Cheap D.I.Y. speaker project


Just after I bought an old Ford Bronco II, I upgraded the sound system in it (Crutchfield). I replaced the factory tape deck and 5" x 7" rear speakers. The old deck was junk, but the speakers looked in good shape after vacuuming out all the dust and dog hair. I hate throwing away anything possibly useful, so I stashed them on a shelf. A little later my Dad told me he had some left over 3/4" paper-coated plywood from a furniture project that he wanted to get rid of. Thus was born the Cheap Speaker Project.

I pulled out my books on speaker design. Since I only had two drivers, I wouldn't have to mess with crossover networks, compartment building, etc. I decided I wanted these speakers to be as cheap as possible, but still sound decent and be built with solid acoustic engineering principles. I went with an acoustic suspension design, which is basically a sealed box that the driver works against. Ported speakers can better handle power and give a smoother response, but they are more complex to design. The sealed box is simpler to design and build, and provides an adequate response for the drivers I would be using.

Since my drivers were oval-shaped, and all the tables I had were for circular drivers, I averaged the dimensions to obtain an approximate diameter. This I used to look up the box volume in one of my books. I used another table to calculate box dimensions based on the volume and the 'golden ratio', which is used to help prevent coming up with a square box that has sharp resonance frequencies. I drew out the design, added the thickness of the plywood and adjusted everything so the resulting box would fit together properly, and it was time to build.

Plywood isn't a bad material to build speakers out of. Many people (and most commercial builders) use medium density fiberboard (MDF) instead, since it has better self-damping properties than plywood, but I used what I had on hand. I cut out the pieces with a table saw, to help keep everything square. I used 14 gauge solid copper wire to make a template that would fit the driver, and traced this on to the sound board (the front surface of the speaker enclousure). I then cut the oval-shaped driver mounting holes with a saber saw, or jigsaw as it is sometimes called. On the rear, I made a cutouts for the terminal plates.

I assembled the sides and back of each box using wood glue and 16 gauge brad nails. Since the boxes were relatively small and the plywood rather thick, I didn't bother with corner blocks or internal braces. After the glue had dried, I applied a bead of silicone caulking to every inside joint to help seal against air leaks. When that had dried, it was time to install the stuffing.

Stuffing speaker enclosures helps smooth out any resonance frequencies, damps reflections, and increases the apparent volume of the box. Many different stuffing materials can be used, from fiberglass to polyester batting, but in keeping with the Cheap theme of the project, I chose cotton rags. I swiped a couple of terry cloth bath towels from the rag bag, and stapled a double thickness into each box. The cloth covered the back, sides, top and bottom of the inside of the box. The sound board didn't receive that treatment, and doesn't really need it if the back is covered. The towels were trimmed even with the front edge of the box.

I made one last check of the boxes, then attached the sound boards with more glue and brads. Each box was then sanded and given three coats of flat black paint. A wood veneer would have looked nicer, but I was going for cheap. With normal plywood I'd use a stain or other transparent finish to show off the wood, but paper-covered plywood looks like cardboard, so an opaque finish was called for.

While the paint dried, I soldered lengths of 14 gauge stranded copper speaker wire to the two terminal plates. I cut a small slit in the stuffing cloth, fed the wire through, pumped more silicone caulk into the hole, and screwed the plates in place. The other ends of the wires were then soldered to the connectors on the drivers, keeping track of polarity. After one last check of the box interior and the wiring, I sealed the drivers to the speaker fronts with more caulk, and used screws to secure the drivers in place. One last bead of caulk around the edges of the drivers assured a good seal with the box, and then it was just a matter of letting them dry long enough before trying them out. After the caulk and paint had dried completely, I added some stick-on rubber feet to the bottoms.



Here are the completed speakers. They're not very pretty, but do produce nice, even sound with no spikes, buzzes, or rattles. The drivers are mounted on the front of the soundboards, since they have better dispersion that way. The fabric covers on the drivers are stock: they're to keep dust out when installed in an automotive application. I thought about removing them but decided not to bother. Car audio speakers always sound weaker in this sort of application, due to the fact that even a small room has a lot more volume to fill with sound than the interior of a car or truck, but even so these speakers are pretty good at handling the power a desktop stereo system can produce. In the system I use the speakers with, I added a small powered subwoofer, designed for use with computer audio systems. This rounded out the response nicely, giving plenty of 'thump' when needed.

All in all, I'm quite happy with the way the speakers turned out. They won't put any Polk Audio speakers to shame, but since they were assembled out of junk parts and leftovers, they were essentially free. They produce sound of adequate quality and volume for the location I use them in.

The emphasis of this project was to keep things cheap. While most people like they way they sound, some of my more craftsmen-type friends cringe at the look of the speakers. Looking back on it, I probably should have sanded the paper off the plywood, rounded the edges with a router, and used a stain or hand-rubbed finish instead of black paint. I do always have the option of adding a wood veneer to pretty them up. Of course, I could just use the looks of them as an excuse to plan and build a new set of speakers... :)