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Car Audio Subwoofer Enclosures

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.

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. Some people believe that a certain type of enclosure can magically increase the output of a woofer. The best an enclosure can do is to allow the woofer to produce what it's capable of producing. A poorly designed enclosure essentially works against the speaker to prevent it from producing its maximum output. A good enclosure works with the speaker to allow it to produce its maximum output. It's also a bit of a myth that bandpass enclosures make woofers produce far more output than other enclosures. They cannot. Look at it this way... If you had a bucket of sand, you could pile it high or spread it out over a wider area. It can't be both tall and wide. There's a limited amount of sand. It's essentially the same with speakers. You can build a bandpass enclosure to produce a tall peak or a wider flatter response. You can't have both.

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.


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.


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 = 0.03 ft3
10 inch = 0.05 ft3
12 inch = 0.07 ft3
15 inch = 0.10 ft3

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

Speaker's Resonant Frequency (fs)? Hertz
Qes?

EBP =

In the calculators that follow, if you are using internal measurements, make sure the wood thickness is set to '0' (zero).

This is for rectangular sealed enclosures.
Height? = Inches
Width? = Inches
Depth? = Inches
If the above dimensions are OUTSIDE dimensions, enter the thickness of the wood below.
Wood Thickness = Inches
 
Total box volume = Ft3


Enclosure Calculator:
This is an enclosure calculator for sealed enclosures. It is designed to help you calculate the size of the pieces of MDF that are needed to build an enclosure.


Click HERE to make this applet fill this window.


For those who are new to this site and have found this page via a search engine, this page is part of a larger site. If the site is properly loaded, there will be a directory to the right of this page. If there is no directory, click HERE to load it properly. This page is #110 in the directory.


Triangular Enclosures:
The first type of triangular box has a right angle which will make the calculations a little easier. For a right triangle, you simply multiply the height by the depth by 1/2 of the base. The diagram shows the height and depth (base). The length is the dimension not shown on the diagram. You can see why you use 1/2 of the base by looking at the diagonally bisected rectangle and how it relates to the triangle.

This is for sealed triangular enclosures.
Height? = Inches
Width? = Inches
Depth1? = Inches
Depth2? = Inches
If the above dimensions are OUTSIDE dimensions, enter the thickness of the wood below.
Wood Thickness = Inches
 
Total box volume = Ft3


The next program will calculate the proper port length for a given box volume at the desired port tuning frequency.

  • Suggestions For Using This Calculator
  • If you pick a tuning frequency above the lowest frequency that you intend to drive into the speakers, the speaker may well be damaged due to the lack of damping. Below the tuning frequency, all of the control will depend on the speaker's suspension. If you drive the amplifier into clipping, the problem will be even worse. If you're not sure about the proper tuning frequency, use a sealed box or at the very least keep an eye on your woofers for the first few times that you drive them hard with very low notes (and for the idiots out there... DON'T put your eye on the woofers).
  • This program automatically subtracts the port volume from the internal box volume.
  • You can not have a port length shorter than the thickness of the wood that you're using.
  • I don't recommend using a port diameter of less than 3 inches for subwoofers.
  • The port length must be short enough to allow a clearance of the port's diameter between the end of the port and the wall of the box. If you're using a 3 inch port, allow at least 3 inches between the back end of the port and any wall of the enclosure.
  • If the port length is negative, the tuning frequency needs to be lower or the port diameter needs to be smaller.
  • If you want to use square ports, use a port who's height multiplied by its width equals the 'port area' on the calculator. The port length will be the same as the round port.
  • If you have or need an enclosure with all right angles (no slanted sides) set depth1 and depth2 to the same value (whatever the depth of your box is).
  • This calculator subtracts the port volume from the enclosure volume then re-calculates the port again for the smaller enclosure volume. This means that there are many calculations that are not seen in the form's output. If any of the internal (hidden) calculations returns an invalid value, you will get an error message and suggestions to help remedy the problem. When there is an error message, disregard all of the output calculations.

