Building the Space-Age Sub

Building the Space-Age Sub

Pictures and Plans

Picture of the Space-Age Sub (small, 33k jpeg)

Picture of the Space-Age Sub (large, 65k jpeg)

View of the inside without stuffing

Sub with stuffing on floor

View of the inside with stuffing

Figure 1. Space-Age Sub Plans

Figure 2. 3D view of Space-Age Sub parts showing front and back bracing

Figure 3. Space-Age Sub side brace installation

Figure 4. Space-Age Sub cross brace installation

Entire thing in MS Word format (sans color pictures, but easier to read the dimensions!)

The Design

I got the idea for building the Space-Age Sub when I cut apart one of those cheap interior doors that are so prevalent in newer homes. These doors consist of an outer pine frame covered with a plywood skin, and an interior filled with a serpentine narrow length of cardboard glued on edge between the sheets. This is remenicent of the honeycomb skin used in modern aircraft. The Space-Age sub employs this same concept of a stiff-yet-thin material on edge reinforcing a thicker skin material. The cabinet is made from 23/32" (3/4" nominal) five-ply BC grade pine plywood, and is braced with 1/4" three-ply BC grade pine plywood and 1" x 3" (3/4" x 2.5"actual) pine. By gluing the braces on edge, a high degree of stiffness can be obtained from a small amount of wood. Stiffening the sides of a sub enclosure eliminates cabinet resonances by raising these resonance modes above the operating range of the driver. Box volume is 2.87 ft^3 after bracing volume is subtracted.

The Driver

The Space-Age Sub is a subwoofer based on the NHT 1259 12"poly cone driver available now through Madisound. This is a really great driver, designed to work in a three cubic foot sealed enclosure. Sealed enclosures make ideal beginning sub projects, since there are no ports to fiddle with, the design is relatively insensitive to driver spec variation, and a minimum of test equipment (none, actually) is required to get it right. Ask for the bass list discount!

Joints

I used yellow wood glue and drywall screws to hold the thing together. The external joints were glued and screwed, and the screws were removed when the glue was dry. By using several fender washers with each screw, I was able to keep the head of the screw away from the external wood surface, and the clamping pressure provided by the screw was also distributed over a larger area. This left only a small hole that was easily filler later with wood putty. I used a lot of glue, and I mean a lot. After the joints were dry, I squeezed more glue into the interior corners of the joints, and then formed it with my finger to get an even distribution, much like the process of caulking. This made the joint a little stronger, but more importantly made the cabinet air-tight. I did several applications like this on every joint, even the brace joints.

Tools

This project can be undertaken with a minimum of hand tools. I used a circular saw, a router, and a palm sander. For the router, a 1/4" single flue bit, a 1/4" radius ball bearing rounding over bit, and a ball bearing laminate flush trimming bit were used. All were carbide as high speed steel tendsto burn, and the ball bearings protect the wood surface beingused as a reference. A hand saw was also used, but is not necessary.I also used a file to knock off burrs and such, and for final fitting.

Assembly

Please note that the assembly should be undertaken in a specific order so that all of the braces end up inside the cabinet! I built the thing many times in my head before actually putting it together, which helped catch any of these major types of common-sense mistakes, but a couple almost got me. Watch out! Also, take a little time to dry fit everything before gluing. Yellow glue sets up surprisingly quickly, giving you almost no time to correct anything during the gluing process.

Also a word on the grain of the wood: all of the parts in the parts in the plans are aligned with the grain of the wood (the grain of the exterior layers of the plywood) running vertically, except for the small and large braces, where the grain should run horizontally. The direction of the wood grainis very important! The photo should show the direction of this grain if you need a reference.

Day 1

  • First, cut the top, bottom, and two sides. The18 1/8" dimension that goes from front to back is very critical, match this as close as you can for all four pieces. You might get the wood cutting dude down at the place where you buy the wood to cut this with one of those panel cutting saws (drool, drool) which can get these cuts near perfect. These cuts, especially for the top and bottom pieces, should also be square. The 19.25" dimension on the sides includes 1/8" of excess wood, which will be trimmed off later, so this dimension is not nearly as critical. Route the corner joint recesses and the 1/4"wide slots for the braces. Make sure the 1/4" braces will fit in these slots easily, but do not cut out or mount the braces at this time. Note that all routing is done to a depth of exactly two plies of the plywood, or about 1/4" (see detail, figure 1). My 1/4" router bit is a bit narrower than the wood, so I ended up doing these cuts to the width of the bit, and then placed a piece of cardboard between the router and the router guide for the final pass. This made the slot the right width. Experiment with different thicknesses of cardboard till you find one that works well. Glue and screw the top and bottom pieces to the side pieces, taking care to keep everything even and square. Allow the glue to dry overnight, then remove the screws.

Day 2

  • Cut out the front and back pieces of plywood. Set them up against the top/bottom/sides assembly and mark with a pencil the inner dimensions and the locations of the brace slots. Route the front and back to these pencil markings, including the brace slots. Cut out the 11 1/8" dia. speaker hole (Sears router jig #925179 works well) in the front, and the binding post mounting cup in the back. Wrap a piece of medium sandpaper around one end of a 1 x 3 and use this to level out the edges of the top/bottom/sides where the front and back will attach (I learned this trick building acoustic guitars). Glue and screw the front to the top/bottom/sides assembly, making sure that the bracing slots are aligned. Remove any excess glue from the brace slots inside.
  • Cut out the small and large braces from 1/4"plywood. Note that the grain of the outer plies should be oriented with the long dimension of the braces for them to be stiffest (has anyone told the stiffie joke yet?). It is probably wise to cut the long dimension (18 1/8") a bit shorter (~1/16")than the depth of the sides, just to ensure that the braces won't keep the back from seating correctly. Also note that two of the large braces can be cut from one rectangular piece. Study figure 3to see how the braces will be oriented in the final design.
  • Install the small braces only! Put lots of glue in the slot, then position the brace, redirecting any glue squeezed out back to the joint with your finger, as described above. Allow to dry.
  • Cut some 1" x 3" and install the horizontal braces on the back of the speaker baffle, using the small braces to locate them (see figure 2). Use glue and screws (without fender washers) for attachment to the baffle, and glue also to the small braces. Cut the vertical baffle braces from the same stock, and glue and screw them to the baffle also. Note that the screws should go through the braces and grip in the plywood here. Use 3" long screws.
  • Now glue in the large braces, again using copious amounts of glue. Allow to dry overnight.

