Subwoofer

The subwoofer is an interesting speaker. It exists solely because the reproduction of low frequencies is very difficult without large diameter drivers mounted to large enclosures. This is impractical for most full-range speakers because it would make them very large, heavy, and more expensive. The solution is to have one speaker that is dedicated to reproducing very low frequencies and simply redirect the low frequencies for all of the speakers in your system to it. This works well because low frequencies are hard (if low enough impossible) for your ears to localize, so having the low frequencies produced in a different location than the rest of the audible spectrum doesn't cause problems with the source materials "sonic image" (that is to say the apparent aural "location" of things). Because a subwoofer usually consists of one or two large drivers and no crossover it is a very easy speaker to design. However, because of the large enclosure sizes required and the thick, well-braced construction that is needed to make a good subwoofer enclosure it can be a very difficult speaker to build.

I wanted to achieve a flat in-room response all the way down to at least 20Hz with as little distortion as possible (this is really the goal of any subwoofer). The subwoofer also had to be aesthetically pleasing and integrate well with my existing setup. I had already decided that I wanted to use my stereo Leach Amp to power the subwoofer. In order to use both channels of my Leach Amp to power a single driver I would need a driver with dual voice coils (commonly abbreviated DVC). I also knew that I wanted to include some line-level filters to allow me to adjust the subwoofer in-room response, the first being a Linkwitz Transform and the second being a graphic equalizer. In order to use the Linkwitz transform circuit correctly you must be using a sealed subwoofer (as opposed to a ported). I chose the Dayton 15" DVC driver because I felt it was the best driver available in my price range (<$150) and because it had dual voice coils I could use both channels of my Leach amp to power it. I already had a position for the subwoofer picked out, so I figured out the maximum enclosure size that would fit comfortably in that space. I used WinISD to simulate frequency response and decided on an enclosure (internal volume) of about 2.8 ft^3. This allowed enough room for me to build an enclosure with 2" thick walls and took into account the volume consumed by the bracing and the driver itself.

With any speaker, but especially with a sealed subwoofer, the enclosure is subject to large changes in pressure caused by the vibration of the drivers it houses. As the drivers move it either "pushes" or "pulls" on the walls of the enclosure. When this happens the walls will begin to vibrate inversely with the driver. If the vibrations are intense enough they can produce audible noise, this is known as "panel resonance" and is a form of distortion. The design I chose involves a layered construction. The first two layers are of 1/2" MDF and create the internal braced box, the third layer is 1/2" neoprene foam rubber, and the fourth layer is 3/4" MDF. The basic idea behind this enclosure design is to attempt to completely mechanically isolate the outmost layer of the enclosure from any vibrations. Above is a document detailing all of the panels involved and their positions in the construction of the enclosure.

 

Construction

Since this was a particularly complicated speaker to construct I wanted to take this opportunity to really detail my speaker construction methods. In the most basic sense the speaker enclosures I build are 6 MDF panels bonded together in order to create a box. In order to create a nice, tight box you need to be able to accurately cut panels of MDF. A table saw is the ideal tool for this job. When I first start building speakers (around August of 2003) I bought a Ryobi Table saw, which worked very well. Unfortunately I was a bit short sighted in my purchase, the cutting surface on this saw is very small, so in order to cut larger panels I needed to extend the cutting surface. I bought an adjustable height work bench and used it to extend the cutting surface and then used a level and two clamps as my fence. Before each cut I use a square to check the distance from the inside edge of the blade to the fence and then I check to make sure the fence is perpendicular to the table saw's original cutting surface. This ensures that my panel will be cut to the correct size and that it will be square. I like to buy my MDF from Home Depot because they offer so-called "handy panels" (2'x4') of 3/4" and 1/2" MDF. These "handy panels" are much easier to work with than the large 4'x8' sheets. When working with a table saw it should be common sense to wear eye protection, but especially when working with MDF you should also wear a dust mask. When MDF is cut it creates an abnormally large amount of very fine sawdust. If you want proof you should be wearing a mask go and cut a few panels of MDF without one and then blow your nose.