'Gross' volume is the internal volume of the enclosure before the port volume is subtracted. If you're using external dimensions and you set the wood thickness to something other than '0', the wood thickness will be subtracted.
'Net' volume is the internal volume after the port volume is subtracted from the gross volume. If your speaker needs an enclosure that's 1.5 cubic feet ported at 35Hz, the 'net' volume must be 1.5 cubic feet for proper operation.
This is for PORTED enclosures.
Data Input:
Height? = Inches
Width? = Inches
Depth1? = Inches
Depth2? = Inches
If the above dimensions are OUTSIDE dimensions, enter the thickness of the wood below.
Wood Thickness? = Inches
Speaker Displacement? = Ft³
Port Frequency? = Hertz
Port Diameter? = Inches
Number of Ports? =  
Sq. Port Ratio H:W? = 1:  
 
 
Data Output:
Box Front Angle = Degrees
Gross box volume = Ft3
Net box volume = Ft3
Port Length = Inches
Port Area = Inches2
Port Volume = Ft3
Square Port Dimensions:
Height = Inches
Width = Inches
  • If the angle is a negative number, 'depth 1' is greater than 'depth 2'. The face of the enclosure would be past vertical.
  • The angle at the top of the enclosure (inside) is 180-'box front angle'. If the BFA is 45 degrees, the top inside angle is 180-45 or 135 degrees.



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 Enclosures:
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.


A partial list of updates added to the full version of the repair tutorial between February and June 2014.

  • If you repair the older BD type amps (BD1000A1 and similar), there have been significant updates with regards to simpler troubleshooting. The information allows you to troubleshoot with little or no risk of damaging components if you make a mistake. The troubleshooting is much more step by step than it was before.
     
  • For the relatively new amps like the T1500-1bd, new information and troubleshooting techniques. These allow you to test the drive circuits with virtually no chance of causing damage to the output transistors if there is some un-expected problem with the drive circuit.
     
  • There have been significant updates to the Basic Class D Troubleshooting page. Significantly more information has been provided for the protection circuits as well as other troubleshooting. This applies to many of the Hifonics and similar amplifiers that use the HIP4080 driver IC.
     
  • Many of the graphics (static and interactive) have been updated/replaced/added. Many make it easier to understand the related text.
     
  • Many of the various Tech Tips have been updated. Some new ones added.
     
  • The older Rockford amp page (old style punch 45/150, etc...) has been updated with new, cleaner graphics as well as other information.


Enclosure Construction Suggestions

Before you build an enclosure, you may want to draw it out. This can help prevent making mistakes when cutting the various panels. It also allows you to double-check measurements, which can be taken directly from the scale drawing. For those who are willing to take a few minutes to read through a basic tutorial, you will be able to use Sketchup (a free 3D drawing/modeling application). It's on one of my other sites (asos1.com). The tutorial page is #10 in the directory. The following show enclosures drawn in Sketchup. For those willing to take the time to learn the software, it could prove at least moderately profitable. There are a few people getting paid to draw enclosures on the various car audio forums. Even if you only charge $5 each, you can make an extra $15-20/hour in your spare time. Most enclosures can be drawn in 15-20 minutes.

Borrowing Tools:
Below, you will find suggestions that require tools that you may not own. If you plan ahead, you may be able to find friends or family members with tools you can borrow. If you borrow tools like jigsaws, routers, belt sanders, buy new blades, bits and belts in case you damage them. If the blades, bits and belts are in good condition when you finish the job, return the items you purchased with the tools. When you return the tools, make sure that they're at least as clean as when you borrowed them. It's difficult to find people that will let you borrow good tools because they are rarely returned in good condition. In some instances, people may have tools and won't be willing to let you borrow them but may help you do part of the job. For example, you may not have a table saw. A table saw makes much better cuts (generally) than a circular saw. It may be possible to get the owner of the table saw to help you cut your panels to size.

I'd suggest that you NOT borrow tools from people who earn a living with those tools. If you borrow a circular saw from someone who needs it to make a living and you damage it, you could cost them quite a bit in lost income.

MDF:
Medium Density Fiberboard is a compressed type of 'wood product' It is similar to particle board but is much nicer to work with than particle board. When cut, the edge is not badly pitted like the cut edge of particle board (see below). It also resists breaking out 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 an edge near the end of the board. Many people use a pneumatic stapler and a good quality wood glue to assemble boxes.

The image below shows the cut edge of MDF (top) and particle board.

When cutting the pieces for the enclosure, you may be tempted to simply use the thickness specified by the manufacturer. In most cases, MDF is thicker than the stated thickness. The following is 3/4" MDF. It should read 0.750. It reads 0.761 in the dial indicator. If you make the cuts assuming that the MDF is exactly 3/4" thick, the pieces won't fit properly. For most newbie box builders, this isn't a concern because the cuts will not be very accurate but if you're using good tools (table saw with a solid fence) and are careful when measuring and marking, this will make a big difference.

