(A novices account and guide, by neilb)

I took on this project at the end of the summer 2001. I wanted a reasonable sized compact twin bass cabinet for practice and small gigs that would be easy to lug around. I fitted it with two CELESTIAN 12" Bass speakers that I had in my old 4 x 12" Marshall stack.

Since my last project I made my 4 x 12" Marshall Cab. A little redundant. So I decided to use two of the speakers from the Marshall which were Celestion 100 watt speakers, which are a few years old but in good condition. The requirements of Bass is much different to that of other speaker design. The frequency range is 41Hz (E3) to 261 Hz (middle C), so speakers producing deep bass are desirable. Ideally the speaker should go down to 41Hz, if possible. Most manufactures are quite optimistic with their lower level. It is governed by the resonant frequency of the cone, below which the response falls off by 12dB per octave for sealed enclosures. However that frequency is raised by the effect of the air in the enclosure, and small enclosures raise it more than large ones. So, the actual resonant freq. Is always higher with sealed enclosures than the quoted free air resonance. As it is common practice to specify a response some 10Hz below the quoted resonance, the specified bass response is thus likely to be optimistic. With reflex units the bass is extended and is more like that quoted, but it drops off rapidly by some 24dB per octave below resonance.

For bass enclosures two designs seem to work the best: Sealed and Ported Enclosures. In most Bass applications the vented design (bass reflex) is used nowadays. The advantage of a vented design is the sensitivity at lower frequencies. However, the quality of the transient response is not as good as in a sealed cabinet.

The sealed enclosure design is easier to compute and construct. It also has quite a good transient response. The bad thing about sealed systems is a poor efficiency at lower frequencies. Usually not a very good thing for a subwoofer.

For this project I was going to build a Sealed Enclosure. As after a little probing with the speaker Thiele-small parameters, they seemed more suitable to a sealed enclosure design.

For the Box size ratio, I used the Golden Rule ratio: 1: 1.6: 0.6 (width: height: dept) this would help to some degree eliminate standing waves inside the Box.

There are many free software packages out there some seem to work better than others, they also depend upon the amount of information you plug into them. I have found that the software 'SpeakerPro 6.0' worked best for me, so this is what I used.

Plugged in my Thiele-Small Parameters for the Celestion speaker. Obtainable from the speaker manufacture or you local speaker dealer. From this I was able to calculate the optimum Cab enclosure size. However the optimum cab volume was too big to be practical. So I reduced my volume and compromised bass response trying to avoid a boomy sound.

Note: Don't forget to take into account the extra internal volume of the speaker, handles and bracing when calculating the overall volume of the enclosure. I calculated 15 liters for bracing, speaker and internals.

So setting the cabinet volume size at 124 Lts and I've calculated my enclosure dimensions from this and panel sizes using the golden rule ratio. Now what material do I build it from? See below for a layout drawing of the enclosure and panel sizes.

There are many various materials available, however for a musician on the road durability and cost are critical so narrowed it down to M.D.F. (Medium Density Fiberboard) or Plywood. I have used MDF in the Past and it's easy to work with, although it's heavy and the dust is nasty. I choose 16mm high quality plywood.

I first choose one of the Top panels and affixed the bracing. I cut the bracing to run the full length of the side panels and across at the ends. I also decided to reset the front baffle about 25mm back, this will give some added protection to the speaker and visually look appealing. I pre-drilled and countersunk the braces on the opposite sides facing the panel. I spaced out the holes approx. 8cm. I used plenty of carpenters glue on both mating surfaces and screwed down the braces to the side panel. This was then repeated for the other side panel. See Figure 2 & Figure 3. The glued panels were then allowed to set overnight.

The Next step in the process was to add the remaining Bracing to the side Panels. The only Bracing will be along the Front and Rear of these panels. This length is the width of the Top or Bottom Panel minus the side panels and bracing (18 + 18 + 20 +20mm). See Figure 5. Again I left the glue to cure overnight before assembling the Cab-Box. Remember not to pre-drill the brace where the rear panel will be screwed.

Having all the bracing glued and screwed onto the panels. The next step was to assemble all four panels together. This is a bit tricky, you may need someone to hold one panel at 90 deg while you are screwing the other panel together. This is where, having the adjacent sides on the bracing already predrilled comes in handy. Place the Bottom panel at 90 deg. flush against one of the side panels. Ensure the panels are held at 90 deg. angles and simply screw the panels together. Remember to apply liberal amounts of glue beforehand to both mating surfaces. See Figure 6.

This was then repeated with the other Top and Side Panel. Then screw and glue the two sub-assemblies together. When assembled, the cabinet was very accurate (getting the panels cut properly also helps!) and the cabinet was almost square (measure between the top and bottom opposite corners for squareness, should be the same distance) So with a little tweaking I got the cabinet set up right. With the cabinet assembled and fixed, it was left to set overnight. See Figure 7.

