Building the Enclosure
When building an enclosure, the enclosure's walls should be as rigid as possible. Any flexing in the enclosure will drastically decrease your speakers' performance. Also, all of the joints and walls in a speaker enclosure should be airtight, including screw holes and wire holes. Any leaks or flexing will cause cancellation, resulting in reduced output.
Enclosures should be built with very dense and heavy wood. We recommend MDF (medium density fiberboard) or Medite (high density fiberboard), They are rigid, heavy and not porous like some particle boards. We recommend 3/4" (19mm) MDF although 1/2" (12.5mm) MDF is acceptable for woofers 8" and smaller.
Most enclosures are built for multiple drivers and require separate chambers for each driver. Dividers are a very important part of box building because they create strength in the box and provide an airtight seal between the speakers. Keep in mind that no two things are created equally (even speakers of the same size and model!), This will cause drivers in an open chamber to react differently, substantially reducing output and power handling. When assembling the box it is very important to glue all joints. Screws or staples should be placed approximately every 4" (10cm). Drywall screws work better if they are counter-sunk. Countersinking makes it easy to fill over the screw heads for paint or carpet, and improves the appearance of the enclosure.
A quality air stapler has been proven to be a good substitute for counter-sunk drywall screws. Use 1-1/2" (38mm) or 1-5/8" (41mm) staples at least every 3" (7.5cm) along every seam. Don't forget the wood glue. This is much faster and smoother than using screws.
Glue joints all the way across the wall to provide an airtight seal. We recommend Titebond or Elmer's Wood Glue.
It is always a good idea to use corner braces, also called glue blocks, on each joint in an enclosure. Corner braces are usually made from left-over enclosure wood and measure approximately 1" (2.5cm) wide. The braces should be glued and screwed or stapled to the walls and caulked on both edges where they meet the walls. Use a silicone caulk to ensure the airtight seal. Don't use "bathroom tile" type caulk, it won't do the job!
Not all corner braces will be the length of the wall they're attached to. For example, if a wall's length is 36" (1 m), but there is already a corner brace on the adjacent wall, we'd need to deduct one inch (2.5cm) so that the braces fit together - not overlap. Deduct two inches (5cm) from a brace's length if there are braces on both adjacent walls.
When adding braces to an enclosure always add the displacement of the extra wood to the gross box volume as it is designed.
Another type of brace, called a cross brace, should be used in any span that is 12" (30cm) or more to prevent panel vibration. The most common application will be from the front baffle to the rear wall and between the top and bottom walls. This type of brace is usually made of 3/4" x 2" wood. The brace will contact the enclosure only on its ends where it should be glued and screwed or stapled. Place the brace slightly off center for maximum rigidity. A perfectly centered cross brace can actually increase cabinet flex and resonance (at a higher, more audible frequency).
Designing General Enclosures
The fundamental Thiele-Small calculations can be performed using a scientific calculator and a little knowledge of algebra. However, it is important to note that the basic calculations have some factors averaged or removed for simplicity, and the answers they give are only approximate. Your best bet is to use known enclosure design data such as given here, or to use a computer program such as LEAP 4.5 along with our published driver specifications to assist you in designing enclosures.
When you have completed your enclosure, it is important to make sure the speaker is at least close to your design specs. Fori 00% certainty that everything is correct, you may want to measure the box tuning frequency for vented enclosures. If you did the calculations by hand or with a simple computer program (any program that requires only Qts, Vas, and Fs for enclosure performance calculations is simple), you will definitely want to measure tuning frequency because the vent dimensions given by the calculations could be off enough to reduce the speaker's performance. Be sure to measure the tuning with the enclosure in the vehicle, the acoustic environment of the speaker can also affect its tuning.
When designing an enclosure, it is best to follow what we call "The Design Sequence." This is a simple, three step process that can save you a lot of time and hair pulling!
The Design Sequence
1. Determine size of enclosure.
2. Calculate and subtract driver, brace and port (if ported) displacement from enclosure volume.
3. Finalize driver selection.
Step One: Determine size of enclosure.
This is the easy part. Get a tape measure and get in the vehicle. Consult your customer on exactly how much space he or she is willing to forfeit for their system. Measure this area and write down the height, length and width dimensions in inches.