Stapler:
Previously, staples were suggested as fasteners. The stapler below is an example that works well. This is a Senco model SKS stapler. This is a professional quality stapler which means that it's a bit more expensive than the run_of_the_mill staplers you see on eBay. It's important to know if a stapler can drive staples into MDF before buying it. Many can drive their longest staples into plywood but can't fully drive them into MDF or particle board. This stapler was used to assemble home speaker cabinets from MDF when I was building speakers. The staples below it are the ones that were used. The yellow adhesive that holds them together melts as it enters the wood and increases their resistance to pulling out. These and wood glue in the seams make a very strong enclosure.

Adhesives:
It's important to make good, straight, square cuts when building a speaker enclosure. If the cuts are good, you need nothing more than a good quality wood glue. Adhesives like Gorilla Glue may be a bit stronger but since normal wood glue is far stronger than the wood itself, the added strength is useless. Standard wood glue is easy to work with and if you get it on your skin, it peels right off after it's dry. Gorilla glue may remain on your skin for days before it can be removed completely. When applying wood glue, apply a thin strip along the center of the edge of the board. If you apply the right amount, a 'small' amount will be displaced along each side of the board when the boards are fastened together. If too much is displaced, apply a bit less. If there are places where none is displaced, you need to apply a bit more. You can remove the displaced adhesive with either a paper towel or simply with the edge of a piece of scrap wood.

You've likely heard that the glue holds strong enough that you could remove the screws and the enclosure would remain strong. It's true that the glue is that strong but the glue is only bonded to the surface. For material like MDF (which is essentially highly compressed layers of cardboard/paper), the surface can be pulled up fairly easily. The screws prevent this from happening and add significant strength to the enclosure.

If you make a slight mistake when cutting (often happens when using a circular saw without a guide) and there is a small gap that wood glue won't fill, you can mix sawdust and wood glue and use that to fill the gap.

The old type silicone adhesive/sealant 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. If you have to use silicone, use 'Silicone II". 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). If the cuts are good enough, you won't need any sealant other than the wood glue.

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 may well be flammable (and may be explosive when in a confined space). 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. Liquid nails is another substance that you don't want on your skin. If you remove it immediately with a solvent like acetone, you can get most of it off. If you let it dry, expect a week before it's completely gone.

While we're on the subject of solvents... Too many people use gasoline (petrol) as a solvent to remove adhesives and such. This is IDIOTIC. If you ever see a gasoline fire, you'll understand why. Gasoline doesn't burn like it does in the movies. Gasoline burns very quickly and very hot. Do NOT use gasoline as a cleaner.


This photo 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. Counter-sinking greatly reduces the chances of breaking out the edge of the board and it produces a better looking enclosure. The holes were also pre-drilled with a 3/32" drill bit to prevent splitting the wood.

This is the end of the board where the screw holes were pre-drilled. 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 pre-drill 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. Remember, wood glue is not designed to fill large gaps.


When building an enclosure, if you don't have a lot of experience, you probably won't make perfect cuts. If you have a choice of making the panels slightly shorter or longer, make them slightly longer. In the image below, you can see that the one that's slightly longer extends beyond the face of the enclosure slightly. This is correctable with either a belt sander or a router, using a bit with a guide bearing or bushing (the first router bit below is one example).

If you have trouble getting straight cuts, use as many of the factory edges as possible. If, for example, you can have a panel with one factory edge and one bad (poorly cut) edge or one with two bad edges, cut it so that you use the factory edge.

A belt sander was mentioned previously. If you've never used a belt sander, you should understand that it's not as easy to control as an orbital palm sander. When using a belt sander, you must have a good grip on it and be on good footing. If you're slightly out of balance, it can pull away from you. You should also understand that it can cut FAST. With a 40 or 80 grit belt, it can cut 1/16" off of your enclosure (or your skin) before you can react.

It's been suggested that you clean-up the glue that's displaced when you screw the panels together. This is especially important if you're going to have to sand the enclosure. The glue will clog the sandpaper and make it useless. This can happen with a new belt the first time you try to sand over an area with a lot of glue. You can sometimes clean the paper with a wire brush but the belt won't work as well and will likely have to be replaced. When you go to buy belts for your sander, you'll need to know the size. 3" x 24" is common but there are other sizes. If you don't kneed, bring the old belt with you. The size is sometimes on the belt but it's generally worn off by the time the belt needs to be replaced.

