Box Building 101

on 02 December 2010
Created: 02 December 2010

Box building can be a profitable part of any installation. Knowing how to sell it and being able to build it quickly will demonstrate your skills and knowledge as a certified installer. These are the steps used to build a subwoofer enclosure for a specific vehicle that is tuned to a specific frequency.

The total design and build took 90minutes.

Materials cost $40.00 plus labor cost.

A shop built enclosure in the southeast region would sell for approximately $220.00.

Box Design

Parameters are as follows:

Box Volume (includes Port and Speaker Displacement) 3.03 Cubic Feet

FB (Tuning Frequency) 41Hz

F3 (-3db roll-off) 23.7Hz

Ripple (Frequency Response Curve) 1.91

Box Size

32” Wide x 15.5” High x 13.76” Deep

Port size

2.5” Wide x 14” High x 17.65” Deep

Step 1

Panel Saw is used for precuts only, this tools allows us to slide a full sheet of ¾” MDF (Medium Density Fiberboard) directly onto the saw with out any type of heavy lifting. It also allows for a much smaller table saw. As we all know space is very valuable.

Step 2

All final cuts are done on the table saw. The table saw can be very dangerous. Be sure you have a guard on the saw and use a push stick when ever working with small or thin pieces of material.

Step 3

Now that the box is cut take the sides of the enclosure to the router to rabbit a lip around the edge. The router should remove about 1/8” of MDF and should leave a lip all the way around the outside of the side pieces. This lip will give us an area to tuck our carpet in once the enclosure is built.

Step 4

Assembling the box is the next set. We teach that it is best to attach the top and bottom to the sides of the enclosure first. This will allow you to tweak the box to match up perfectly with the front and back of the enclosure. Please note the routed lip must face away from the enclosure and should look set into the box. A pneumatic crown stapler and wood glue are the proper methods to attaching MDF, the staples hold the enclosure in place until the glue has time to dry. If you are liberal with the glue you can create perfectly sealed seams that will not leak air.

Step 5

At this point of the build only the front of the box has been attached, our next step is to mount the port into the enclosure.  The port is what will tune this box. The tuning frequency (FB) of this enclosure is 41Hz’s. Before it is installed into the enclosure you should round over all the sharp edges, this helps reduce air turbulence inside the port.

Step 6

Installing the port into the enclosure is quite easy, for this box we choose to build a slotted port over a round. If you have designed a port that is square it must be installed before the box is completely built.  One of the advantages to a slotted port is you can design the port to match the internal height of the enclosure however if a slot port will not work for your enclosure you can put a round port in. You must make sure that the surface area of the round port equals the surface area of the slot port to maintain the same tuning frequency. Below are the equations to convert slot ports to round and round to a slot.

Slot to Round Port

Internal Width x Internal Height= Port Surface area

Example:  2.5” Width x 14” height = 35” squared (Surface Area)

**If the round port does not match available sizes on the market you would need to recalculate the port length based on what is available.**

Round Port to Slot

Example:  Round Port 6.67”

Divide 34.923 by internal height of box 14” = 2.4945 or 2.5”

Step 7

When installing the port into the enclosure we used two 2.5” spacers made from MDF which were the exact width of our port. This is a quick and easy way to ensure your port is the same distance away from the walls of the enclosure. With the spacers in place a square was used to transfer the lines of the port to the out side of the enclosure. The port was then glued and stapled into place.

Step 8

Braces were added to the box to help prevent unwanted vibrations, the more an enclosure vibrates the less output you create. Once the brace was mounted the back of the box was glued and nailed into place.

Step 9

Next we marked the location of the port on the front of the box including the ¾” Port wall. Now it is time to lay out the location of the two JBL P1224 woofers. An easy and quick way to find the center of any square or rectangle it to line up a straight edge on opposite corners and drawing a line, duplicate that line on the other corners, where the two lines intersect is the center of the shape. Then a straight edge was used to draw a line down the center of the enclosure, you can then use the same technique to find the location of the center pin for the perfect circle.

Step 10

Use an 1/8” drill bit and drill out the center of the two subs mounting locations. The 1/8” bit is used for the pin on the perfect circle router. The perfect circle is an attachment that is mounted on to the base of a router and can cut perfect circular holes. When using a perfect circle you must find the mounting diameter of the woofer and divide it by to two find the radius. Then measure from the outside of the straight cut router bit to the center of the 1/8” pin. You should always double check your math at this step.

Step 11

Now that the holes of the subs are cut and they fit, we can cut out the port. Find the center of the port and use a drill bit larger than ½” and small enough not to come close to the walls of the port. We will be using flush trim bit to route out the inside of the port, so it is very important that the walls of the port are not damaged.

Step 12

Next the entire box was rounded over using a ¾” round over bit. This helps the overall look of the box and takes almost no time at all. A ½” round over is used on the inside of the port and set a little higher than normal so it will leave a 1/8” groove around the edge of the port to tuck the carpet.

Step 13

For cosmetics we used flat black spray paint and painted the inside of the port. Once the paint dries tape up the port and trim the tape flush with the routed lip on the edge of the port.

Step 14

When measuring for carpet you can take a tape measure and wrap it around the box. Then transfer the measurement to the carpet needed to cover the enclosure, you should always give yourself a few extra inches just in case.

Step 15

The glue used to attach carpet to the box is Dap Weldwood upholstery glue. This glue can be found at any local upholstery store. The box and carpet should be well sprayed, be sure not to build up puddles of glue because it can take a long time to dry and even soak though the carpet. It is a good idea to spray the box while on the carpet it helps control over spray. Once the box is sprayed you should remove the tape that is covering the port, so it will not stick to the carpet when you lay it down.

Step 16

Wrapping the carpet is fairly easy. When the box is fully wrapped you have the carpet over lapping each other, use a straight edge and cut though both pieces of carpet then remove the free pieces. When you lay the carpet back down on to the box you will have a perfect seam. Be careful not to pull and stretch the carpet or your seams will no longer match.

Step 17

Now, remember the lip that is in the sides of the enclosure, we are now going to glue and tuck the carpet into it. A quick and easy way to push the carpet into the seam is to use a piece of ABS plastic about 3” wide and a light weight mallet. Make sure to pull and stretch the carpet to remove any wrinkles. Once all the carpet is tucked into the edge cut off excess carpet with razor blade.

Step 18

We can now cover the side of the box with a different color carpet using the same technique. To remove the extra carpet, run the razor blade down the center of the routed lip, to avoid any MDF showing through the carpet.

Step 19

We can also use the same tool to tuck the carpet into the side of the port, and then trim off the extra carpet. Take your time at this stage, you want to use a new razor blade so the cuts to look straight and perfect. Then a heat gun can be used to remove (melt) the frays in the carpet. Don’t get the heat gun to close to the carpet it will burn and melt very easily. 

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