Build Plans: Home Theater Subwoofer

In this post I am going to share with you plans for a home theater subwoofer with some interesting design elements.

Parts Used*

2 Design Goals

The hardest part of building a speaker is making it look good, especially if it is a budget build. Wood veneer is a simple solution that that looks great, but it can really run up the cost of a project. It is no more expensive to use cabinet grade as compared to veneering MDF. The problem with plywood is the exposed layers on the edges. Some people like the look, I don't. So, this design is going to make use of overhangs to cover as many edges as possible, and edge banding to cover the rest.

As far as the sound goes, the goal is a a relatively flat response that reaches as low as possible, without exceeding X-MAX. The design will make use of 2 3" ports with flares on both ends. The port area is not big enough to prevent port noise, so flares are used to fix this problem.

Cut List

Baffle (2): 15" X 21.25"
Back (1): 15" X 21.25"
Sides (2): 21.75" X 21.25
Top/Bottom (2): 22.25" X 17.5"
Window Brace: 15" X 19"
Port Support: 15" X 7"
Ridge Brace: 15" x 7

Dimensions

You are free to cut up your plywood anyway you like. But, take some time to plan ahead. Unless you have a large table saw with some big in-feed and out-feed supports you will want to have the plywood cut down to size when you purchase it. This will make it easier to handle and transport. Another alternative is to get a Kreg Rip Cut and some Styrofoam insulation.

I am very fortunate because the guys at my local big box stores are very accurate when they make cuts. But don't count on that. Ask them to oversize the cuts by at least 1/4", possibly more. Plan this out before you go pick up the plywood. In my area they will make two free cuts, but if they guy is in a good mood he will do more. The best advice I can give is to make all of the cuts of a given dimension at the SAME TIME. That way if you are off by some small fraction (1/16" or less), it will not make any difference in the finished product.

Here is an example of how you might break down the material. This assumes that you can get two free cuts at your big box store.

Start with the largest dimension is the top and bottom piece, at 22.25" (22 1/4"). Have the store cut a piece that is 22.5"x 48". Then when you get it home you can trim it down to size to 22.25" Then you can set the saw to 17.5" and cut out the top and bottom. That will leave you with a 22.25" X 12.875" piece of scrap. Save that piece, you can use it for the port support later.
The next largest dimension is 21.75. So ask for a 22" cut. Again, trim this down to 21.75" at home. Then you can make the two 21.25" sides. When finished you will have two side peices plus a 21.75" x 5.375" piece of scrap. This one will not be very useful. But it may work great for the port rings, so save it.
Now with the saw still set on 21.75 take the last piece (this one will be large you may need some help if you are using a table saw, at the very least you will need some rollers if you are using a table saw) and rip that down to 21.75. Then set the saw to 15" and make three cuts for the double baffle and the back piece.
With the saw still set to 15" take the remaining plywood and rip it down to 15". If you want to grab the scrap from before and make the port brace do that now. Set the saw to 19" and cut the window brace, then to 7" and cut the port brace and ridge brace. The ridge brace was added after the fact and even though it did raise the tuning frequency it stiffened up the box quite a bit.

Port Design

The ports are made from inexpensive flared port tubes available from parts express. The tubes themselves are not long enough to reach the desired tuning frequency. So extensions were fabricated to lengthen and connect the tubes. The internal tubes are connected to a brace inside the box and the tubes are mounted using 5 minute epoxy. You will have plenty of 3/4" scrap from your plywood to connect and extend the ports. When you take into account the fact that 3/4" plywood is undersized by 1/32" then four layers of plywood rings will get you very close. Here is a video showing the fabrication process and the mock-up:

Assembly

You can use any method you like to assemble the enclosure. Keep in mind that the wood glue does all of the real work, any mechanical fastener that you use in the build does the same thing that a clamp would do, it holds things in place until you can Typically I build enclosures out of MDF, and I just use wood glue and brad nails. One of the goals is to show off that nice expensive birch veneered plywood, so you will need to take care that you don't damage the surface veneer. Therefore clamps and glue will work just fine. But I am going to make use of some additional tools. As described here.

You can start by laminating your two baffles pieces together, before you make the speaker and port cut out. If you go this route you will need a rather large rabbiting bit in order to recess the speaker, or you can forgo the speaker recess. If you don't have the proper rabbeting bit you can cut the circles before you laminate the pieces. Here is a video with some tips to help make that process easy.

For the ports I made several test cuts in scrap material in order to get the best fit, and I used a round-over bit to match the curve of the ports. Make sure to align port holes on the brace support with the port holes on the baffle. To do this I clamped them together, made sure that the pieces were square to each other and drilled the pilot holes for my circle jig at the same time. My original plan was to recess the ports, but that did not go as planned so I just inserted them in the port holes and epoxied them into place with 5-minute epoxy. I then up the ports, the port support and the window brace an connect them all together. Here is a video that will hopefully shed some light on the process.

That video also shows how I used pocket holes and pocket screws to connect the internal parts, as well as the process for the window brace. The top, bottom, and sides overhang a bit. This was an important part of the design as helped hide most of the plywood edges.

Finishing

The goal of the finished product was to hide as much of the plywood edges as possible. This was done by paint the back, bottom, and the bottom edge with black duratex, and spray painting the subwoofer recess black. Then iron-on edge banding was used on the top and the front of the sides. The entire thing was finished with Polycrylic.

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