The image below shows two different types of drywall screws. The coarse threaded screw is, in my opinion, 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 images. The galvanized screw below is a #6 drywall, and is 1 5/8 inches long (the preferred screw for building enclosures).

When using screws with either phillips or Torx head screws, you need to make sure that you have a bit that fits properly (preferably one supplied with the screws). You also need to keep the bit inline with the screw. If you have a bit that fits properly and is inline with the screw, there is virtually no chance for it to slip. If it does slip, it will likely damage both the bit and the screw. If the bit is damaged, it may make it difficult to finish the job. It's good to have extra bits on-hand.

The following two images are of a countersinking bit. The first is the bare bit. When using the bit in this way, it's difficult to get consistent depth. The second image shows the bit with several washers taped to the bit. This make is really easy to get consistent depth on the holes.

It's possible to use a drill bit to countersink the heads (after pre-drilling with a 3/32" bit) but you must be very careful. Most drill bits tend to pull themselves into the material being drilled. It's very easy to go too deep when using standard twist type drill bits. Using bits with a flat cutting edge will allow you to drill without a risk of the drill bit un-intentionally going too deep. The bit below is a standard drill bit that's been ground to have a flat cutting edge.

Cutting Openings in Speaker Enclosures

Cutting Holes with a Jigsaw:
Most people, especially those building their first enclosure, will use a jigsaw to cut the holes for the speakers. Most anyone can cut a hole with a jigsaw but there are a few things that will make the job easier and produce better results. When cutting, don't get in a hurry. When building your first few enclosures, you just want to hear what it's going to sound like as soon as possible. This will generally mean that you will try to force the jigsaw to cut as fast as possible. This can lead to broken/bent blades and holes with sides that are bevelled instead of square (perpendicular to the top/bottom surface). If you take your time, it will take only slightly longer and the quality of the cut will be much better.

When selecting a blade to make the cutout for woofers, you should choose a blade that's relatively deep (front to back). This will make the blade stiffer which will help keep the blade from deflecting to the side (which causes the sides to be bevelled instead of square). The blades that are thin from front to back are better for cutting tight curves but you don't need that for cutting woofer holes. A blade with larger teeth with significant side-set will cut faster but may make a rougher cut (especially if you try to cut too fast). If you're not on a tight budget, buy a multi-pack of blades so you can try several (on scrap material) to see what works best.

A standard jigsaw simply moves the blade up and down. More advanced jigsaws have an 'orbit' setting. There are typically 3 orbit settings and one straight setting. The orbit feature makes the tip of the blade follow an oval path instead of going straight up and down. When making rough cuts, the orbit setting may be the best option but it's not generally the best option when making precision cuts. When using the orbit setting, the area in front of the blade will typically get chewed up pretty badly (tearing away the line that you're following). When using the non-orbiting setting, the line will remain largely intact until the cutting edge of the blade reaches it. Practice on a scrap piece of wood to see which works best for you. If you don't have much experience with a jigsaw, set the jigsaw to the straight setting.

Many jigsaws have a variable speed control. It's generally not necessary to use the jigsaw at full speed. If you have a sharp blade, the saw will be able to cut more quickly than you can react if the saw begins to go off of the line. Using a slower speed will make it easier to react before you go too far off of the line.

When marking the hole layout, you'll want something that will make a ledgible line in one pass. A red fine point Sharpie marker works very well. You'll need to go slow when marking (so the color remains intense) so you only need to make one pass. It's difficult to make additional pass precisely on top of the first pass which leads to multiple lines or a thickened line.

Cutting Holes with a Router:
Before we get into the use of a router, you should understand that a router can do a lot of harm VERY quickly. Too many people give too little respect to routers. When using a router, you should have your hands on both handles from the time you switch it on until the bit comes to a COMPLETE stop. Even if the bit has only a few more rotations before it stops, if you make contact with the bit, expect to lose whatever contacts the bit.