As you can see from the picture I use my table saw with the blade guard removed, I really don't recommend this because it makes it very easy to cut your finger(s) off. After I've made my cut I check it with the square to make sure that it is square and is the correct dimensions, then I label it with the measurements.

For the construction of this particular enclosure you will need to laminate some panels together. Start by spreading regular white (yellow) wood glue all over one panel with a puddy knife. The clamp the two panels together and give the glue 15 minutes or so to dry.

For all of the braces I drew out 1" wide edges and 1" wide crossbars. I used a 1/2" drill bit to make all of the holes and then a jigsaw to do the cut-outs.

Before each panel or brace it put into place I always sand down it's edges. This allows the panel to fit tightly against it's mate and creates a stronger bond with wood glue.

In order to create the complicated, braced inner-box I used layered panels. To assemble each panel I would layout all the pieces, including the bracing, and trace around them. Each panel has a 1/2" margin around the edge to allow it to bond to the other panels. Once I had all of my lines traced I could laminate each piece onto the panel individually until the panel was complete.

Once I had completed all of my panels I could begin to assemble the inner-box. As you can see I would draw a line 1/2" into the overlapping panel, spread glue along the seams of the panels, and screw it into place using the line as a guide for screw placement. On corners I would often using a square brace (shown in the middle picture, I got it from Home Depot) in order to ensure my panels were square.

Once I had three sides assembled I could glue and screw the bracing in place. Occasionally it was necessary to "help" the braces into place with a rubber mallet.

Once the inner-box was completed I used my router along with the Jasper Jig and the bit shown to make the cut-out for the driver.

The next step was to cover the entire inner-box with 1/2" Neoprene Foam Rubber (from McMaster Carr part number #85175K29, I needed 4 feet). I glued it into place with some 3M spray adhesive (from Home Depot).

Next the outer cabinet had to be assembled. I did this in much the same fashion as I built the inner-box, but made it out of 1 layer of 3/4" MDF. Remember to put the inner-box inside before closing the outer cabinet =). A larger hole must be cut in the front baffle of the outer cabinet to accommodate the entire frame of the driver. I also added T-Nuts which I bought from Home Depot so that machine screws could be used to mount the driver. All of the cabinet edges have to be sanded until all sides of the cabinet are flat and the edges are pretty smooth.

This is a 4'x8' sheet of Wilson Art black gloss (#1595-1) laminate which I purchased from a local laminate dealer, but which can also be purchased from Home Depot. The cost for a 4'x8' sheet is around $70. I used my table saw to cut it into panels that were 2" longer and 2" wider than the side of the cabinet it was intended for.

Once all of the pieces were cut I applied a liberal amount of contact cement, 2 coats, to each surface I was going to bond together. Once the contact cement had dried for the final time I pressed the two surfaces together and used a rolling pin to ensure there was good contact between the surfaces. The Wilson Art laminate is easier to work with than veneer because it is much thicker and won't wrinkle or bubble, applying it is much easier than applying veneer.

After each side was applied I would use my router along with a laminate "flush trim bit" in order to trim off the excess laminate. The object in the other picture is a base I made to accompany the subwoofer. It is the exact same length and width of the subwoofer and is made from 2 3/4" pieces of MDF laminate together. The entire base was covered with laminate as well. I placed black spikes on the bottom of the subwoofer and used the included metal discs to protect the finish of the base from the spike points. Paint thinner (also called "Mineral Spirits") can be used to clean off excess contact cement from your cabinet and your "flush trim bit".

When the enclosure was completely assembled and finished I drilled 4 holes into the back of the enclosure for the binding posts (1 pair for each voice coil). Because this enclosure is over two inches thick I had to pre-solder on some "extender" wires because I couldn't find any binding posts long enough. This enclosure has approximately 3 1/2 pounds of "poly-fill" or "polyester batting" (available from craft stores) stuffed into it, for an explanation on why this is useful read this. When I mounted the driver I put a thick bead of black silicon sealant (from Home Depot) between the driver gasket and the enclosure, this was to ensure there were no air leaks around the driver.