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Under-Seat Subwoofer Boxes

Because of the overall design of the `61-`71 Dodge Truck Conventional pickup cab, and because the gas tank for these trucks resides behind the bench seat, it is difficult to place speakers in the cab without cutting the sheetmetal or fabricating custom speaker enclosures. For those of us who would like to have some extra sound or who already have speakers installed in their Sweptline Era pickup and would like to enhance the low end bass from their sound system, this article provides a possible solution.

There isn't a whole lot of room to add woofers in the cab of a `61-`71 Dodge Truck if you still have the gas tank in the cab and have already added speakers to the doors of these trucks. Also, for those out there who choose not to cut into the precious sheetmetal of these trucks, there are few locations where speakers would fit without making large holes in the doors or cab.

Since most pickups have a ton of room for a speaker box and large woofers behind the seat, there is no simple alternative for Dodge Truckers unless they turn to placing speakers under the bench seat. The only way this is possible is through the use of small, long throw woofers that can handle enough power to sound good in a truck.

To pull this off, you'll need at least half a sheet (4' x 8' in half) of particle board that is 3/8" thick. The small thickness allows more room for speaker and speaker box internal volume. You'll also need small wood screws (#6 x 3/4" long work well) and the capability to drill and countersink for this size of screw. Wood glue is nice for a air-tight seal and some paint for the boxes will protect the boxes from swelling due to moisture. Adequate speaker wire for the speakers is necessary (16 gage for low resistance) and you'll want to drop by the local electronics store for a pair of those flush mount quick-release connectors so you can wire up the boxes once their built. Soldered connections for the internal wiring is best, but you can certainly use solderless connectors and crimping pliers with no loss in sound quality.

The heart of these under-the-bench-seat speaker boxes are the speakers themselves. The easiest way to get these are to purchase or scrounge up two sets of computer speakers. These computer speakers have to be the type that include a subwoofer box along with the two "satellite" speakers that sit on your desk. Many companies make these speakers, but the most common are manufactured by Altec Lansing. They offer computer speaker sets that include a four inch, long throw subwoofer installed in the subwoofer box with a 15 watt power handling capability. It is not cheap to buy the new speaker sets just for the woofers; however, it may be feasable depending on how badly you want to do this or how often you drive and listen to music in your truck.


Included on this page are detailed drawings that describe the size of the wood pieces that need to be cut to make the speaker boxes. Once you have cut all of the pieces out, the parts are simply glued and/or wood screwed together to make the box. Not shown in the drawings are cutouts for the flush mount quick-connectors for the speaker wires that go to the boxes, these are best added in whatever location the user desires based on the location of the wires in the truck.

The drawings should be pretty self explanatory, even though they do not include an exploded view of the parts. Each box has a top panel, a bottom panel, two identical side panels, two panels at the front (subwoofer panel and front panel) that are glued together to make the front of the box and finally a back vertical panel. Here's a list of the panels to make based on the drawings:

Refer to drawing sheet 1 for a pictorial description of the parts. Drawing sheets two through four describe dimensionally each individual panel. In order to print these out, change your printer setup to "Landscape" paper orientation and they should print out as desired.
| SHEET 1 | SHEET 2 | SHEET 3 | SHEET 4 |

Because the side panels and back panel do not include complex angles along any of their sides, they should be easy to layout on the sheet and cut. But the rest of the panels for this design require that fancy angles be made to the sides of each piece. These can be achieved in one of two ways; cutting with a table saw or jig/scroller saw, or the panels can be rough cut and then filed to achieve the best fit with the rest of the box.

Once all of the pieces have been cut and shaped (or rough cut) you can start to assemble the speaker box(es).

STEP 1: Assemble the bottom and back vertical panels using glue. Use clamps to ensure perpendicularity. Once the glue has dried, drill and countersink for a few of the #6 wood screws.

STEP 2: Add the side panels by gluing them to the existing assembly and add screws along the mated edges.

