Construction Tips; Speaker Cabinets
This is a fairly complex subject, but once you establish the method that you plan to use, and you've made a few, it will become easier to manage. I'll reccomend how I build them, however other people and companies may build theirs differently. I'll try to discuss the variations and explain how these variations change some of the measurements. You can do it any way that makes the most sense to you.
Unlike most home audio gear and cases, any type of box that you take on the road will need to be able to survive an occasional fall down some stairs, being dropped, having something big and heavy smash into it and rain/snow falling on it while you are loading or unloading your gear. You need something that holds up to random abuse and doesn't develop rattles or exterior damage and at the same time protects its contents and/or retains acoustical qualities.
When sizing a speaker cabinet or case, you need to consider if it will have internal bracing or not. For cases for guitars and keyboards, which are relatively thin, I don't use additional internal bracing, but for every thing else, the sides are glued in place and have 3/4 inch (18mm) wood bracing running against all edges. These are both glued in place and held in by wood screws for all areas where a panel is not removable. These braces will be used glue any sides or to eventually attach removable panels.
Some production cabinets use a Dove Tailed Joint - these remove parts of the wood in such a way that 2 sides fit into each other like a jigsaw puzzle piece. This is very strong but requires tools and the skills to do it. I have built cabinets like this before, but I no longer do. Unless you are using woods where you want to showcase the material, I would suggest the more common 'Butt Joint' with internal bracing.
One of the problems that you will encounter is the desire to use a router to round the edges of the cabinet after assembly, but wanting to hold the cabinet together during assembly by using nails or wood-screws in the corner edges. If you were to put nails or screws in the wood in a place where a router bit will need to shape the wood, its possible to damage the router bit, the cabinet, tear out the nail (or screw) and send it flying and/or get injured. None of these things are desirable. There are a number of options:
I tend to choose the last 2 options most frequently - it allows me to make certain that the box is actually square or rectangular before the whole thing sets up. Depending on how accurately you cut the lumber, it may be very difficult to square things up if you assemble too tightly.
The side effect is that the cabinets are fairly flexible during the early stages of construction and you need to have an area where you can set your cabinets aside to dry once they are squared up. This may leave gaps in some areas - all the more reason not to use a 'Dove Tailed' joint approach. Not all musicians are exceptionally skilled in the area of wood-working and carpentry, but most people can get things fairly close. If you plan to cover the results with indoor/outdoor carpeting, no one will ever see the added filler material you put on, and since you used screwed/glued internal bracing, the cabinet will be extreamly strong (stronger than many production cabinets, some of which are stapled together). Allow at least 8 hours for the glue to set before attempting any additional work on the cabinets.
Since you are working with wood, a good quality wood working glue should be used. Epoxy may seem like a good choice, but it really isn't necessary and it will end up slowing your construction efforts down.
A good brand (in the United States) is 'Elmers Glue'. There are 2 types that we are interested in:
Either will work fine and you will often find that some stores will carry their own store brand of the same sort of glue at a reduced price. Buy the one that is the best deal. I tend to use the Yellow Glue, but have used both and the end result is the same.
NOTE: Don't use the Elmers School Glue - this is for children's projects. It will probably work, but its not as strong as the other glues. Its designed to be easily washed out of clothes. It costs about the same.
If you plan to cover your cabinets later with indoor/outdoor carpeting, or fabric backed vinyl (such as Tolex), you should probably buy the quart (or 1 liter) size of glue. You will use a fair amount of glue assembling the pre-cut pieces for your cabinet. If you will be making many cabinets at once, you can get the largest size, but the container tends to get in the way when assembling things. I find that the Pint size (1/2 liter) to the quart size (1 liter) is the easiest to handle.
When assembling the cabinets, you will be applying a narrow bead of glue to the wood bracing and sides, so you may want the glue to be in a bottle with an narrow applicator tip. When covering the cabinets, you tend to pour glue onto the wood, so a 1/2 inch (13mm) or larger opening in the glue bottle will be necessary. if in doubt, buy glue in 2 containers - A small one and a large one. You can always refill the smaller one from the larger one.
You can mix the 'Elmers White and Yellow Glue' together - it won't hurt anything (I do this when I run out of one or the other, but still need to finish something right now). The Yellow Glue is stronger and is designed to be sand-able, but for our application, the White Glue is plenty strong and we won't be doing much sanding.
A 'Butt Joint' is the simplest. All you do is place the end of one piece of wood up against another. Its also the weakest - which is why the bracing is added. It requires no special tools or skills and helps make the cutting of parts and assembly go much quicker. You can use other joints if you have the desire, tools and skill, but I won't cover them in this write up.
