Now it's time to build the subwoofer. If you're only interested in using Speaker Workshop to design speakers you can skip this whole section.

These speakers are intended for use in a living room / home theater. As such I wanted them to be pretty. This means they need to either be made with a veneer or with solid wood. I prefer solid wood for its appearance and longevity. BUT solid wood has two serious issues.

1) Solid wood shifts over time due to changes in humidity.

2) Solid wood can be resonant - and resonance issues cause imperfections in the sound.

Due to these issues most speakers are built from MDF (medium density fiberboard). MDF has been shown repeatedly to have great characteristics for speakers. MDF is made from wood chips and glue and as such it is very non-resonant. It also is extremely stable - it doesn't change size with humidity changes. MDF does, however, fall apart if you get it extremely wet and MDF looks mediocre no matter what you do to it. You can veneer MDF but I don't much like veneers - at least in my house eventually they chip and the whole thing ends up looking sloppy.

When I built speakers for my computer, I built them from MDF and sprayed them with TextureLac - a very pretty textured spraypaint. I don't consider that attractive enough for a living room. 

I'll get back to my solution to the wood issue, but first...

The first consideration when designing the subwoofer is size. I wanted a volume of approximately 4-5 cubic feet in order to satisfy the sealed box constraints. I also wanted something esthetic. I ended up with a speaker that is internally 30" tall and approximately 16" square. This produces a volume of about 30x16x16 or 4.4 cubic feet. The amplifier is mounted in the speaker as is the driver itself - both of which reduce the volume to about 4 cubic feet.

In general you don't want square speakers. The internal reflections can affect sound and when you make square speakers you increase the reflection effects - however in this case it's important to understand the issue. 16" as a wavelength is approximately a frequency of 600Hz and that is so far beyond the frequency of interest for a subwoofer that internal reflections are really not much of an issue. I would never make the main speakers square but for a subwoofer it's fine. If I were making a squat subwoofer then square could be an issue - and I would never make a speaker a cube.

Now, the wood solution.

I decided to make the top and sides solid cherry (about .9" thick). The front and back are a sandwich of 3/4" MDF and 5/8" cherry (actually the front also has two strips of 1/2"x5/8" rosewood laminated in for appearances). With speakers the resonances from the front panel are where most of the sonic distortion occurs. By using a sandwich I solve two problems - resonances and moisture-based size distortion. The sides and top are aligned with grain running the same direction (left to right for the top, up and down for the sides) so with gross humidity changes the speaker may actually change depth but the sides and top/bottom will expand/contract in synch and the speaker will not tear itself apart as humidity changes. The 1 3/8" front sandwich is very heavy, dense, stable, and non-resonant due to the presence of the MDF. 

For a finish I coated the inside with two heavy coats of polyurethane (to help with moisture and to help with the air-seal). On the outside I prefer about 6 coats of shellac. Shellac is a great moisture insulator which helps with the humidity-size issues. For my customer's subwoofer I used 3 coats of polyurethane on the outside. I used poly rather than shellac because poly is virtually indestructible whereas shellac can be more fragile.

The top and sides are connected via dovetails - which looks really neat although it isn't that necessary - butt joints would work nearly as well. The MDF is attached to the sides with a tongue and groove to add some stability to the front and back. The MDF sheets are flush with the top/sides and the cherry front/back are glued flat to the MDF and to the exposed side portions of the sides and top / bottom. All of the dimensioning is accurate to about .003" so that glue joints are solid and no air passes through the speaker enclosure. 

Here's a picture of the subwoofer during construction. If you look closely you can see the MDF/cherry sandwich for the front. Click on the image for a zoom.

The speaker looks good and should survive many years. As an example I recently ran some wiring to my woodshop and the electrician asked if he could stand on my current subwoofers to run the wire through my ceiling. I had no problem with that - always design for serious abuse.