Using a jigsaw to cut holes for speakers works well enough for most people but if you want perfect holes, you have to use another option. For those who think that they can cut perfect holes with a jigsaw, you've never seen holes cut as described in this section. There are two common ways to cut perfect speaker holes in the baffle of a speaker enclosure. Some people use a circle cutting jig that mounts onto the router. These work well and can work for cutting holes in baffle boards but you have to be very careful at the end of the cut if you're cutting the holes on an assembled enclosure. If you have to cut a lot of holes the same size, I'd strongly recommend that you use the circle cutting jig to make a template and use the template to cut the holes in the baffle of your enclosure.

  • In this section there are three main components
  • Circle Cutting Jig:
    Mounts to the router base
  • Template Making Base:
    This is the base where you will screw down the template blank to be cut. It will also provide the pivot point for the circle cutting jig.
  • Speaker Cutout Template:
    The template is what you will use to very quickly cut holes in the baffle-board of a speaker enclosure.

If you're going to use a template to make the holes in the enclosure, you don't need to buy an expensive circle cutting jig. You can make one in about 10-15 minutes and since it's only going to be used a few times, it doesn't have to be anything special. It simply has to attach to your router base-plate and allow you to have a place where you can drill a hole to set the diameter of the hole that it will cut.

Router Basics:
You'll probably find more information on routers in this section than you'd expect on a page about speaker enclosures. That's because they're very useful, if you understand the basics and avoid the most common mistakes.

If you don't have a router, you should try pawn shops as a cheap source of routers. Before buying from the pawn shops, call the ones near you to see what models they have and get their prices. Then check the prices in the regular outlets. It's possible to get a good deal in a pawn shop but many pawn shops charge near-retail prices for used items so it pays to do your homework before buying from them. Of course, this doesn't only apply to routers. It's only mentioned here because it's the tool that you're least likely to have.

There are two different types of commonly available bases for routers. One is an adjustable, fixed base. This allows you to make adjustments to the cutting depth before the router is powered up but you can't change the depth of cut while the router is in operation. The router below has a fixed base.

The second type of base is a 'plunge' base (see below). This allows you to start the router without the bit protruding through the base. After the router is up to speed, you plunge the bit down into the material that you need to cut. The plunge base is required to use the circle cutting jig. You can use a router with either type of base when you have a template.

This shows how the guide bushing fits onto this particular router. This one is held in the base by two screws. Others are held in the base by a nut on top of the base. The outside diameter of this bushing is 3/8" (for a 1/4" cutting bit). This is why the speaker template hole has to be slightly larger than the actual speaker hole. The router's bushing diameter will dictate the actual size differences between the template hole and the speaker cutout hole. The bushing is installed after the router is removed from the circle cutter jig (for those circle-cutting jigs that mount rigidly to the router's base).

When using the guide bushing with a template, there are a lot of choices for router bits. Cheap bits are made of 'high-speed steel'. HSS bits will not last for more than a few cuts in MDF or particle board. If you buy an HSS bit to cut the holes in MDF for two 12" woofers, expect to burn through the last part of the second hole because the bit will have dulled.

There are three basic types of router bits. The first is the high-speed steel bit. They are the cheapest but also dull more quickly than the others. There are solid carbide bits. Carbide is much harder than high-speed steel and will hold a sharp edge much longer than a high-speed steel bit. They are also the most expensive of the three types. The third type has a steel body with a carbide cutter brazed to it (below). Steel bits with carbide cutters are typically cheaper than solid carbide bits and last about as long. If you're searching for a bit similar to this one include the following terms: plunge pilot panel router bit 1/4"

Using a large router, you can easily cut the entire thickness of 3/4" MDF in one pass. If you intend to do this, you need to make sure that the cutting edge is at least 3/4". The two most common choices for 1/4" diameter bits is 3/4" cut and 1" cut. If you have only a smaller router that doesn't have the horsepower to cut 3/4" material in one pass, you can cut in 2 or 3 passes, cutting deeper with each pass. Using a template or a good quality circle cutter will produce a perfect opening, even after multiple passes.

Note:
The bit above is only useful (for the purposes mentioned on this page) for plunging in and cutting boards that have space beneath them. This bit cannot cut when the cutting edge is only part-way through a board. Since there are shaft sections above and below the cutter, the cutter has to positioned vertically so that it has at least a tiny bit of the blade both above and below the top and bottom surfaces of the board.

For bits that cut straight slots, there are two main types. One is a bit with a straight cutting edge (like the one above). The other type is a spiral bit (below). It has a spiral cutting edge. The spiral bits are used where the edge finish needs to be nearly perfect (for cutting soft metals or plastics, like acrylic). For cutting MDF, straight bits will cut well enough. If you want a spiral bit, the Bosch 85903MC and the 85911MC work well and are readily available for about $20 each.