Now we'll learn how to calculate volume for a cabinet from the dimensions we just took. Let's say the space we have available is: 14" High x 41" Long x 14" Wide
The formula for figuring volume is pretty simple. Memorize it now, you'll be using it a lot!
Height x Length x Width = Total Cubic Inches
Total Cubic Inches / 1728 = Total Cubic Feet
Total Cubic Feet / # of Drivers = Total CubicFeet Per Driver
What is 1728 and where did that come from? 1728 is one cubic foot or 12 x 12 x 12. Dividing by this figure converts total cubic Inches into total cubic feet. You'll want to divide total cubic feet by the number of drivers to be installed in the enclo¬sure, usually two, to get the total cubic feet per driver. This is how the manufacturer usually lists specifications.
When calculating an enclosure's volume, you will work with EXTERNAL and INTERNAL dimensions. External dimensions are what you have after measuring the space available in the vehicle. Internal dimensions are what you'll want to go by when selecting a speaker for the enclosure. What's the difference? The thickness of the wood used to build the box. For example, if using 3/4" wood, you'd subtract double the thickness of the wood from each dimension to get internal dimensions.
Let's look at our example again and figure the internal dimensions using 3/ 4" wood.
3/4" + 3/4" = 1.5"
14"-1.5" = 12.5"
41"-1.5" = 39.5"
14"-1.5" = 12.5"
12.5x39.5x12.5 = 6171.88
INTERNAL cubic inches
6171.88/1728 = 3.57
INTERNAL cubic feet
3.57 / 2 drivers = 1.79 cu. ft. per driver
Step Two: Calculating Displacement
Displacement refers to the space used by things inside an enclosure that influence total volume and is mostly associated with the drivers in an enclosure. However, other factors will also influence an enclosure's total volume, like ports and bracing. To calculate exact enclosure volume, we'll need to consider port, bracing and speaker displacement.
Port Displacement: Calculating the amount of space taken by a port will test your memory of high school Geometry! The formula itself looks pretty simple: Area x Length = Volume. First you'll need to know the port's dimensions. For an example we'll use a 12" long port with a 2" diameter. Now we need to calculate the area of a circle the same size as the port, 2" in this case. (This is where Geometry comes in!) The area of a circle is the radius squared multiplied by "pi" or 3.14". Area of a Circle = r2 x 3.14 The radius of our circle is 1" (half of the diameter). 12x 3.14 = 3.14" Next, we'll multiply this by the port's length, 12". 12" x 3.14" = 37.68 cu. in. So, to get the accurate volume of the enclosure, we'd need to deduct 37.68 cu. in. from the total volume.
Brace Displacement: After figuring port displacement, brace displacement is relatively easy. Braces are discussed in detail a little later so trust us for now. We'll use a 1" x 1" x 20" brace for an example. All we do is multiply the dimensions together (1" x 1" x 20" = 20 cu. in.) and deduct this amount from the cabinet's total volume. Piece of cake! Remember to do this for all braces in the enclosure, there may be quite a few of them!
The driver also takes up air space inside the enclosure. If you use Kicker speakers, we've made calculating driver displacement real simple for you, just look at the chart on appendix page 25. The recommended enclosure volumes we give in the charts already have driver displacement deducted. If you use our speakers and go by that chart, you won't have to figure driver displacement. Let's get back to our example again. We currently havel .79 cu. ft. per driver before displacement. This size box might work for a 12" driver or a 10" driver. Let's calculate driver displacement now to where we are.
12" driver displacement = 0.0538 cu. ft. 1.79 - 0.0538 = 1.736 cu. ft. per 12" driver
10" driver displacement = 0.0334 cu. ft. 1.79 - 0.0334 = 1.757 cu. ft. per 10" driver
Now we'll look at braces. Two different types of braces are corner braces and cross braces. For this example, we are building a sealed enclosure in a simple rectangular box. We'll have twelve corner braces (one for each joint), and six cross braces (one for each wall).
We have eight corner braces that are 1"x 1"x 12.5". Each one uses 12.5" cu. in. of our enclosure's total volume, so eight of them use: 8 x 12.5" = 100 cu. in.
Four of our corner braces are 1" x 1" x 37.5". We deducted two inches from the length so the braces would meet instead of overlap the other corner braces. Each one of these use 37.5" cu. in. of our volume, so multiply this figure by four=4x 37.5" = 150 cu. in.