Day 3

  • Test fit the back to the assembly. I had some trouble getting all of the braces to go into their slots untilI chamfered the sharp corners of the braces with a file.
  • Before the back is installed, cut the two back braces from 1" x 3" stock and place them inside the enclosure near their final positions, but not glued or anything. It is impossible to get these in after the back is installed!
  • Glue and screw the back on.
  • Flip the enclosure on its back, and now glue and screw the back braces to the back. Use some glue between the back braces and the side braces that locate them also (see figure 2).
  • Cut some 1" x 3" stock for the crossbraces, and install these with glue (see figure 4). You may also glue them together where they cross. Allow to dry overnight, then remove the screws and washers used to clamp the back during gluing.

Day 4

  • Use a laminate flush trimming bit in the router to knock off all overhangs. Word to the wise, if you first file off the overhand at the corners by hand, then the trimming bit won't splinter out the wood when it gets to the corners. This was my first experience with this bit, and I was impressed with how clean the whole thing turned out.
  • Fill the screw holes with wood putty and allow to dry. Sand with a palm sander using medium and then fine paper.

Finishing

I radiused the corners using a 1/4" rounding over bit, though a larger radius would probably look better, such as 3/8" or so. I then final sanded, then applied an acrylic varnish (Minwax Polycrylic clear satin) using a foam brush. This stuff cleans up very easily, but goes on really thin and requires many coats. I wish I had stained the thing first, or better yet, had used one of those stain/varnish combos (stains don't liketo "take" to my glue joints, no matter how clean I keep them). A dark color would make this (not so small) sub appear to be smaller, so a dark stain could improve the SAF of the device, and help it fit into the color scheme of most stereo equipment and speakers (basic black). Laminate would also be a good finish. It is more durable, and would probably go on faster than varnish. Don't round the corners if you plan to laminate the cabinet (duh!).

Buttoning Up

Fill the cabinet with polyester batting (pillow filling down at WalMart). You can experiment with how much by testing the frequency impedance peak of the sub, and stuffing until this frequency is the lowest. Or not. Seal the driver by using a single small roll of clay compound called "Mortite" between the speaker and the baffle board. Don't make the mistake of using caulking compound (who started that, anyway?); the clay will form a perfect seal, will be vastly easer to remove later if necessary (it will be necessary, believe me) and doesn't stink like caulk. Mount the driver using drywall screws at 12, 3, 6, and 9 o'clock. These positions have the baffle braces behind them, allowing for more secure attachment. Mount the grille (I got mine at Crotchfield; make sure it is the "bulging" variety so that the rubber surround on the driver has sufficient clearance) using the four free speaker holes. Solder about $100 worth (~3 ft) of fast-electron-drift silver-core low-oxygen Teflon jacketed 0.5 gauge green monster speaker/welding cable between the driver and the binding posts (or use ~$0.25 of dreaded zip cord if you have cast iron ears and can tolerate "skin-effect" induced audio anomalies, or are not otherwise suffering from cable-itis -- I hope it is clear that I am joking here; I used zip cord --) observing correct polarity. Install crossover (optional, see below). Install cup, again using "Mortite" to seal. Use screw-type brown plastic feet on the bottom mounted about 1" in from the edge.

Crossover

I built the crossover as described in the NHT 1259 FAQ. It consists of a 12mH inductor and a 220uF non-polar electrolytic capacitor. The cap goes directly across the driver terminals (in parallel with it), while the inductor is placed in series with the "+" lead going to the driver. Is that clear? The cap belongs across the driver terminals, not across the connector cup terminals. Got both parts from Madisound. The inductor is from the "Sledgehammer" ferrite core line, and the cap is a Bennic. The sub rolls off at 77Hz (-3dB, -6dB @ 100Hz) now that it is installed.

Results

The frequency response of the sub with xover is (close-miked and measured with my modified RS SPL meter):

  • -6dB @ 30Hz
  • -3dB @ 37Hz
  • 0dB @ 50Hz
  • -3dB @ 77Hz
  • -6dB @ 100Hz
I have plans (but only partial schematics and some ideas) to build a class B amplifier (a la the basswave)and 4th order LP filter, but have not done so yet. At the moment the sub is simply in parallel with my left front channel (is everybody wincing yet?) but the sound is great, and the speaker goes down to 26Hz or so if placed in the corner of the room. This configuration is not that far from ideal, and it allows me to use and enjoy the sub until I build the correct equipment to power it. The sensitivity is about the same as my sats (RS LX5's, what a crappy woofer and crossover!). I am listening to my CD collection with the bottom two octaves enabled for the first time, and I am in heaven! In particular, the second track ("I love you") of Sarah McLachlin's Surfacing CD has some deep bass; and Sheryl Crow's CD Sheryl Crow also has a rumbling intro to track six ("Redemption Days") that shakes the room -- great for showing off the Space-Age Sub to friends!

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