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Building Your Own Bass Cabinet

There are a huge range of bass cabinets and combos available on the market, both new and second hand. There is a box to suite every budget and ear. All this said there is nothing quite like pointing to a massively impressive quad that sounds fantastic and saying "I built that and it only cost me $x". With a minimum of woodworking skill and a soldering iron it is amazing what can be achieved.

First of all there are several decisions to be made.
  • How big do you want it? 4x10, 1x15 or anything in between, even 4x8 is acceptable. Larger boxes with more drivers can be more efficient and have greater head room but are a pain to cart around. However the smaller boxes will not fill a large venue.

  • How much do you want to spend? More money means better quality drivers and more of them. This will be the major expenditure.

  • What power do you want? This is also related to cost. A 50W 10" driver is going to be far cheaper than a 250W unit of the same quality.
Once you have made these decisions then it is time to go shopping for drivers (the actual loudspeakers).

Choosing drivers
This is the most important decision to be made. This can make or break the whole deal so choose carefully. Once the box is designed then it is difficult to fit other drivers without some sacrifice.

Instrument speakers have an entirely different design than HI-FI speakers and although I have used HI-FI speakers in both of my bass cabs instrument speakers are much more rugged and will stand far more abuse as a rule. In addition instrument speakers tend to be more sensitive.

The drivers also need to be more sensitive to get the sound into the audience. Look for drivers that are up around the 93 db spl mark. Remember each 3db increase is double sound level so a speaker of 93 db spl will sound twice as loud as a speaker of 90 db spl.

Whatever you buy make sure that you have the Thiel and Small parameters. These are two Australian engineers who did a lot of research into loudspeaker characteristics in the late sixties/early seventies. When they published their work it revolutionized the design of speaker boxes. They developed a standard nomenclature for describing loud speaker characteristics. You will find that all reputable manufacturers publish the Thiel and Small parameters for their drivers.

Designing the box
The drivers will typically be designed for either vented of seal enclosure. This may be stated in the specifications, if not then you will have to experiment with the box design. Vented enclosures are more popular but you will find that with many modern loudspeakers a sealed enclosure will be perfectly adequate. I built a sealed enclosure to suite a 12" driver, however the design is not particularly efficient and I may have been better off using a more efficient driver in a vented box.

There a number of excellent speaker box design programmes. The one I use is freeware and is called winISD. It does not have some of the features of the commercial programmes but it will more than suffice for our basic requirements. It is very easy to use and produces adequate results in a very short time. It has a library of standard drivers but you can add drivers to its library for your own use.

Once you select your driver choose the number of drivers and whether it is sealed or vented. The programme then works out the box volume and port size automatically. It also gives you the cutoff frequency and the frequency response plot. You can play with the various parameters but you will probably find that the automatically chosen dimensions will be pretty close to ideal.

Once you have the volume you can then decide on the shape. This can be rather personal. I typically choose the traditional square front with relatively shallow front to back, but you can (within reason) make it whatever shape you like. Because my woodworking skills are rather limited I stick to the simpler designs. Whatever you do make sure there is sufficient room on the front for the drivers, the port(s) and whatever other hardware that is required. If you are building an amp inside then you have to consider access to the controls. These may be on the front, recessed into the top or on a sloping panel at the top front edge of the box.

There are a number of ways of protecting the drivers but I use the following design, again because I can, it is simple effective, and does not require a great ability and is easy to cover errors! I recess the front panel on which the drivers are mounted by about 20mm. Rroute a 5mm dia around the front inside edge. Screw a 12mm square cleat into the edges of the recess. With larger boxes add a horizontal piece across the center of the front panel as well. To this screw an expanded steel mesh which provides protection for the speaker. If there are front panel controls run a piece horizontally about 50mm from the top and cut the mesh 50mm short to leave a gap at the top for the controls.

If you are building a ported box then make sure there is enough depth for the port!

For the remainder I will assume that this is a standard rectangular box with no sloping sides. Make the top and bottom the full width with the sides fitting in between the top and bottom. The front and back fit inside the top, bottom, and sides. Cleats will run around all inside edges. This reinforces the box and makes it more rigid and more rugged. 20mm square shoud be more than adequate. However 16mm square may me used for smaller boxes.

Building The Box
The best material these days for making boxes is mdf. I have only had to buy the material for the box once. On all other occasions I have managed to find sufficient second hand materials to save me the cost. In one instance I used a pallet that was used to transport an HP minicomputer rack from the US. This was VERY substantial timber being 32mm plywood. If you are patient and a bit of a scrounge then it is surprising what you can find.

