DIY Audio Speaker Box Building Guide / FAQ
First, you obviously need your speaker drivers. These will determine the size of the box. They will also determine your basic budget, since most of the other costs are fixed. Next, you will need 1 or 2 sheets of MDF. Remember, you will need extra wood to brace the box. Other necessities: screws, wood glue, polyfill to stuff the box, silicon to seal the edges (optional), and wood sealant to seal the wood itself on the inside of the box. Any wood is porous to some degree so some like to paint the inside of the box with some type of sealant. Warning: Silicon fumes can eat away at the speaker. If you do seal the box edges with silicon, allow it to dry at least overnight before adding the drivers. You will also need wire and speaker terminals (the plate mounted on the box where you connect the wires to), and a crossover. The crossover materials will be handled separately. Other materials may be necessary depending on the type of finish you want on the box. I am using finished 1/4" oak veneer glued directly onto the MDF, with a stain and polyurethane. If you choose this method, you will want to test the stain and the poly first on a separate piece of wood to make sure that they don't react. Black high gloss paint also looks nice. The MDF provides a nice smooth flat surface for a glossy paint finish. For tools, you will need a drill, a table (or circular) saw to cut the wood, a jig-saw to cut the larger holes, and a router to help countersink the speakers.Why use MDF?
The speaker box should be constructed from Medium Density Fiberboard (MDF). See the MDF Board FAQ for more information on obtaining and using MDF, along with safety issues. You should use minimum 3/4" MDF which is sold in 4x8 sheets. You will probably need 2 sheets. This can cost up to $40 a sheet, but it is worth it. MDF is a somewhat like particleboard in that it is made up of wood fibers glued together. The difference is the density of the fibers in the wood. Fiberboard is denser, harder, and stiffer over plywood and particleboard. It is a dead wood that won't resonate. It does not have the problems of screws being easily pulled out or easily warping when exposed to water like particleboard. Compare the top sheet, which is MDF, to the bottom sheet of particleboard. You can see that the MDF is much more dense.
Look up the manufacturer specs for the optimal volume for each driver and the displacement for each driver. If this information is not provided, use the Speaker Box Volume Calculator to calculate the speaker box volume based on the driver specifications. Any dome tweeter will not need its own chamber because it is independently sealed. Add to the optimal volume the amount of space that will be displaced by the speaker and the space taken up by bracing. Use the Speaker Driver Displacement Calculator if you were not provides specs for driver displacement. From this volume, calculate the dimensions of the enclosure. Remember that these are the internal dimensions, and to add the thickness of the MDF to get the outer dimensions. Also, remember when using butt joints that only half of the time will the sides of the box be cut to the size of the outer dimension. Use the Speaker Box Volume Calculator and most of these calculations will be done for you automatically. Also be sure to read the Help for the Speaker Box Volume Calculator for more information.How do you screw together the box?
Obviously, if you are attempting a project of this magnitude, you know how to use a screw driver. Still, there are some important issues dealing with box assembly. The first is selecting the proper screw. I would suggest a #8 wood screw at least 2 1/4" long, with the first 3/4" of the screw having no threads. This is required so that the threads cannot hold the two pieces of wood apart. There will be no threads in the first piece of wood, and when tightened, the screw can pull the two pieces of wood together. Also, look for a screws that uses a square head (Robertson) bit. These work much better when using power tools, and are less prone to slipping. There are also screws that are prelubricated, which decrease the chance of wood splitting and allow the screw to go in easier.
You should have both a square drill bit for a drill and a screwdriver (probably size #2). You should always drill a hole first before putting in the screw. The hole should be about the size of the screw without the threads. I also like to put a little wood glue in the hole and on the screw before putting in the screw, but this is not necessary. But, you should definitely put wood glue on the edges of the wood before putting pieces together. When the glue dries, it will become as strong as the wood itself, to the point where you could remove all the screws and the box will still stay together. You will probably want to counter sink the screws for a flat surface, and this will not be possible without drilling a counter sink hole. MDF is too strong to counter sink a screw by strength alone. There are special counter sink tools, or you can use a second drill bit the size of the head of the screw. Note: For a more finished look (without gluing on veneers), some people will build speaker boxes with wood glue alone. This will work fine, but you will need some long clamps to hold the wood in place until the wood dries.Why is bracing necessary?
Bracing refers to the support structure inside the box that prevents the box from flexing. When a speaker box flexes or vibrates wildly it causes distortion. This distortion can be eliminated by simply adding a brace or two inside the box. Just remember to subtract the volume of the wood used in the bracing from the total volume of the box.How do you brace the box?
Here are a couple of ways a speaker box can be braced.
