Subwoofer Project

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This page is here to provide a brief description of this loudspeaker project, for those few people who might be interested in such an undertaking.

The cabinet was built as two boxes, one inside the other. The inner is constructed from 3/4 in. MDF, the outer is 1/2 in. plywood. The interior is braced with MDF cutouts in one direction and dowel rods in the other two. It's about 70% filled with polyester hollowfill batting.

Click image for a larger view.

The cabinet is an acoustic suspension design to save space. The driver is Madisound's NHT 12 in. Subwoofer. The cabinet was built entirely enclosed, after which I added external rails to the back to support a Keiga KG-5150V Amplifier.

The cabinet dimensions follow the "golden ratio" rule of 0.6:1.0:1.62 in depth:height:width. Cabinet volume was selected for a low frequency of 20 Hz and follows the closed-box design described in David Weems' Designing, Building, and Testing your Own Speaker System, ISBN 0-8306-8374-7.

Click image for a larger view.

Since it was planned for our living room, a lot of attention was given to appearance. The top is a tile mosaic with two 12" square marble tiles interleaved by decorative trim tiles. Quarter-round wood trim was used to border in the tiles and finished to match the cabinet.

Originally a full, cut marble surface was planned, but getting one the correct size and color proved to be very expensive and time consuming.

Click image for a larger view.

All plywood surfaces are stained and varnished, although only the left and right ends are visible (unless the front cover is removed). The bottom is supported on 2.5 in. reinforced MDF rails, painted satin black. This makes it somewhat easier to lift.

The front cover is a 3/16 in. piece of red oak with an oversize hole cut for the speaker. It was spray painted satin black and covered with acoustic fabric, attached by staples. It has eight quick-connect fasteners. The fabric and fasteners are from Parts Express.

I'd like to add special thanks to Kevin S. (he knows who he is) for the advice and help he provided for this project. Thanks Kevin!

Making a Large Round Hole - on the Cheap

DISCLAIMER: The following description is not a recommendation. It presents a solution I found to the problem of creating large round holes in wooden boards. The author accepts no responsibility for anyone else's use of this information.

All work that involves power tools requires precautions and safety equipment, and imposes personal risk. Please follow the instructions and warnings provided with the power tools to minimize that risk. Seek the advice of someone experienced in woodcraft if you have any doubts before attempting a wood construction project.

Anyone who's set out to make a large round hole for a speaker in a piece of MDF or plywood knows that it's very difficult to cut a smooth and accurate hole using a jigsaw, and that tools made for this purpose are expensive and hard to obtain.

After experimenting with some homebrew tools, I found a way to build a satisfactory hole cutter using some 3/8" threaded rod, two 1 inch flat wood drill bits, some scrap wood, and assorted common hardware. The following picture captures the essence of the design.

In order to use this tool, a slightly smaller hole than the one desired must be cut first using a jigsaw. This tool may then be used to cut a more precise and smooth hole around it.

The next picture shows how the threaded rod that forms the drive shaft for the tool is supported from below. A piece of 1" x 1" scrap has been drilled for the 3/8" threaded rod, and washers were installed on both sides to hold the rod securely. The washers have been drilled and attached with wood screws. The 1" x 1" piece was then clamped into the WorkMate as shown.

The tool is constructed from two identical pieces of wood scrap screwed and glued together. Holes were drilled between the two pieces for the 1" drill bit shafts, just a little narrower than the drill bit shafts. The wood above each bit was then carved at an angle on either side of the bit, so that the flat portion of each bit would recess into the tool. This kept the drill bits perpendicular to the hole while the tool was in use.

A 3/8" hole was drilled at the center for the threaded rod, which was secured in place with bolts, flat washers, and lock washers. These had to be tightened very securely in order to keep them from loosening while the tool was being operated. A thread locking glue would have been effective, but would also have made it difficult to re-use the hardware later.

As shown in the picture, the bits were held in place by spring steel clips from my spare hardware bin. These were perhaps the weakest part of the design, since the vibration of the tool in use caused these clips to loosen after each 1/4" or so of progress cutting the hole. The clips had to be tightened before further use. An improvement would be to use appropriately sized shaft collars to secure the drill bits into place.

A downside of this tool is that it is only useful for cutting one hole size. For the subwoofer project described above, I chose to make the hole in the 3/4" MDF layer support the driver, and made the hole in the outer 1/2" plywood layer large enough for the driver to fit within. This allowed the speaker surround to be fully recessed, and required a larger hole to be cut.

As a last point, I found that the round wood shaver tool shown here to be very useful for making round holes cut with saw blades more even and smooth.

Scott Chilcote 8/31/03

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