In choosing the timber remember that the more substantial the timber the less colouration there will be but the heavier it will be. However, internal bracing can stop the box vibrating and affecting the sound.

There are standard sheet dimensions so I would recommend the following technique to selecting the sheet size and shape. Using graph paper make scale drawings of the box sides, top, bottom, front, and back. Draw the sheet sizes to the same scale on the graph paper. Layout the box pieces on the scale drawing of the sheets to find the one which fits the best. This will also show you how to cut up your sheet of timber. You may have to alter your box dimensions to achieve an optimal fitting but make sure that the volume remains correct.

Now cut all of the pieces of the box from the sheet. At this point beg borrow or steal an electric screwdriver if you do not have one. You will have roughly 100 screws. I got through screwing the cleats onto one side before a huge blister formed in the middle of my palm. I told everybody it was stigmata, and one person actually believed me!

Cut the holes for the drivers and any controls in the box front. Because the front is recessed it is much easier before assembly. You may wish to cut the hole for the speaker terminals at this stage as well. Do not do as I did and cut it the wrong way. The speaker terminals are sideways in one of my boxes.

Cut out, glue, and screw the cleats to the top and bottom, four each. Use three screws per cleat. I used a bit of scrap to gauge the distance of the cleat from the edge. Once that is done, glue and screw the sides onto the top and bottom. The fromt and back should now squeeze in nicely to the box. Glue and screw the back. If there are any internal components such as crossovers or amplifiers then mount then now. It can be done once the front is on but it can be extremely awkward. Now glue and screw the front. The back and front should be screwed as soon as possible after the sides while the glue is still soft. There will inevitably be some slight adjustment in the box as the front and back go in and if the glue is allowed to dry this will be more difficult and may also create a crack in a joint.

It is here that I cheat a little. Real cabinetmakers will cringe so for those pros out there skip onto the next paragraph if you do not want to wake up with nightmares. My joints are always less then perfect, due mainly to lack of skill. It is imperative that the box be airtight. To this end I run a bead of glue along all of the joints and force it in with my finger. If you are using old scrounge wood then it may be less than perfect and this may be mandatory to achieving a seal. I have achieved a perfect seal with the most imperfect of timber using this technique. In one case I was using weathered chipboard with about " bow in it and with the above construction technique produced a perfect seal.

Route a 5mm dia edge around the inside front. Glue and screw the 12mm square around the inside front. Rout around the outside edges of the box. The dia depends on the corners you use and the box size. Use the same diameter as the corners.

If it is a vented enclosure you will now want to mount the port. It will need to be sealed. If you are using a home made port out of polly pipe then make sure the outer edge is rounded using sand paper. Sharp edges can cause the box to woof at high powers.

The front grill can be made from expanded metal cut to size from a metal vendor. I use steel, mainly because it is strong and relatively cheap.

When the glue is dry paint the front and the grill matt black. You can use a spray can. One can should provide several coats. You probably want to prime the surfaces first for a more durable finish. You can also use blackboard paint. Fill the screw holes with wood putty and sand smooth.

Carpet the outside in 3mm speaker carpet. Use contact cement applied with a cheap 1" paint brush. This makes the box rugged and hides any imperfections in the construction. Roll the carpet over the front edge to meet but not cover the 12mm square. Run 10mm x 3mm adhesive backed black foam around the front of the 12mm square. This will cushion the grill and stop it vibrating.

Solder all of the electrical components together and screw the terminals and speaker et al into place. I seal the speaker terminals with hot melt glue and the speaker with 10mmx3mm adhesive backed foam.

Now screw the grill onto the front of the box.

It is now time to find the centre of gravity of the box. You need this to locate the handles on the side of the box. This way the box will hang straight when it is carried. Find a round pole. Some " water pipe or similar will be ideal. Place the box sideways on the pole and move it back and forth until the box is perfectly balanced. Make a note of the position of the pole with respect to the front of the box. This will be the center of the handles. Using this line as a reference cut holes for the handles about two thirds of the way up the sides of the box. Again I use hot melt glue to seal the handles. Now screw the corners and castors if you are using them.

Testing The Result
Now for the acid test. Make sure that everything is set and dry before applying power. Connect up your amp and guitar and riff away. If there is any breathing you have a leak somewhere and you will have to find it. If you listen closely then you can locate the general area and then move your hand around where you think it is coming from. You will feel the air escaping from the box at the location of the leak. You will have to seal the leak. If it is under the carpet then you will have to cut away a section and reseal the area.

John Hancock is a local church bassist from Adelaide, Australia.