STEP 3: Glue in the subwoofer panel, let this panel dry in the assembly and then add screws through the sides (pre-drill and countersink, of course) and into the subwoofer panel to enhance the rigidity of the box. Drawing lines on the side panels can help you to locate where to drill since the subwoofer panel doesn't mate flush along its long edge with the side panels.

STEP 4: Glue in the small front panel. Add screws only through the side panels if desired. Because this panel is small and may be difficult to cut exactly to match the drawings, it may be necessary to file down the top edge so that it mates well with the top edges of the side panels. Try to match them as closely as possible to ensure a tightly sealed enclosure.

STEP 5: Add the subwoofer to the front face panel, and wire it to the flush mount quick connectors you bought (not shown here). Finally, add the top panel using pre-drilled and countersunk holes for the #6 screws. Glue can be used for a tight seal, or you can use silicone gasketing adhesive from the tube to really make for an airtight seal.

The speakers used in this buildup were chosen because of their size and power handling capabilities. Also, the speakers are of a long-throw design which allows them to move more air than a standard woofer. One could also consider small woofers from a stereo shop or from an electronics outlet, however, they may not be designed with a long-throw. If anyone knows of or can find a more suitable 4" woofer that would work in this application, please send in an Online Response.

For most automotive applications where a thumpin' bass sound is desired, the interior volume of the speaker box is crucial to the performance of the box and speaker combo. This is true for sealed enclosures only, as vented or bandpass boxes allow the speaker to move the air in and out of the box. Sealed enclosures that have been properly designed and built are known to have a tighter thump to them than boxes that have a vented enclosure. Conversely, vented speaker boxes can produce more sound than vented enclosures since the speaker is working to move air in and out of the box. In this way the back side of the speaker is not enclosed and thus can make the sound from the back side of the speaker vent out of the box.

As the diameter (or size) of the woofer increases, so does the required volume for a sealed enclosure. The ideal volume for a sealed enclosure is typically listed for a subwoofer by the manufacturer. There are also some formulas available to calculate the necessary sealed enclosure volume for a given subwoofer. The enclosed volume for each speaker box designed here was easy to calculate using the solid volume capabilities in the CAD program used to design the boxes. The sealed enclosure volume determined in the CAD program for these boxes is 397 cubic inches or about 0.23 cubic feet.

Since the volume was not given for the Altec Lansing 4" Long Throw Woofer, there are two ways to determine the required volume. A linear regression on data for larger woofers and their required enclosure volumes gives the result of approximately 0.3 cubic feet. The other way would be to calculate the optimal volume theoretically, once I get my book back I will post the result of this calculation.

This means that these boxes (as designed in the above drawings) do not have adequate volume for a sealed enclosure! There are a couple of ways you can go with this from here. First, you could just build the boxes and try them out--this is what the author has done, and they work good. Or, you could build the boxes to be big enough to have the necessary 0.3 cubic feet of volume--this merely requires making the boxes 13" wide instead of 10" wide. To do this, simply add three inches to the width of all the panels except for the two side panels and correctly center the subwoofer hole by adding 1.5" to the length of the circle's center dimension. Finally, you could add a vent to the box. To do this, it is recommended that a small slot be cut in the box in the desired location, then add an extra piece of wood within the box to create a tunnel for the air to pass through. See also the books offered by Radio Shack on speaker enclosure design.

As clearly stamped on the back of the magnet of the Altec Lansing woofers, the max power rating for these woofers is 15 watts. It is assumed that this is an RMS value. Testing with the prototype boxes and speakers as rated has shown that the speakers can handle more than this rating without any problems. The prototype speakers have been run at an estimated 50watts RMS on a 4ohm circuit with no failures after many hours of use.

THIS PAGE, INFO, AND PHOTOS ARE COPYRIGHT 2001 by Kris Wickstead for the `61-`71 Dodge Truck Website. No part of this page may be reproduced without permission.

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