Spiral bits come in two styles, upcutting and downcutting. The upcutting bit pulls the waste material up and out of the slot. The downcutting bit pushes the waste material down. If you're using this to cut the holes in a baffle board that's mounted onto the enclosure, a downcutting bit will throw most of the sawdust into the enclosure (which can be easily vacuumed out). The upcutting bits are better when you're making a cut that doesn't go completely through the material or when you have a vacuum attachment on the router.

All of the bits mentioned/shown here are 1/4" cutting bits (they cut a 1/4" slot). Larger bits take much more horsepower to turn and make more of a mess (more sawdust). Smaller diameter cutters are typically easier to break.

Installing the Router Bit:
When inserting the bit in the collet of the router, you'll insert it until it bottoms out and then pull it out about 1/8". When you tighten the collet nut, the bit will go slightly farther into the router and you don't want it to bottom out. If it bottoms out, you may not be able to tighten it properly and the bit may slip while cutting.

Stuck Router Bits:
Some routers don't properly release the bits when you loosen the collet nut. In most routers (those without problems), the collet nut will initially loosen but the bit will still be held tight. You have to unscrew the collet nut a few more turns (where it will again become difficult to turn) and then unscrew until it again turns freely before the bit is released.

Some routers are more stubborn. When you loosen the collet nut, it pulls the collet out of the spindle and, when all is right, the collet remains free in the nut. In some routers, the collet gets wedged in the nut and the entire nut/collet/bit assembly has to be removed to remove the bit. When this happens, place the assembly on a solid surface. Place a deep socket (the type used with a ratchet) over the bit. It must be completely clear of the bit and must rest solidly on top of the collet. Tap the top of the socket to force the nut down. When the nut is free, the bit should pull out freely. It's important that you not leave router bits in the router for long periods of time. They can become stuck and be very difficult to remove.


Circle Cutting Jig

There are quite a few commercially available circle cutting jigs. The Jasper circle cutting jig is probably the most well known. Most of these jigs mount rigidly onto the router base. The following 'home-made' jig has a hole for the router guide bushing that's the same size as the outer diameter of the bushing. This keeps the router and jig together instead of screws and it allows the router to rotate freely on the jig.

The following image shows a more typical jig that mounts to the router rigidly. If you buy a jig like this, make sure that it has holes that will match the holes in your router base. Some routers have 3 holes and some have 4 holes in the base. Some jigs are made for either 3 or 4 hole routers but not both.


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It's not important that the pivot-point holes in the circle cutting jig are through the center-line of the jig. All that's important is that the hole is the proper distance from the bit.

When making a template, the hole in the template must be slightly larger than the required speaker cutout to compensate for the diameter of the guide bushing. The hole in the template must be the diameter of the speaker cutout plus 1/8" for a guide bushing that's 3/8" in diameter and when a bit with a 1/4" cutting diameter is used.

The circle cutter here is triangular in shape but the shape isn't important. It could have been left rectangular (with the corners slightly rounded to make them less likely to injure the user). If you want to make one from aluminum, you can buy 1/4" aluminum plate from eBay. You should be able to buy a pre- cut piece of a suitable size. If not, some of the sellers offer to cut to size for a small fee. If you can't find the size that you need, it can easily be cut with a jigsaw. This piece of aluminum was cut with a Bosch blade designed to cleanly cut laminate floors. The teeth have no side-set so it leaves a very clean cut. You will find photos of these when you visit the upcoming 'Working with Acrylic' page of the site.

When making the pivot-point holes in the circle cutting jig or when making templates, each will have to be made for each individual speaker for the best fit. You can probably make a generic 10" template for 10" woofers but if the hole is too large, there may not be enough material for the screws to bit into and will require a bit more effort to get the speaker perfectly centered. Some owners (those for whom you're building the enclosure) can spot a speaker that's slightly out of line and could reject the enclosure for that alone.

In the following demo, you can start and stop the rotation of the jig. Clicking the second button converts the jig from a rigidly mounted one to a floating jig. As you can see, the floating jig will make it easier to keep a secure grip on the router. It also makes it easier to keep the power cord safely out of the way.


