This page will touch on a few new topics (all of which are covered in detail on upcoming pages). It will attempt to help you get what you want from your audio system and your subwoofer in particular.

Make up your mind: Many people don't realize that, unless you have virtually unlimited resources, there are going to be trade offs when designing a system. If you want a system to produce a lot of SPL (but have limited power or number of speakers), you'll probably have to concentrate a lot of the acoustic energy within a small band of frequencies. This would allow you to generate the desired SPL and properly impress your friends. It would, however, leave you with a system that would have less than perfect frequency response and marginal performance at the low frequency end of the spectrum. You could design a system that would produce the desired SPL at all frequencies but it would require significantly more power and speakers than the aforementioned system with the narrow peak.

Manufacturers Recommendations: Many manufacturers recommend an enclosure that's going to give you a peak like we mentioned earlier. This will give you good results for rap or other bass heavy music. If you listen to those types of music, this system may perform satisfactorily. If you listen to all types of music, you'd likely want an enclosure that would give a flatter response. To get a flatter response, you could increase the size of the enclosure but the overall response wouldn't be perfectly flat. To build a system that had a perfectly flat frequency response (generally desired by audiophiles), you'll have to take the vehicle's transfer function into account (this will be discussed later). When a manufacturer recommends a particular enclosure, you should ask yourself (or, better yet, the manufacturer) what the enclosure is optimized for.

Enclosure Types: When laying out a system, you need to decide what type of enclosure to use. No enclosure is magic.

A sealed enclosure will be the smallest (for a given response shape) and will have good low frequency extension but may not have the best low frequency extension. If space is limited, this may be your best choice.

A ported enclosure will generally have a better low frequency extension for a given response shape (alignment) but would require a larger enclosure. If you made the enclosure as small as the sealed enclosure but ported it to gain the low frequency response, the output would deviate from the desired flat response.

A bandpass enclosure can sound good and give you a flat response but most of the generic bandpass enclosures are not designed for a flat response. They are designed to impress you in the stores. This means that they are built to produce a large peak at some frequency near 60hz. These enclosures will work well with something like rap music but generally won't sound good with other types of music. If you use a bandpass enclosure, it should be designed specifically for your speakers.

If you're building your first system, I'd recommend a sealed enclosure. It is the simplest enclosure and will be the easiest to get right. A sealed enclosure only needs to be the right size and well sealed. Alright... Enough of my opinions. Move on to the next section.

Calculating Speaker Enclosure Volume

As you already know, for speakers (especially woofers) to work properly, they must be in the proper size enclosure. The manufacturer can give you the required enclosure volume but can't give the exact dimensions of an enclosure that will work in all vehicles. So that you can build an enclosure of the proper size for your vehicle, this page will explain how you calculate the total volume of the enclosure.

Magic number:

OK... It's not magic but the number is 1728. If you forget it, just remember that the number is 12 inches * 12 inches * 12 inches. 12*12*12=1728.

Square or Rectangular Enclosures:

These boxes are the easiest to calculate the internal volume. You simply measure the height, width and depth (in inches), multiply them together and then divide that number by 1728. If the box has internal measurements of 6" high*18" wide*12" deep then the volume of the box is 1296/1728=.75 ft^3. The diagram below shows how you would measure the dimensions of the box.

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Warning:

Use the following calculators at your own risk. Since I've never seen two port calculators that would give the exact same port length for a given box volume and tuning frequency, the port calculations from this calculator will probably be slightly different than other calculators. Do not spend large sums of money on exotic enclosure materials until the enclosure has been fully tested with a simple MDF enclosure.

If you want to compensate for the volume taken up by the woofer, you can use the following approximations. Keep in mind that these are for 'normal' woofers. If you're using a competition woofer with a huge frame and magnet structure, refer to the manufacturer for the actual displacement volume of the woofer. Even if you're not using competition woofers, most high quality manufacturers provide this spec in the woofer's datasheet.

8 inch	.03 ft3
10 inch	.05 ft3
12 inch	.07 ft3
15 inch	.10 ft3

Use this calculator to determine whether your speaker is best suited for use in a sealed or ported enclosure.

