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The Subwoofer Project v1.0, Part One

September6

Sub thumbnailI’ve known for quite awhile now that my Behringer 2031’s lack sub 60Hz punch. Having a desire for the low-end reproductive capabilities of a subwoofer, I decided to build one. The following article details my process.

The project started out with the idea that I would build an integrated amplifier and subwoofer enclosure. I went through the whole process of designing an amplifier and laying out the circuit board for it but after putting together and pricing a parts list, I was going to do just as well to buy an amp. At this point, my attention turned to the enclosure.

I’d pretty much settled on this 10″ driver for several reasons. One, I’m space-limited at the moment. I don’t have the space to locate a giant box nor do I need a larger driver to make the bass. Second, economics dictated that I not spend a fortune on a larger driver. Third, as it turns out, I couldn’t have fit a larger driver with the scrap of MDF that I built the box out of. That leads me then to the design of the box.

After hunting around a bit on the internet for some guidance on how big the enclosure needed to be, I found a handy bit of free software called, appropriately enough, “Subwoofer Simulator.” It isn’t fancy, it isn’t super-easy to use, but it works. You can plug in your speaker’s T-S parameters and it can either tell you what the internal volume of your box should be for maximally flat response or you can punch in a volume and it will calculate the response. Not only does it do sealed-box, it also does ported, passive radiator, 4th order bandpass, 6th order bandpass, and 8th order bandpass. Did I mention that it is free?

It told me that I needed an enclosure of about 8.8 liters which is about 0.3 cubic feet. I was surprised that a box so small would do. I think mine ended up being about twice that, but after punching in that number, the response didn’t change much. The main change is that the resonant frequency of the box and driver system drops from about 57Hz to 53Hz or so.

Construction on this guy is intentionally simple. The corners are butt jointed, no miters, nothing fancy. To reinforce the corners, I used blocks of hickory cut to 3/4″ square. The top and bottom panels are 13.25″ square and the sides are 11.75″x13.25″. The front and middle panels are 11.75″ square. The dimensions are odd because I had to shrink everything by a quarter of an inch to get it to fit into the limitations of my scrap MDF.

I took great care to make sure that I cut everything of the same designed dimension with the same setup of the table saw. That way, for example, all of the pieces cut at 13.25″ would all be the same.  Because the MDF is slightly thicker than 3/4″, I also made sure that the front panel would fit into the opening. Two sheets together plus the front should equal the width of the top. It took a few passes to get this right and I still had to trim it after gluing the sides and top together.

Sub 01

Those are cargo straps, by the way. Only the best clamps in my shop. On the bottom, though, is a frame clamp. The fact that it had been hanging on the wall for ten years without ever once being used might to some indicate uselessness but here it was perfect.

You can see the corner blocks in the photo above. I glued and tacked these in with brads while it was in the clamps. There was no good way to clamp them, so the brads were necessary. You should also notice that they do not extend all the way to the back of the box – they stop where the back panel of the enclosure will be. The space behind that is for the amplifier.

While the glue was curing on the box, I went about cutting the hole in the front panel for the driver. I made a simple circle cutting jig out of a piece of plywood. I drew a line down the center and drilled a hole of about 1.25″ diameter on the line a short distance back from one edge for the spindle of the router to extend through. I used the stock base from the router to locate three holes for screws to hold my jig onto the router. As far as where the pivot hole goes, it is simply a matter of measuring from the router bit and marking where to drill your hole. It can be anywhere on the jig so long as it is the right distance from the router bit. Just remember you’re marking the radius of your hole, so divide the diameter you want in half.

Sub 02

For my pivot, I drilled a 1/8″ hole in the jig and the front panel and used the same bit to pivot around. The crossing of lines drawn across opposite corners of the front panel told me where the center of the panel was and where to drill.

I did use a plunge router and a spiral bit for this. You could try it with a fixed-base, but I wouldn’t recommend it. I don’t know how you’d get the bit into the work and keep it perpendicular. The spiral bit is recommended because it is very good at removing large amounts of material cleanly and without chatter. I would also recommend some form of dust removal. MDF makes a mess when you cut it.

I did one circle at a diameter of about 10.5″ at about 1/4″ depth to inset the flange of the driver and one at about 9.5″ for the cutout. Notice that the inner cut is at the same depth. This is for two reasons – one, it wouldn’t be a good idea to remove all of the material in one deep cut and two, if I were to make the whole cut, I wouldn’t be able to do the next steps.

Sub 03

I had to drill some extra holes in my jig to remove the material in between the two grooves I cut. After doing that, I could begin to deepen the innermost cut to make the whole for the driver. I made several passes of increasing depth until I had just a few hundreths of an inch of MDF holding the inner circle to the frame.

Sub 04

I didn’t want to cut full depth with the router because there is workbench under there and also because once I cut the center loose, my router is no longer fixed to the workpiece – it would be free to wander. I finished the cut with a utility knife from the backside.

Sub 05

Here you can see the paper-thin piece that was holding the center to the frame.

Sub 06

After a trim and a bit of sanding, this is what it looks like.

I unclamped the box and went about installing more interior corner bracing. I dry fit the back panel and used it to position and install braces. I removed it and fit the front panel in and did the same. The front panel will be screwed on to the corner braces behind it. The back panel was be glued in after I was finished working through the hole it fills. I also put in some angle braces. In retrospect, I should have made these longer.

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Again, the corner and angle braces were tacked in. Be careful and don’t use brads that are too long. I did and two of them shot through the outside of the box. Because my nail gun was too large, I had to switch to a smaller stapler to put the angle braces in. With a box this small, there isn’t much room to maneuver big pneumatic tools.

That’s it for now, tune in later when I drill the front and install the driver.

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