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Note:
If you only need to cut a couple of holes, you can use the circle cutter without the template base. To do this, you would need to secure both the outer part of the baffle board and the center of the cutout. The center of the cutout will be secured when you put a screw through the pivot-point of the circle-cutting jig. The screw will have to go through the baffle board and into the board that you're using as a base (scrap wood or your workbench). This is for when you are cutting the baffle before you fasten it to the rest of the enclosure. If the screw through the center of the jig isn't a tight fit in the hole of the jig, you need to keep constant outward force on the router. This will make the hole as perfectly round as possible.


Template Making Base

To make the template, it's best to make a simple base (below is one example). The center pivot will be the anchor for the circle cutting jig. The blocks on the bottom of it are to be clamped in a Workmate or other vise. If you want to use this on your workbench and don't want to screw it down to the bench, you can use a material made by Duck (as in Duck Tape, the trade name for one brand of duct tape). It's called Easy Liner (photo on the Working with Acrylic page). This will keep the base from moving. If you have another way to keep it steady, you can use whatever works best for you.


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When you make the template, you should make the 90 and 45 lines before you drill the center hole that will go over the pivot. This will ensure that the screw holes align perfectly with the center of the template. It also gives you reference points when placing the template on the enclosure. You'd draw lines on the baffle of the enclosure, crossing at the point where you want the speaker to be centered. You'd then align the marks on the template with those on the enclosure.


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When selecting the wood to make the templates, don't use inferior wood. 1/4" material works the best. 1/4" MDF is difficult to find but makes nice templates. 'Good quality' plywood also makes good templates. Luan is OK but birch and oak (oak is preferred if you're going to use plywood) are a bit better. If you use cheap plywood, the inner layers become loose and dislodge making the inner diameter of the template uneven. If you're using a cheap plywood, apply a thin layer of glue around the inner diameter (wiping away as much as possible). That will keep the inner layer secured. When you buy material for templates, you don't generally have to buy a full sheet. Many of the home improvement stores have sheets cut down to smaller sizes that are less expensive and easier to handle if you don't have a truck to haul it in.

When marking the template, you should include the center points for the speaker mounting holes. While the template is on the jig, make a tool similar to the one in the following image. It's simply a thin (~1/16" thick) piece (metal, plastic...) that has a 1/4" hole to fit the template maker's center post and a second hole that aligns with the center of the speaker mounting holes. To find the center of the holes, you can measure from center to center of the holes on opposite sides of the frame and cut that in half. If it's easier for you, you can measure from the right side of the hole to the right side of the hole. That's sometimes easier than judging where the center of the hole is.

The template base should either be mounted in a Workmate type vice or otherwise secured so it can't move. When cutting, it's difficult to get the bit to cut completely through the template blank without cutting the base slightly. If you'd like, you can insert a sacrificial piece of wood between the template blank and the base. The cuts in the base won't be deep if you're careful and generally won't cause any problems. Remember, this is to make templates. You'll use the templates until they wear out or get damaged. 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 blank on the dowel, you will have to secure it with a few #6 X 1/2" or #6 X 3/4" 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 cutting jig can pass over them.


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For those who don't know what flat-head screws look like... You can also use drywall screws.

This is what's left after the hole is cut.


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When using t-nuts or when pre-drilling holes for speaker mounting screws, you need to confirm that the dust cap logo is aligned properly with the screw holes. If the logo is square to the holes, you can pre-drill the holes square to the enclosure 90 and 45 to the top and sides of the face of the enclosure). If the logo isn't square to the holes, you'll have to line up the logo, mark the hole locations then drill the holes.

Setting the Router Bit Depth when Using the Template:
When setting up the bit, confirm that it can cut deep enough to cut entirely through the piece being cut. The depth beneath the base of the router will have to be the thickness of the template plus the thickness of the material. The bit needs to be chucked up so that as much of the bit shank as possible is in the collet but not so far down that it won't cut deep enough or so deep that collet hits the plate for the guide. If you check this before you begin, it will save a lot of time.

Some people like to set up the router so that it cuts approximately 99% through the piece being cut and then use a utility knife to cut the rest free. I recommend using a sacrificial piece, setting up is easier. The sacrificial piece will end up with multiple rings in it but it does no harm and rarely ever needs to be replaced.

You will need to screw the template down to the enclosure so that it cannot move when routing it. You will use the lines on the template to line up the template with the desired center of the opening in the enclosure. Remember, this template must be used with the guide bearing. If you attempt to make the cut without a guide bearing on the router, the router will cut through the template and the enclosure.