The Speaker Cabinet has now taken shape. The speaker driver cut-outs now need to be cut out on the front baffle, as well as the cut-outs for the jack socket plate on the rear panel, this is also a good time to do the cut-outs for the handles. To do this I used a Jigsaw. Yip, it's noisy and makes a lot of dust but it's quick and simple. I measured & marked up all my cut-outs according to the manufactures specifications. I positioned the 12" drivers at equal distances apart on the centerline of the enclosure. After cutting, I gave the cutouts a quick sanding and brush down to remove any loose particles. I then mounted the front baffle by gluing and screwing it to the cabinet batting and braces, this is where an electric screwdriver comes in handy! . PS: again use plenty of glue to get a nice seal. See Figure 8. for the layout drawing.

Caulking or sealing the interior of the enclosure is critical in a Sealed enclosure. I applied a standard sealant to all the interior corners of the cabinet and used my fingers to rub it in. Make sure when you fit the speakers that you use a seal, which should come with the speakers. If you decide to use a Silicon sealer, make sure the sealant has completely cured before fitting the speakers as the fumes from the silicone can attack the rubber surrounds on the speaker!

Since I didn't have a router, which would have really finished off the box edges to a nice rounded professional look. I just sanded and somewhat rounded all the Box edges with rough sandpaper and smoothed them down with finer paper.

To achieve the brace I created a very simple type of Brace, in the shape of a '+'. There are lots of ways of creating braces, so experiment around. I designed this type, as I was limited to tools and material. It also worked out fine and was easy to build. I first cut two pieces of 50 x 35mm high-grade pine. One the internal width of the cabinet and the other the depth (which will brace the front and rear panels) the width (Internal) of the enclosure. I measured and predrilled holes in the cabinet walls where the faces of the braces will be attached.

Then attached the Brace running from the two internal walls. Glue was applied to the ends and the brace was screwed from the outside using two screws on either side.

I mounted the speakers from the front . I used speaker mounting kits which I purchased from a specialist store. Make sure to use a gasket between the speaker and the front panel to get a perfect seal. Additional holes were then drilled for the clamps which hold down the speaker grills, another must to protect those drivers from damage.

The final stage of the project was to add the hardware. For me this was fitting the handles, corner protectors, which are another must. I bought the speaker driver clamping kits from Maplin, these ensure the speaker grill does not become loose as they allow you to really tighten the driver grill. To keep the Cab from sliding around the stage I screwed in 4 rubber feet on the base, I used rubber door stoppers which can be easily bought and seem to work fine. The jack socket recess plate was screwed into the rear Panel.

Wiring one speaker is easy, just keep the positives (hot wire) and negatives (ground wire) Together. However when you have two speakers you have more options on how to wire them up. Check out this link for all your wiring and power matching requirements.

 I use heavy gauge speaker wire. I also allowed plenty of slack so if I need to take off the back cover at any stage I'll have enough cable length to move the back panel out of the way. I also stapled the speaker cable just after it comes from the Jack socket to the brace to stop any accidental strain on the cable whenever the rear panel is off.

These have two main purposes, one to damp panel vibrations and secondly to damp internal air resonances. Lining the enclosure is usually one of the last procedures in Cabinet Building, basically you are also fine tuning the enclosure It is essential to stop unwanted waveforms from canceling and interfering with your sound. Again like the cabinet material there are all different types of acoustic wading available. Namely fiberglass, BAF wadding, Rockwool as well as the various hi-tech materials. I purchased a simple polyester acoustic damping material, from Maplin Electronics. It's simply called Acoustic Wadding, it's a very light mixed polyester fiber about 3/4" thick. This material was easily available and is not very expensive.

 I glued two layers of wadding inside the box on all walls, Then I proceeded to fill the enclosure with the rest of the material. This creates an effect on the volume of the enclosure. It effectively increases the volume of the enclosure by anything from 10 - 20%. This only works for sealed enclosures. However take note, if you apply too much wadding or pack it in too tightly you will reverse the effect and end up with a smaller volume, thus reducing efficiency and sound.

Adding the wadding is also like fine tuning the bass cabinet. Try playing around with different amounts of wadding in the enclosure and see if you notice a difference with the bass sound!

Looks great and sounds great. A small cabinet but loud enough with lots of Bass and punch to play at small gigs and practice. Here are some photos taken with my webcamera, quality is crap, in reality the cab is a deep gloss red!

Looks and sounds great. Very tight with lots of punch. It doesn't have the same low bass as my other cabinet but it's compact and relatively lightweight. It's still difficult to keep the weight factor down, due mainly to the amount of wood used and the Speakers. Another alternative I may try in the future is to use a lighter plywood and attach a thin layer of dense damping material to the walls. It's been proven to have the same effect if not better than using a heavier thicker wood. Let me know if anyone has tried this out!

Here are some Speaker building Page Links: there are hundreds of thousands of pages out there. Surf around and get what you need, good luck!