Each Butt Joint needs to have a brace. How long should it be? It depends on how you plan to cover your cabinets. If you are planning on painting them black, then the sizes are just the actual dimensions of your lumber. If you are going to cover them with a thick material, like indoor/outdoor carpeting, you need to add up the additional thicknesses of it that occur at any joints of junctions of removable panels. Grill Cloth (as used on the front of some speaker cabinets) thickness must also be accounted for, otherwise you won't be able to assemble front boards without tearing it.
You need to decide how you plan to cover your cabinets/cases and come up with a consisitant plan and method - this will allow you to decide what works best for you in the long run.
NOTE: The Front panel shown here is not covered in any form of thick material - Grill Cloth might be used in this configuration.
While many people build cabinets with a Full Wrap, it adds an additional thickness to the panels that you need to compensate for. It also prevents wood panels from pressing up directly against wood braces - since carpeting can collapse over time, if you don't get a wood panel to wood brace junction, your panels will become loose and leak air (a bad thing for certain types of Speaker cabinets). I find it and more reliable to use One Side Wrap and not go thru the additional steps of compensating for additional thicknesses of carpeting. I strongly suggest using One Side Wrap for all panels covered in carpeting. For thinner covering (such as vinyl/Tolex), I suggest using a Full Wrap.
One thing that you'll find is that these cabinets use an awful lot of internal bracing material. Consider that you need pieces for the front, back and sides. I typically buy 1 1/2 inch (approx 36 mm) wide by 3/4 inch (approx 18 mm) Furring Strips and cut them down the middle with a table saw or band saw. This gives me 3/4 inch (approx 18 mm) square bracing strips. The edge you cut doesn't need to be perfect, you will have 3 other sides of the bracing that should be flat. Pine furring strips are usually fairly cheap to buy at the lumber yard (they are not high grade lumber).
The next step is to figure out the inside measurements. You have to account for the front panel and rear panel (if this is a speaker cabinet, this would be the Grill and the Back), as well as any material thickness (such as indoor/outdoor carpeting, which is 1/8 of an inch - approx 3 mm - thick) that might be used on the cabinet or case.
I like to use some scraps of indoor/outdoor carpeting and scrap material used for the front/rear (or Grill/Back) to lay onto the sides of to see just how much room and thickness is taken up by everything. You have to account for any material that will have eventually have multiple thicknesses of indoor/outdoor carpeting (an example of this would be where the rear panel has carpet on it, and the bracing has carpet on it. Minimally, this is to layers, but if the carpeting wraps around to the back of the rear panel, it now becomes 3 layers).
Once you are sure where the braces will go, you can cut them to size. Always start at the braces that are used on the 'Butt Joints. These should be as long as they need to be to reach the the full length of the inside minus the space taken up by the front/rear panels. Pre-drill for the wood screws to prevent the bracing from splitting as you put the wood screws into the corners. You need to drill 2 sets of holes since you are attaching 2 sides together. Make sure that you have at least 1/2 inch (13 mm) between any drilled holes, and that you are drill closer that 1 inch (25 mm) to either end of the brace. Always use at least 2 screws per side attached (that would be 4 screws total) - 3 per side (6 total) is preferred.
You must be careful not to get wood screws that are too long, otherwise, you will get part of the wood screw popping out of the cabinet. When I make these cabinets using 3/4 inch (18 mm) braces and 5/8 inch (16 mm) side material, I use 1 1/4 inch (approx 31 mm) wood screws. These are long enough to hang onto the wood but too short to stick out the other side once screwed down.
NOTE: DO NOT pre-drill the screw holes into the side material - this will simply make them come loose and have your case/cabinet fall apart.
If you plan to put wheels/casters on the bottom of your cabinets, you really need to add additional wood to the side panels where these will screw on. If you don't do this, you are likely to find that your cabinets start falling apart in where the wheels/casters are attached. The internal damage to the bottom material is often difficult to repair after it has occurred.
Fortunately, this is quite easy to deal with. Simply cut a piece of plywood that is at least 2 inches (5.0 cm) larger (in both directions) than the screw plate of the wheels/casters. It can always be larger than this, but it should never be smaller. Glue this in place and attach with wood-screws as you would the point supports/braces. You will add one of these for each wheel/caster that you plan to apply. I sometimes cut a plywood piece that fills the entire end (both corners), rather than 2 separate pieces of plywood for that end.
© 1999 Shavano Music Online