…and then some

My Custom Subwoofer Box

A friend of mine gave me a great deal on a Kenwood XR900-5 eXcelon 5 channel car amplifier and I’m having it installed soon.  It’ll be wired to current door speakers but I’m lacking a subwoofer and a box to put it in, so I thought I’d rectify that.

Another friend of mine, and also roommate, used SketchUp to work out the dimensions and details of the box.  I didn’t need anything fancy, but the resulting diagram we printed out gave us a great visual of what to expect.

We also worked out exactly what cuts we would need to make out of a 4’x4′ sheet of MDF (medium density fiberboard).  This way when we were ready to cut the pieces it would be super simple and less prone to mistakes.

Unfortunately, Home Depot didn’t have any 4’x4′ sheets of MDF so I had to settle for two 2’x4′ sheets, which mean re-thinking the cuts anyway.  Oh well.

Originally I didn’t want to use a sub box because our little 2018 Kia Soul doesn’t have a ton of cargo space as it is and I didn’t want to take up any more of it.  There is a round section on the side wall that is designed to hold an 8″ subwoofer (some models of this Kia come with a nice sound system already built in, but ours did not), and I figured I could just put one there.


However, after doing a bit of research I discovered a post by someone who put a small subwoofer box in the storage tray area below the floor board.  I thought this was a great idea and decided to do the same.  After a lot of brainstorming I decided we didn’t even need to use the storage tray and I’ll have much more room if I just use the entire spare tire cavity under the floor board.

Putting a box in the storage tray would only give me about 5.5″ of vertical space and I think about 11″ width.  By removing the tray and setting the box in the spare tire cavity, I’d have 12″ of vertical space… and tons more horizontal space.

I didn’t like the bare metal and other visually unappealing surfaces of the cavity so I got some semi-thick felt and covered it all.

It’s not perfect, but it serves its purpose.

Anyway, back to the box.  I brought the two 2’x4′ sheets of MDF home and made the cuts.  Oh, and I’m using .75″ thick wood.  Despite not being able to follow the SketchUp, all cuts came out quite nicely.

I started assembling the box…

The tricky part was going to be the circular hole.  I didn’t have any suitable tools for the job.  I have a buddy who does woodworking and he said he would cut it for me, but it was a bit more of drive than I was thrilled about.

I ended up just getting a really good open-box deal on a jigsaw at Harbor Freight, so I attempted to cut the hole myself.  At this point I had already ordered a 10″ subwoofer, and though it had not yet arrived, the spec sheet told me the mounting hole should be 9.2″ in diameter, IIRC.  I drew this out with a compass, drilled a starting hole, and took the jigsaw to it.

It’s worth noting this is the first time I had ever used a jigsaw.  In fact, I had to look up online how to attach the blade properly.

I’d say the hole came out okay for a first time jigsaw cut.  It’s certainly not perfect, but it didn’t have to be.  It just had to hold the speaker securely.  With the speaker hole cut and the all sides attached, the box was structurally complete.  Oh, and I also lined the inside edges with hot glue.  You can probably notice that more in later photos.

Recently I built a small shelf (you can partially see it in a couple of the above photos) and my wife and I went to home depot to get black paint for it.  I decided to use some of the paint for my box.  Since the box would be out of view and out of reach most of the time I didn’t see any reason to line the outside with carpet.  A layer of black paint would be just fine…

Well, black paint is what we asked for.  Gray is what they gave us.  Sigh.  It’s fine.  I’ll keep it like this.  As I said, it’ll be out of view anyway.

The speaker finally arrived.  I say “finally” but it took less than 48 hours.  Thanks, Amazon.

I’m not super familiar with BOSS but this speaker had a lot of great reviews and it was only 30 bucks.  Anyway I set the speaker into the hole but it was just a tiny bit too snug.  Totally fine, though.  I purposely did it this way just in case.  I can always sand/grind it out a bit more, and that’s exactly what I did.


Most speaker boxes are not designed to be opened.  Generally you use the speaker hole to do whatever you need to do inside and then attach the speaker.  This wasn’t really going to work for me so I designed the top to be easily removable.  I used metallic insert nuts so that the top screws can be repeatedly screwed on/off without excess wear and tear on MDF.

The removable top gives me easy access to everything inside the box, which will come in quite handy.  Also you may notice a small hole in the center of the bottom of the box.  I’ll explain that later.  First, let’s wire it all up.

I cut out a small hole on the back so I can use a wire terminal.  For some reason I had a lot of trouble cutting this hole, but I managed it.

This is a dual voice coil subwoofer so I had the choice of connecting the wiring in parallel for 2Ω or in series for 8Ω.  If you don’t know the difference, don’t feel bad.  I didn’t either.  I still don’t totally understand it, but I did know that I wanted to go parallel, and the handy dandy manual shows how to do that.

So it’s all wired up inside now.

Now I’ll explain the little hole on the bottom.

As I mentioned, the box is going into the spare tire cavity in the car.  This cavity is shaped to fit a tire and in the center is a threaded “hold down clamp bolt thing” that secures the tire in place.  I decided it would work quite nicely to also hold my box in place.

While this clamp does a great job holding the box in place, unlike a tire, the bottom of the box is perfectly flat, and because the center of the cavity rises up a bit, this means the box sorta “teeters” on the center.  Does that make sense?  Anyway, to provide a wider area of support, I got a foam pad and cut out the center so the box can rest on it.

The clamp will hold the box in place and the foam pad will prevent it from wobbling around.

Anyway, at this point I discovered something that triggered my OCD quite a bit.  Turns out that the spare tire clamp hole isn’t exactly centered between the left and right sides of the vehicle, which means that when I clamp the box down, it’s not centered…

It’s actually about an inch to the left.  This bothers me quite a bit more than it should, but seeing as it has absolutely no effect on the performance of the speaker or the box and is going to be hidden under the floor board most of the time anyway, I’m going to have to just let it go…. for now.  Maybe someday I’ll go back and patch up the hole in the bottom of the box and drill a new one about an inch over.

Anyway, I tossed a few handfuls of polyfill into the box to help scatter the sound waves inside the box a bit.


Now to put the top back on…

I had some felt left over so I just laid it over the top of the box so that I wouldn’t have the wood of the box directly up against the plastic of the underside of the floor board…

And finally, this is all you see when you look in the back…

I still need to make a few adjustments.  You can’t tell in the above photo but the box is actually a tiny bit too high.  The clamp can push it down a bit more (I didn’t tighten it very much), but I think I *might* have to make a new box top out of .50″ MDF instead of the .75″ that I’m using now.

Anyway, the sub is all ready and now I just have to wait about a week for the amp to be installed.  And yes, I know I need to take the proper steps to break in the subwoofer before amping it up too high.  Don’t worry.  I got this. 👍

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