- small triangular wedges connecting two sides in the corners
- small triangular wedges connecting two sides in the corners
- a rectangular piece of wood connecting 3 sides of the box
- a rectangular piece of wood connecting all 4 sides of the box with several circular holes cut out
- a rectangular piece of wood connecting all 4 sides of the box with a single large rectangular hole cut out of the middle
When cutting holes in the bracing, leave at least 1" between the hole and the edge of the brace, and when using multiple circular holes it is best to have the holes staggered for more strength. Also, don't create a brace so big that it will restrict the airflow of the speaker. Basically, make the holes big enough, or use a lot of them. Finally, notice the double layer of wood used for the baffle where the woofer will be mounted, for even more durability where it is needed most.What are battens?
A batten is another type of bracing that is used to secure all of the edges of the box. It is a thin piece of wood that runs along the joints of the box, as seen on the left. You can usually get strips of wood in 1" x 1" or 3/4" x 3/4" sizes in either pine or hard woods. The batten does not need to run the entire length of the edge. It is fine to cover 2/3 of the edge with the batten. Screw the batten into both sides of the box, making sure that you don't hit another screw. It is not necessary to countersink these screws. You should use a #6 1 1/4" screw for the batten, which will allow the screw to go deep enough into the MDF without coming out the other side. Again, pre-drill the hole and use plenty of wood glue.
This is an optional step. Proper bracing is much more important than sealing the box, and this is usually reserved for large woofers. Once the box is assembled and braced, silicon all the edges in the box including the batten edges, as seen on the left. Remember that silicon fumes can damage the speakers, so wait a day to put the speakers in the box after doing this. The silicon will prevent any possible air leaks the may exist at joints. Still, the wood itself is going to be porous to some extent. You may want to seal the inside of the box with some kind of water seal. I have also used fiberglass resin to do this. Now, the only place air can escape is around the speakers and around the speaker terminals on the back of the box. You should use a gasket for both of these. Basically, the gasket is a rubber/plastic ring that goes around the hole. The speakers and terminals should both come with gaskets, but if not they can be found on-line.Why do you want to flush mount the drivers?
Be sure to flush mount all of the drivers. Not doing so will cause small spikes in the frequency response produced directly in front of the speaker. For that matter, any protrusions on the surface of the speaker box can produce these spikes in frequency response. There are two ways to flush mount a speaker. The first is to use a router and cut a groove around the outside of the driver hole that is as deep as the driver edge. The simpler method is to add another piece of wood of the right thickness with holes for the drivers directly over the baffle. Be sure to do this correctly. If the front of the driver is not flush with the baffle, then all this work is wasted. Another way to cut down on noise spikes on a rectangular box is to round all the edges of the box. Usually this is impractical with MDF. It also makes laminating or veneering the box more difficult. One option is to build most of the box out of MDF but put a full 1" thick hardwood on the front surface of the speaker. Then veneer the rest of the box with the same hardwood for a matched look.Driver spacing?
Try to keep the dimensions of the mid and tweeter chambers less than the length of a wave at the crossover point. The table below lists wavelengths at different frequencies. Obviously, if you do this, then you can't use same depth for woofer and mid chambers. You should also keep the centers of 2 drivers within the length of a wave at the crossover frequency. Also note that a speaker producing frequencies of a wavelength less than its effective diameter will be very directional at those frequencies.
|5000 Hz||2.7 in|
|3000 Hz||4.5 in|
|1500 Hz||9.0 in|
|750 Hz||18.1 in|
|500 Hz||27.1 in|
|300 Hz||45.2 in|
|200 Hz||67.8 in|
|100 Hz||135.6 in|
Polyfill is basically fiberglass, like the pink insulation stuff. There are variations of polyfill, like "Polyfil", that can be found in arts & crafts stores. "Polyfil" will work almost as well as fiberglass, but without the nasty problems you run into when working with the pink stuff. By using polyfill in a sub box, it will artificially make the box act larger than it is, which will improve the deep bass response. Note that actually making the box larger than the driver is designed for will make it sound like crap. The speaker manufacturer or distributor will be able to tell you how much you should use. Otherwise, the only way to find out is by trying it yourself. Add about 1/2 pound at a time, until you like what you hear. When using polyfill, try to spread it out evenly, and try to make sure it doesn't touch the driver. If you are using a ported box, then only line the sides of the box with a 1" thick sheet of polyfill. You can buy polyfill that comes in sheets or loose.What are toe spikes for?
When the woofer is located near the floor, there will be more bass due to reflection and vibration of the floor. Sometimes, this extra bass is desired, but it is not accurate reproduction of the input signal. The easiest way to eliminate the vibration is to put the speaker on something that will not vibrate at all, like a rock. A more realistic solution is for the speaker to make direct contact with the floor by way of a spike. If a spike is used, then the speaker will rest on the subfloor, not the rug, and the speaker will not be able to vibrate as much. This will eliminate much of the extra bass.