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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. As an example, using a multi-hole template, it is possible to cut out holes for a tweeter, woofer and port in approximately 20 seconds using a large plunge router. If you make all of the 8 screw holes in the template, you can mark and pre-drill them so that the speaker can be mounted perfectly straight the first time. Don't drill through the holes in the template. It will make the holes slightly larger each time because you can't prevent touching the template with the drill bit. Use a fine point Sharpie to mark the hole locations, remove the template and then drill the holes. For best results, use router bits with carbide cutters. MDF will cause a high speed steel cutter to dull very quickly. Sometimes, non-carbide cutters will dull after just a few holes.


Notes for Using the Router with the Template
  • 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. When cutting the holes, you must go in a clockwise direction. If you were cutting around the outside of a template, you'd go in the opposite direction.
  • Don't allow router to come out of the jig, it will destroy the jig. Make sure that the router remains flat on the template at all times. If it tilts inward, it will damage the perimeter of the cutout in the enclosure. If it tilts outward, it could slip past the template and destroy the template (possibly damaging the enclosure. It's easier to keep the router flat if there is a lot of material (at least 1/2 of the width of the router base) between the cutout and the edge of the template.


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 cross-pieces 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.

Brace Volume Calculator
First brace

Brace Height ? = Inches
Brace Width ? = Inches
Brace Thickness ? = Inches
Opening Height ? = Inches
Opening Width ? = Inches
Number Of Openings ? =


Total Solid Brace Volume = Ft 3
Total Brace Opening Volume = Ft 3
Net Brace Volume = Ft 3
Second brace

Brace Height ? = Inches
Brace Width ? = Inches
Brace Thickness ? = Inches
Opening Height ? = Inches
Opening Width ? = Inches
Number Of Openings ? =


Total Solid Brace Volume = Ft 3
Total Brace Opening Volume = Ft 3
Net Brace Volume = Ft 3

Box Volume ? = Ft 3
 
Net Box Volume = Ft 3
Total Net Brace Volume = Ft 3


Using T-Nuts:
If you have very heavy woofers or will need to remove your woofers frequently, you'll need to use T-nuts. T-nuts are threaded metal fasteners that are used to provide a solid mounting point in wood. They are inserted into the back side of the baffle board. A hole is drilled just large enough to accept the cylindrical part of the T-nut. You insert the T-nut into the hole and (if possible) give it a good solid whack with a hammer. If you can not get to it to hit it with a hammer, they can sometimes be pulled into place by simply tightening the screw. This works fine on softer woods but won't always work with MDF. Sometimes the threads strip or the screws break before the T-nut is fully seated. If you're having trouble getting them to pull all of the way down, use a C-clamp to seat them. If you don't get them to seat fully before mounting the speaker, the screws will continue to loosen as the T-nuts continue to pull down. Only after they are fully seated will the screws stay tight. If you have a problem of them falling out, apply a bit of Goop or E6000 to the cylindrical part of the T-nut before you insert it. The photo below shows a couple of T-nuts. They come in many different sizes. The diameter of the screw that's to be used with them is important because the screw must be able to fit through the hole in the frame of the woofer. The depth of the T-nut isn't really important but you should use one that's closest to the thickness of the wood as possible. This provides more threads and reduces the chances of it stripping out.

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

The following threaded inserts are also available but they take up a bit more real estate than the T-nuts and can't be installed where there isn't sufficient clearance between the center of the hole for the insert and the edge of the hole for the speaker. They can however, be used in other locations on amp racks and such. This allows you to remove and reinstall panels without getting dust around the holes (screws threaded directly into wood always pull out a bit of dust when you remove them. This can be difficult to clean up if the material covering the piece has fibers that tend to hold the dust and wood particles.

The next insert is screwed into the wood with an allen wrench (hex key wrench).

The next two examples are hammered or pressed (C-clamp) into the wood. For heavy loads, the one with the flange should be used (flangs on the back side of the MDF). For the greatest resistance to pulling out (inserts without the flange), use the ones that are the same thickness as the wood. Longer inserts have more cleats which makes them harder to pull out.

The following two images show what the inserts look like after being installed.

These are brass inserts that you screw into the wood with a large flat-bladed screwdriver.


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:


Sealing Around the 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.



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