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PostPosted: Wed Jun 11, 2003 7:17 pm 
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If you are not certain that you have the woodworking skills, tools or desire to build your own box, we recommend purchasing a prefabricated box with the correct internal volume.

You can follow these steps to determine the correct dimensions for you subwoofer box:

1. Measure the depth of the woofer and add 2 inches.
This is the minimum depth of your box (in this article, the depth dimension refers to the front-to-back dimension, with the woofer being mounted to the front). Make note of this dimension, as you may need to refer to it later when figuring your box dimensions.

2. Determine the minimum height and width
Measure the frame diameter of your woofer to determine the minimum height and width for the front of your box. Be sure to allow for any additional space that may be needed to mount a grille, if applicable.

3. Determine the available space in your vehicle
Measure the height, width and depth of the vehicle space that you are willing to devote to your subwoofer. If the box must be wedge-shaped to fit, you will need to know the depth at the box's top and bottom.

4. Sketch your box
Now let's take a moment to sketch the box on paper using the basic dimensions you have gathered. It may look like one of these:


For our example, let's use the following external dimensions:

1. Rectangular Box:
* Height = 14"
* Width = 18"
* Depth = 8"
2. Wedge Box:
* Height = 14"
* Width = 18"
* Depth 1 = 5" / Depth 2 = 8"

5. Determine the internal dimensions and volume
The above steps identified the external dimensions of the box. To determine the internal volume, we must subtract the thickness of the wood to be used for construction. If the wood is 3/4" MDF (medium-density fiberboard, recommended), then 2 x 3/4", or 1 1/2", will be subtracted from each dimension.


The internal dimensions (external minus 1 1/2") for our example:

1. Rectangular Box:
* Height = 12 1/2"
* Width = 16 1/2"
* Depth = 6 1/2"
2. Wedge Box:
* Height = 12 1/2"
* Width = 16 1/2"
* Depth 1 = 3 1/2" / Depth 2 = 6 1/2"

6. Calculate internal volume in cubic inches
Based on the internal dimensions, we can calculate the cubic volume of the enclosure with the following formula:

Height * Width * Depth = Cubic Volume

Let's put some numbers in:

1. Rectangular Box:
* 12.5" * 16.5" * 6.5" = 1,340.63 Cubic Inches
2. The wedge box actually has two depth dimensions, so we must first find the average depth, then we can find the internal volume:
* Depth 1 + Depth 2 / 2 = Average Depth
* Now with numbers:
3.5" + 6.5" = 10"
10" / 2 = 5 Inches Average Depth
3. And the internal volume for the wedge box:
* 12.5" * 16.5" * 5" = 1,031.25 Cubic Inches

7. Converting cubic inches to cubic feet
Since most manufacturers will give you the internal volume in cubic feet, it is time to convert your internal volume from cubic inches. This is done by dividing the cubic inches by 1,728

This is the total cubic inches in one cubic foot, or 12 * 12 * 12

1. Rectangular Box:
* 1,340.63/ 1,728 = .776 Cubic Feet
2. Wedge Box:
* 1,031.25/ 1,728 = .597 Cubic Feet

8. Adjust your box's air volume to match the sub's specifications
Now we can compare the cubic feet of the proposed box to the manufacturer's recommendation. Make minor adjustments to one dimension by increasing or decreasing it in small increments your internal volume matches the recommendation as closely as possible. Make sure that your final dimensions will fit in the proposed location, and that the box's depth will still accommodate the woofer.

9. Determine final exterior box dimensions
Once you have identified the correct internal dimensions, it is time to add the 1 1/2" that we subtracted in step 4, so that we can derive the new external dimensions. The new external dimensions (internal plus 1 1/2") for our example:

1. Rectangular Box:
* Height = 14"
* Width = 18"
* Depth = 8"
2. Wedge Box:
* Height = 14"
* Width = 18"
* Depth 1 = 5" / Depth 2 = 8"

10. Build a model to verify fit in your vehicle
If this is your first effort at building your own box, this step is highly recommended: Fabricate an actual size model of your enclosure out of cardboard. This lets you physically test-fit the box in the vehicle. This test-fit is a great opportunity to determine which side of the box your speaker terminal should be installed into, and to find out if there is going to be any serious interference.

11. Transfer your dimensions to the wood you are going to use
Carefully layout each box panel's dimensions onto the wood. Double check each measurement, and be sure to use a framing square to make sure your angles are correct. Use a chalk line to mark edges that are longer than your straight-edge.

Note: The largest sides of the box should overlap the ends of the smaller sides to provide the greatest strength. For our example, this means that the front and back panels will be cut to the actual external dimensions. The top and bottom panels will be cut to actual external width, but cut one and one-half inches shy of the external depth. The end panels will be cut one and one-half inches shy of the overall exterior dimension on both sides.

The actual cut dimensions for our example:

1. Rectangular Box:
* Front and Rear Panels = 14" X 18"
* Side Panels = 12 1/2" X 6 1/2"
* Top and Bottom Panels = 6 1/2" X 18"
2. Wedge Box:
* Front Panel = 14 1/2" X 18"
* Back Panel = 14" X 18"
* Side Panels = 12 1/2" X (D1) 3 1/2" (D2) 6 1/2"
* Top Panel = 3 1/2" X 18"
* Bottom Panel = 6 1/2" X 18"

12. Assemble your box
Dry fit the panels to confirm that the joints will be reasonably tight. If all looks good, go ahead and glue and screw the box together, making certain to keep the panels properly aligned throughout.
Note: Leave the front panel for the last step. This will allow you easy access to apply silicone to the interior joints.

13. Locate the woofer and terminal cup cutout location
Use the template for the woofer to layout the mounting hole, making certain that the woofer will have sufficient depth at the intended mounting location. Measure your terminal cup to determine the correct mounting hole size, and lay that out as well. Cut out the mounting holes for the woofer and terminal cup.

14. Install the terminal cup
After you have prepared the proper wiring connections, mount the terminal cup to the box. With easy access through the woofer-mounting hole, seal the interior perimeter of the terminal cup. This will ensure an airtight enclosure.

15. Install the woofer
Mount the woofer after securely wiring it to the terminal cup, and let the assembly sit for an hour or so before connecting it to an amp. This will allow the glue and silicone to set up before being exposed to extreme internal pressure.

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