Since half of the goal of this project was to be able to match the output of a full-size sub, I knew that I would need to use a ported enclosure to achieve the necessary low-end extension. Likewise, I could tell from some quick modeling that a single driver wasn't going to be able to match the sheer output capabilities of a full-size sub. So, I started to look at how many drivers it would take to get the output I needed, while at the same time providing a healthy amplifier load and keeping the enclosure volume small. It turns out that four of the W6-1139SGs in a series/parallel wiring configuration provides an excellent balance between output and enclosure size, while still providing a 4 ohm load for the amplifier.

The second half of my design goal was to make the subwoofer physically narrow and compact so it could slide underneath a couch or bed. I began to contemplate what the possibilities were with my given drivers. The main advantage of the W6-1139SG compared to other subwoofers is its shallow mounting depth, so I figured that this is where I needed to steer my design. Needing only about 4" of clearance behind the frame, I could in theory create a finished subwoofer that was only 5" thick. Luckily, even four ported W6-1139SGs still only require anywhere from 1.0 to 2.0 cu. ft. to get some good bass extension out of them.

Planning on a 5" thick cabinet built from 3/4" MDF, this leaves 3-1/2" of internal cabinet width. So, doing themath and shooting for a net volume of about 1.2 cu. ft. not including space taken by the ports, we can see that we need about 750 cu. in. of internal surface area for our "thin" subwoofer. So, the external dimensions can be in any configuration that works for your situation: 29" x 29", 24" x 36", 12" x 74", or any variation in-between. For my design I chose a 24" x 36" size, which will fit handily under a loveseat of mine.

One difficulty in working with small, long-excursion drivers is the port situation. They are difficult to tune because of the long port lengths that are needed to support the high port airspeeds that they can create. For example, in a standard cube with our four W6-1139SGs, we would need roughly a 4" diameter by 24" long port- that's tough to fit in a 14" enclosure! So, the wide, flat design actually makes the tuning of our sub a little easier. I am using twin slot ports, which run the length of the cabinet and are 2" wide. We have essentially two 2" wide, 3-1/2" tall, and 30" long rectangular ports, which result in a tuning of roughly 29 Hz.