I decided to upgrade the stereo in my car by changing out the head unit (to one with blue tooth connectivity) and replacing the stock speakers. Afterwards, I was disappointed in the bass. I don’t have a desire to knock pictures off peoples walls but I would like to actually hear the bass guitar and kick drum. Thus, I need a subwoofer! I started out looking at 8″ subs but finally settled on a 12″. Why? Because a 12″ seems to be very similar in cost to a 8″, I could fit it, and with subs (everything else being equal) bigger is better. I ended up finding an Infinity 1262 dual voice coil driver for $60 which seemed like a good deal.
Most decent speakers come with recommendations on the size of the box to put the speaker in and the Infinity was no different. I wanted to use a sealed box since they require less space than a vented or bandpass. Also, subs in a vehicle get a low end boost from being in the small cabin which makes up for a sealed box falling off sooner than a vented one. Infinity suggested a sealed box that was 1.25 ft^3 but I wanted to double check their numbers. To do this, I used a program called WinISD which is a free box design software that has been around for a while. I’ve used it in the past with excellent results. Below is a plot from the program of the output in decibels plotted against increasing frequency. The sealed box’s response is shown by the yellow curve. More on that gray line in a bit.
At subwoofer frequencies the shape of the box isn’t that critical like it is at higher frequencies. So, I have a fair amount of freedom with the design. My car’s trunk has a tub shape to it that is sloped on the front and back. I took some measurements and made poster board templates until I found a profile that fit. Next, I went to LibreCAD and sketched up my design.
With my sketch in hand I headed out to the driveway to make sawdust. My metal working machinery has me wanting to keep as much sawdust out of the garage as possible. I ripped some MDF into appropriate sized strips with a circular saw and then used the tablesaw to cut everything to final shape. The front and back pieces on the box have beveled edges that are cut at 6.5 degrees and the end pieces have a keystone shape with straight sides at 6.5 degrees.
After I cut all the pieces out, I dry fit them to make sure I’d not screwed anything up. Happy with the results, I liberally coated the appropriate edges with a glue and clamped it up. Speaker boxes need to be airtight so they don’t leak air. So, excessive glue is allowed and squeeze out inside is ok. Note I’ve left the top of the box off because it needs a hole in it.
Bracing in a box is also a good thing to keep the sides from vibrating which can negatively affect performance. On a box this side its probably not required but better safe than sorry. Below you can see some bracing on the long sides of the box. I also added some bracing on the ends of the box which can be seen in one of the following pictures.
I’d left the top off of the box in the glue up above because I still need to put a hole in it for the sub. The tolerance on this hole is relatively large since the driver has a flange on it that will sit on the surface of the top. Cutting it a little large is preferred to too small since the circle jig for my router is screwed to the center of my circle. The router makes hole cutting super easy but you can also use a jig saw with good results.
I mentioned above about how a box should be airtight. Another way to do this is the caulk the box with regular window caulk. In my case it’s simply extra insurance. No one sees the inside of the box (unless you post pics online) so you don’t have to make caulk job perfect.
Once again, I used a lot of glue to attach the top of the box and clamped it to dry. Then I used my finger to smear caulk around the inside of the top since my caulk gun wouldn’t fit in the box at this point. To get the signal to the driver, I used a terminal and drilled some holes to run the wires through. Normally, I’d suggest black and red wires, but I have a lot of 12 gauge red sitting around from my shop circuit build. Instead I just tagged one of the ends of the wires with black electrical tape to indicate negative. Finally, as far as woodworking goes, I rounded the edges of the box over to help it fit in my car better.
When it came to finishing the box I used a simple coat of paint. It’s pretty well hidden in my car and I’m not showing it off. So, paint is good. I crimped a couple quick disconnect terminals onto the wires and stuffed the box with polyfil which has the same effect as making the box a little bit larger. To hold the sub in place you can either screw it to the wood or use T nuts to allow the use of machine screws. Finally, I stuck down the supplied gasket and installed the sub.
Remember that other curve in the frequency response plot above? I’ve been wanting to replace the crappy sub in my surround sound system with a real one for a while. I fiddled around in WinISD and found a design I liked. I had most of the sheet of MDF left over and decided to see how this sub would do in the home theater roll. I ended up going with a vented (ported) 4 ft^3 box tuned to 21 Hz. A vented box will usually go deeper than a sealed box and the port allows you to further tune the response of the box. Of course there are exceptions to this as some drivers do much better with a certain box type. By changing the sizing of the port you can change the resonance frequency of the box. For my box this required a 4″ port that was ~16.75″ long. If you look at the plot above you’ll see the sealed box (yellow line) is at -3db at 40hz but the vented one (gray line) makes it all the way to 20hz before hitting -3db. (As an aside, a drop or increase of 3 db corresponds with a halving or doubling of the perceived volume.) In a home theater sub you want to go as low as you can to capture all of the T-Rex stomps, explosions, and superhero battles.
Anyways, I designed this box to fit under the bunk bed in our “game” room which is relatively small. As a result, it’s very shallow and wide at approximately 11″ x 37″ x 26″. The port comes through the short side since you don’t want the opening in close proximity of a perpendicular wall. It’s a simple rectangular box design that also gets reinforcement bracing. Once again, all the joints were caulked. Obviously, there’s a big leak in the form of the port but other leaks can shift the tuning of the box which is bad. I used the router to cut the speaker opening as before. I also used a smaller router to cut the opening for the port. The port’s hole needs a much tighter fit since I’m using PVC as my port. Thus, I did multiple test cuts in scrap until the fit was perfect. To glue the port in place, I put a ring of caulk on it and pushed the port into place. It’s mostly blocked by a clamp in the picture, but the back of the port is also supported to keep it in place. Once all the glue and caulk was dry the clamps were removed and the top installed.
I tested the box and was very happy with the response demoing it on a couple of movies with my 130 watt amp. The Millennium Falcon in the new Star Wars had some impressive rumbling associated with it and the Hulk vs Iron Man fight in the second Avengers movie had some good explosions and an enveloping bass drop. Yup that’ll work. After that I rounded the edges, ordered a terminal cup, another sub, and painted the box. Final wight on this monster…75 lbs. It gets transported with a dolly.
There’s another issue with a vented box that I need to address. Below the box’s frequency, the driver’s cone can easily be driven beyond its limits and damage the driver. To counteract this most amps come with a subsonic filter which blocks “really low” frequencies. What “really low” frequencies are depends on the filter but most are probably around 20 hz. Most sub amplifiers also have a low pass filter that will filter out frequencies above a limit which can usually be set by the user. Together these two filters keep the sub playing safely and efficiently. I was given the amplifier I’m using by my dad and it doesn’t have a subsonic filter or low pass filter. To fix this I’ve ordered a graphic equalizer with variable subsonic filter.
Sealed subs may or may not need a subsonic filter due to the box design. One wouldn’t hurt though. Regular music usually doesn’t get into the really low frequencies those that are found in movies. Of course some classical, music with a double bass, the Overture of 1812 with actual cannons, or specialized “break your subs” bass tracks can. If you’re curious what frequencies are out there you can find software that will analyze the frequency spectrum in music. There’s also apps for your phone or tablet that will perform a real time analysis on what they hear (subject to how good your mic is).