Sonosub - An Inexpensive Powered Subwoofer (page 3)

Building the Speaker (continued)

• Complete the top endcap.
  • I used two 3/8 inch diameter metal rods to secure the two endcaps to the tube and to one-another. Having rods means that the top and bottom should be aligned relative to each other. Mark a line on both ends of the tube. Make sure these marks form a line that is perpendicular to the top and bottom circular plane.
  • Align the top's inner (smaller) circle with the glued up bottom assembly and temporarily clamp in place. Drill from the baffle side (to avoid splitting the good outside surface) a 3/8 inch hole through all layers. I used a drill press to get a nice perpendicular hole. Use a brad point bit to minimize damaging the entry hole. My holes were 1-1/4 inches from the outer edge of the baffle. Make sure to place the holes to avoid obstructions with the driver, port, tube, etc.
  • On the bottom baffle, the rods stick out and that's ok. On the top, they can't stick out for aesthetic reasons. The nuts on the top endcap must therefore be hidden between the top two circles (which is why they've not been glued together yet).
  • Remove the top's inner (small) circle from the clamps. Now clamp it to the top's outer (larger) circle and mark the center of the 3/8 inch holes on the outer circle. Use another 1/4 inch dowel for alignment.
  • It's probably a good idea to label the 2 rods, especially if they are not exactly opposite each other on a diameter. I marked all holes with a "1" and "2".
  • Mill a recess between the two circles that make up the top endcap. The nuts for the rods will be hidden here. On the top's inner (smaller) circle, use a 1-1/4 inch diameter Forstner bit to drill a recess about halfway into the plywood. This is on the side that will meets the top's outer circle. The recess must be wide enough to handle a flat washer.
  • On the top's outer circle, use a 3/4 inch forstner bit to do the same, using the previously marked center as a guide. This recess is narrower since it only has to conceal the 3/8 inch nut, not the washer. The total depth of both recesses must hold a flat washer, lock washer and nut.
  • Since the top's smaller circle is now thinner at the hole due to the recess, I added reinforcement. I glued an additional layer of plywood to the inside plywood face and drilled a 3/8 inch hole through it.
  • Cut down the steel rods to the desired length. I did a dry fit of everthing to get just the right length. I used a hacksaw, and filed the rough end smooth for safety.
  • If cutting the rod leaves that end unusable for threading a nut and you can't fix it with a grinder, insert a nut, lock washer and washer from the other end. The bad end will be the end that's enclosed between the two top circles and will remain unseen and unused. Thread the rod from the top, through the top's small circle. From the other side, add another washer, lock washer, and nut. Tighten the nuts such that the top nut assembly fits in the cavity previously drilled. [Top endcap assembly - inner circle with rod attached (standing on end), outer circle with recess for bold head, and parts layout in foreground; 7,570 bytes]
  • With both rods in place and secured, the top's larger circle can be aligned with the smaller circle and the recesses milled into it should cover the protruding 3/8 inch nuts. If the recesses are too small, take out the drill and recess deeper, being careful not to cut through to the other side.
  • Align, glue and clamp the two top pieces together. What I forgot to do is to fill the void around the nuts assembly with filler - silicone caulk would have worked. I didn't realize this until much later and it was too late to take the assembly apart.
  • Drill 3/8 inch hole over the 1/4 inch dowel that's sticking out of the top piece (outside face). Make 3/8 inch plug from the appropriate wood using a plug cutter to plug the hole. This step is purely for looks.
  • Add 6 layers of roofing felt on the inside of the top assembly. These are held by caulk and a few screws. This adds additional damping on the surface directly opposite the driver. Total thickness is about 3/8 inch. Why 6 layers you ask ? Because that's what I had on-hand !
  • What I should have done was to attach bracing to the top endcap to stiffen it. Then attach the roofing felt to any remaining flat surfaces. This is probably the single most important step I omitted during construction.
  • The top assembly is now done. [Top endcap assembly with roofing felt and two rods sticking out; 15,802 bytes]
• Complete the bottom endcap (the baffle).
  • Now it's time to work on the bottom endcap, which is also the baffle. Plan the placement of the driver, port, terminal cup, bracing and feet onto the baffle.
  • Route a circle into the baffle for the driver's flange. This is 10-1/8 inch in diameter and 3/16 inch deep. The depth allows for a gasket to be added.
  • Route the driver opening.This is 9 inch in diameter. For safety, do this in multiple passes, leaving about 1/4 inch at the bottom. Remove the remaining 1/4 inch with jigsaw for safety.
  • Mill 3-5/16 inch diameter hole for the plastic port using a T-shaped circle cutter on a drill press. Like the driver opening, cut as far down as possible without going through, remove remainder with jigsaw.
  • Cut a 2-7/8 x 2-1/8 inch opening for a rectangular terminal cup. Do this with a jigsaw. [Outside surface of bottom endcap (baffle); 10,542 bytes]
  • Glue and screw a hardwood bar on the inside of the baffle. This bracing helps stiffen the baffle, compensating for the loss of so much material that's been removed. Depending on the layout of holes on the baffle, there may not be room for any bracing, or multiple braces may have to be used. The particle board I used was pretty good material. Void-free plywood would have been structurally stronger. [Inside surface of bottom endcap (baffle); 10,062 bytes]
  • Add alignment marks to both the top and bottom endcap assemblies. These will be used to align top and bottom with the mark on the tube.
• Work on the cardboard tube.
  • The cardboard tube's outer skin is a spiral layer of thick paper/cardboard. Each winding of the spiral overlaps the previous one by a little bit. To make the tube completely smooth, these overlapping regions must be made flush with the rest of the tube. Shave (with a block plane) the overlapping regions of the cardboard tube. Sand as needed.
  • Use vinyl spackle to patch up all rough areas and other dinks, scratches, etc. (this was after all a surplus tube). Sand the result to 220 grit sandpaper.
  • Prime the exterior tube surface with a spray primer.[primed tube; 11,483 bytes]
  • Spray the exterior with 2 cans of Rustoleum's Stone Creations (greenish tint). Seal with one can of Rustoleum American Accent matte finish clearcoat. [sprayed tube; 12,047 bytes

29-September-2000

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