So all bracing combined uses 250 cu. in. Now we'll divide by 1728 to convert to cubic feet.
250 cu. in. / 1728 = 0.0289 cu. ft.
Now we go back to the enclosure volume figures we got after figuring driver displacement and deduct brace displacement:
12" driver 1.736 - 0.0289 = 1.7071 cu. ft.
10" driver 1.757 - 0.0289 = 1.7281 cu. ft.
Result - we use 12" Kicker Competition woofers in a sealed enclosure! Another possibility though, is the 10" Freeair driver in a ported enclosure. It can be quite a long process to figure all these different things, but trust us, going through all the trouble is worth it when you get done and have an enclosure that sounds awesome!
Let's say you're measuring a customer's vehicle for available space and it's basically unlimited in one direction. If two dimensions are known and the other is unset, we have another way to calculate volume for a specific speaker. For example, we'll say the customer wants to use our Competition 12" driver. We know that the recommended enclosure volume for a C-12 is 1.75 cu. ft., so for two it's 3.50 cu. ft. Our two known dimensions are 39.5" and 12.5" (internal). First we'll find the total cubic inches on our two known figures, just multiply them together.
39.5" x 12.5" = 493.75"
Next we need to figure the total cubic inches needed for two C-12's. All we do is multiply 3.50 cu. ft. (recommended for two C-12's) by 1728 to convert to inches.
Now we divide the total cubic inches needed (6048) by the cubic inches that we have already (493.75) to determine the missing dimension.
6048 / 493.75" = 12.25 inches for the width
To check this, multiply all three figures:
39.5" x 12.5" x 12.25" = 6048.44 cu. in.
6048.44 / 1728 = 3.50 cu. ft.
To figure enclosure volume on angled boxes, draw a diagram of your enclosure. It will help a lot to visualize what you're doing.
17" High x 54" Long x 5" Top and 13" Bottom
Since the formula to determine enclosure volume does not allow for two different width dimensions (5" Top and 13" Bottom), we must find an average of these two figures.
To find the average, add the figures together (5 + 13) then divide by the number of figures added (2).
5"+ 13" = 18" 18"/2 = 9"
In essence, averaging the 5" width and 13" width gives us a rectangular box to work with instead of an angled one (note the "dotted line" enclosure). Now the formula for figuring volume works.
3.53 / 2 = 1.765 cu. ft per driver
To calculate internal volume, you must subtract the thickness of the wood. We're using 3/4" MDF again, so deduct 1.5" from each dimension.
54" - 1.5" = 52.5"
9"-1.5" = 7.5"
17"-1.5" = 15.5"
52.5" x 7.5" x 15.5" = 6103.13 cu. in.
6103.13 71728 = 3.53 cu. ft
Angled Enclosures With Risers
This type of enclosure is a little more difficult to work with than just an angled enclosure, but we'll help you through. Follow the calculations belowfor volume on an angled enclosure with a riser.
Dimensions: 17" High x 44" Long x 5" Wide (Top) and 13" Wide (Bottom) with a 4" riser.
First, make the box into a rectangle like we did before by averaging the top and bottom width measurements:
5 + 13=18 18/2 = 9
Instead of using the entire height dimension, deduct the height of the riser, 4" for this example.
17" - 4" = 13" High
So the external dimensions for the first part are (see the second diagram):
13" High x 44" Long x 9" Wide
We'll go ahead and figure the volume of this enclosure (using internal dimensions). Subtract only one thickness of wood from this height.
12.25 x 42.5 x 7.5 = 3904.68 cu. in.
3904.68 / 1728 = 2.25 cu. ft.
2.26 cu. ft. 12 = 1.13 cu. ft. per driver
Notice that there is a 4"x44"x13" enclosure left over. This is just another airspace that we'll figure volume for and add to the first part. (Use internal dimensions!) Again, subtract only one thickness of wood from this height.
3.25 x 42.5 x 11.5 = 1588.43 cu. in.
1588.43 / 1728 = 0.92 cu. ft.
0.92 cu. ft. / 2 = 0.46 cu. ft. per driver
Now, add 0.46 cu. ft. to the first figure:
0.46 + 1.13 = 1.59 cu. ft. total per driver This is your net internal airspace for each side of this enclosure.