Triangles without right angles: The easiest way to determine the volume of boxes similar to those below is to split the cross section so that you end up with 2 right triangles. Then you simply do as in the previous example.

Combination boxes: Some boxes are a combination of rectangles and triangles like the box below. You can see that you simply split up the box into manageable shapes and do_the_math.

MDF Medium Density Fiberboard is a compressed type of 'wood product' It is similar to particle board but is much 'friendlier' to work with than is particle board. The wood cuts with less dust and leaves a good clean cut as is seen below. It also resists chipping when screwing close to the edge. You should still pre drill the screw holes when assembling a box with drywall screws because the wood will split if a screw is driven into the end of the board. Many people (myself included) use a pneumatic stapler and a good quality wood glue to assemble boxes. Some people use liquid nails type adhesives to seal the joints but the solvents in construction adhesives can soften the adhesives used on some speakers (which could lead to premature speaker failure if the speakers are installed before the adhesive has dried completely). You should also realize that the fumes are flammable (and may be explosive when contained). If you have a loose speaker connection on the speaker terminals, you may have a fire/explosion hazard if the speakers are played before the solvent has fully evaporated. Silicone adhesive has acetic acid which is released as the adhesive cures. This acid will corrode speaker baskets if the speakers are reinstalled before the silicone has completely cured. The best way to make sure the enclosure is sealed is to make good quality cuts. It will take less time to make good cuts than it will for the sealant to dry/cure (24 hours). Box building samples This picture shows what the screw heads look like when they are countersunk into the face of the board. The counter bore recess was simply drilled to approximately 1/8 inch in depth at the center of the hole. The drill bit was just slightly larger than the head of the screw. This greatly reduces the chance of chipping the edge of the wood. The holes were also predrilled with a 3/32" drill bit to prevent splitting the wood.

This is the end of the board where the screw holes were predrilled. You can see that the wood didn't split.

This is the what the screw looks when you don't countersink the screw holes.

This is what can happen when you don't predrill the holes.

This is what the edge of the box should look like. There should be no gap between the two pieces of wood. The strength of the enclosure depends largely on the accuracy of the cuts. Wood glue is not designed to fill large gaps.

The picture below shows two different types of drywall screws. The coarse threaded screw is, IMO, a better screw for box building. They go in more quickly and don't strip out as easily but may be more likely to cause the wood to split. Try both types and use the one that works the best for you. I used the coarse threaded screws in the previous pictures. The galvanized screw below is a #6 drywall, and is 1 5/8 inches long.

Sealing around speaker: If the speaker has no gasket and the speaker box isn't covered in carpet or vinyl, You can use an open cell foam weather stripping around the cutout in the baffle. The weather stripping should be about 3/8 to 1/2 inch wide and 1/2 inch thick. The weather stripping in this example is 3/8 inch thick and 1/2 inch wide. You need to make sure that the area around the cutout is clean and dry so that the weather stripping will stick. I recommend wiping it down with solvent and allowing it to dry before applying the weather stripping. If you REALLY want it to stick, apply a single coat of contact cement to the area around the hole and allow it to dry for 10 minutes or until it no longer sticks to your fingers when you touch it. When applying the weather stripping to the coated area, you get only ONE chance to lay it down in the right place. As soon as the weather stripping touches the contact cement, it's not coming back up.

This is the weather stripping applied around the cutout. I didn't go all of the way around for this example but you DO have to go all of the way around.

This is a closer view of the same thing.

This shows the adhesive backing. Some weather stripping has a really thick backing that won't allow you to bend it around the cutout. You need to get something that is similar to what is shown here. .

Terminal Cups: If you want some nice terminal cups for your enclosure, the following are available from Madisound. They will accept large gauge wire or banana plugs.

Close-up:

Front and Back:

Circle Cutting Jig

This is the base for the template cutter. The center pivot will be the anchor for the circle cutter jig. The blocks on the bottom of it are to be clamped in a Workmate or other vise.

Top View
Edge View

This is the circle cutter jig. The router mounts to this piece. The three holes are drilled to match up with the holes on the router that you are using. The holes that are lined up on this diagram are to make templates with different diameter holes.

