wood/glue will be sanded off so the tiles will sit flush against the shell

The 4th side to be put on sees all the other side's minute miscalculations - in this case the overhang was high enough to saw it off with my Nokogiri (Japanese-style saw).

1.5 hours later...

The final two bottoms are put on - the support fits into the box and the outer side fits over the assembly. It's screwed on for extra reinforcement.

Notice the recess for the driver - the outer diameter of the Rythmik DS-15 is 15.5", the same diameter as the circular cut on the outer shell. The inner cut is 14" in diameter allowing a .75" wide surface for attaching the driver.

One completed box.


Despite the tile overlay, I want this box completely sealed - all joints, seams, and screw holes are plastered to fill any possible glue holes. I'm using plaster here instead of wood filler because I want it to dry in the joints.

I'm a big fan of the pink-turn-white DAP

One completely airtight box. Next, sanding it all down...

Cutting the Cube...

Cutting the Tiles

I calculated that in order for the Rubik's cube to maintain scale it needed a 1/2" round-over, however I couldn't accurately cut this on 1/2" MDF. This was when my design partially changed - all tiles needed to be 3/4" MDF in order to accomidate the round-over.

Because the .75" MDF adds an additional .5" to the outer dimensions, the shell needs to be increased in size - otherwise when put end to end the tiles will hang off one side by .5" MDF. Here's the calculation:

20.75" starting surface + 2(.75") MDF tile thickness = 22.25"
22.25" / 3 Rubik's squares = 7.41667" wide
Since I can't accuarately cut that the larger easier 7.5" dimension is used:
7.5" x 3 = 22.5" - 22.25" = .25" = 1/8" increase on all sides.

Where as adding sheets to the Y/Z/X axis would be the easiest solution, I unfortunately can't obtain 1/8" board!! The other flaw with this method is that it leaves no gap between the squares on each surface - inaccurate to scale.

Instead of adding 1/4" to all sides, I instead added 1/4" to each side. This increases the dimensions of the box to 21.25" all around.
7.5" x 3 = 22.5" for the outer dimensions of the bare tiles and (20.75" + 2(.75) + 2(.25)) for the total dimensions leaves .25" for two gaps, or 1/8" gap between all tiles on all sides, fitting the scale.

Problem Solved.

53 Cut tiles

Plus 20 extras...

Assembling the Outer Cube

Tile Assembly. Done in the same fashion as the shell - all butt joints cut short on certain sides by .75 inches so the tiles line up into perfect squares. Once finished they're going to be sanded , plastered, and then sanded again.

Corners and sides assembled before routing

The two prototype tiles - I finalized one corner and side before doing the rest to verify my method - I think they turned out pretty well!

Routing the tiles with the 1/2" round over bit on my routing table - absolutely necessary for this type of work.

5 tiles need to be trimmed for the amplifier and 8 need to be cut for driver. This piece is backing the top right corner of the amplifier. The cut will be made with my Nokogiri and then routed down.

The first set of tiles rounded over with the driver cutting assembly

In order to cut the circle for the driver into the tiles I attached some scrap wood to my routing table. I then attached my circle cutting jig to one piece and measured a 7.5" radius from the pivot point to where the driver would sit against the tile. Rinse and repeat 8 times.

Dry fitting all the newly cut tiles. All tiles need the same plastering as the shell for cosmetic reasons - painting and sanding will make you forget you're looking at wood...