This diagram shows the router mounted to the circle cutter.

This is what a template will look like. The holes match up to the mounting holes for the speaker. The center hole is slightly larger than the final speaker hole that you need. I will explain later.

This shows how the router and circle cutting template go around to cut out the template. Router not shown.

This is how the edge of this thing looks. The template base mounts into the vice. The sacrificial piece is on top of the template base. It will allow the you to make numerous cuts without damaging the base. The circle cutter goes on top of the sacrificial piece. If you look at the yellow circle, you can see that the router bit goes through the template being cut and into the sacrificial piece of wood. Before you can put the piece of 1/4 inch plywood or MDF that is to become the speaker template onto the template base, you will have to drill a 1/4 inch hole in it so that it will be able to fit over the pivot dowel. After putting the template_to_be over the dowel, you will have to bolt it down with a few #6 X 1/2 flat head screws. If you make the mounting holes in the same place as the holes in the speaker, the same holes can be used to mount the template to the speaker box. You will have to countersink the screw heads so that the circle cutter can pass over them.

Crude representation of a flat head screw.

This shows how the guide collar fits onto the router. The collar is slightly larger than the cutter. This is why the speaker template hole has to be slightly larger than the actual speaker hole. The router's collar will dictate the actual size differences between the two holes. The collar is installed after the router is removed from the circle cutter jig. If you have more than one router, it will save a lot of time. You may want to look in pawn shops as a cheap source of routers. Before buying from the pawn shops, check the prices in the regular outlets. Many pawn shops have really high prices but some are really cheap so do your 'home work'.

This is how the speaker template will go on the box. You might want to center the template a little better than I did though. :-)

This is a side view of the template, router and speaker box. You can see how the collar depth is the same as the template material. The collar rides around the inside of the template. The router bit goes through the baffle of the speaker box and cuts the hole. A plunge router will make getting the hole started a lot easier but I was too lazy to draw a plunge router.

Now, I know this looks like a lot of work but after you make the template base, the circle cutter and a few templates, you can cut perfectly round smooth speaker holes very quickly. I can cut out holes for a tweeter, woofer and port in approximately 20 seconds using this type of template and a large Makita plunge router. If you make all of the 8 screw holes in the template, you can mark and predrill them so that the speaker can be mounted perfectly straight the first time. Make sure to use router bits with carbide cutters. MDF will cause high speed steel cutters to die horrible painful deaths. When cutting the holes, keep forward pressure on the router until the cut is complete. If you stop in the middle of a cut, the vibration may cause the router bit to break.

Note: If you only need to cut a couple of holes, you can use the circle cutter template without the template base. You would screw a long screw through the circle cutter jig, through the baffle board and into a sacrificial piece of wood. Of course, this would have to be done before the box is assembled. With this method, you don't have to use the guide collar and therefore don't have to compensate for the collar's diameter. You would cut the hole to precisely the desired final size. You must also clamp or screw the baffle board down so that is doesn't move as the cut is finished.

Speaker Enclosure Bracing

Most speaker enclosures will benefit from bracing. The diagrams below will give you one example of enclosure bracing.

This is the bracing as viewed from the baffle (where the speaker is mounted). The baffle board and the speaker are obviously not shown. Notice how the brace ties the top of the box to the bottom of the box. This stops the top and bottom of the box from moving along axis 'A'. The brace also connects the sides together. The horizontal part of the brace stops the sides from moving. The open areas of the brace allow the air to move freely through the box and reduce the airspace taken up by the brace. The crosspieces don't have to be really thick because the wood that makes up the brace will not stretch or compress.

 

This is the side of the box with the right side removed. You can see another brace. This brace stops the back of the box from flexing. The back of the box is tied to the vertical brace. When these braces are glued together, the sides and back of the box will be extremely rigid and significantly reduce the resonance in the walls of the box.

 

This is the top of the box. This is simply another look at the bracing.

This image shows their placement in the baffle board. The dashed lines show the diameter of the hole drilled to accommodate the T-nut.