Sixth-Order Band-Pass Subwoofer Enclosure

After building and experiencing what generic reflex (ported) and sealed subwoofer enclosures had to offer, I decided that I wanted to challenge myself by making something more complex.

I designed, built, and tuned a series-tuned sixth-order blow-through subwoofer enclosure for my '03 GMC Sierra. It holds 2 12" drivers, contains about 10 cubic feet of net air volume, and is tuned to approximately 23 and 48 Hz.

4'x8' sheets of 3/4" thick, birch finished plywood

Construction Screws

Wood Glue

Construction Adhesive

Silicone Caulking

Hex Nuts and Bolts

T-Nuts

Threaded Inserts

Adhesive Foam Sealing Strip

Aluminum Tape

Speaker Terminal

Black Wood Stain

Spar Urethane

Accordion Boot

Finding information on how to design one of these took as long as the rest of the process. This type of enclosure is not nearly as common as standard bass reflex and sealed ones. Forums were my main source of information. HornResp was used to model and simulate the performance of the enclosure based on physical parameters. The plots of phase, time delay, output amplitude, and linear driver displacement helped me determine my design.

Once my design with dimensions was sketched up the pieces were cut from 3/4' plywood using my handheld circular saw. Unfortunately I do not have my drawings on hand. An opening was made on the top of the enclosure by offsetting the cut between each layer. Threaded inserts were embedded into the body so that the lid could be installed with threads. As a removeable form of bracing for this feature, a hex bolt was fastened into the floor under a washer with some screws, into which a threaded rod could be tightly inserted and tensioned to reduce acoustic vibration of the removeable panel.

For the enclosure's output to play into the cabin, a 48" by 8" hole was cut through the back of the cabin wall, and through the front of the bed's wall. An accordion boot connected the two sheet metal walls to keep my interior and the port of the enclosure protected from the exterior environment.

The bottom face was covered with a large rubber mat using construction adhesive to distribute pressure, increase friction, and increase resistance from wear against the surface of my bed. Five pairs of hex nuts and bolts were fastened through the floor of the subwoofer enclosure and truck bed, while a steel angle bar lined with self-drilling sheet metal screws and construction screws secured the back lower edge of the enclosure to the truck bed.

Room EQ Wizard was used to analyze the acoustic response of the subwoofers and their enclosure. Physical altercations were made to the enclosure's front and rear ports. The rear ports were lengthened, while the front port was narrowed incrementally. After many revisions, a final tuning profile of 23 and 48 Hz was achieved. Two kerfed pieces of plywood were installed in the front port to diffuse the airflow, reduce chuffing, and for aesthetics. The fundamental frequencies were then verified by manually calculating and plotting impedance vs frequency.

AudioControl's DSP App was then used to adjust phase, time delay, and amplitude response curve. The main goal was to create a smooth and in-phase frequency transition between the subwoofers and the front stage of my system. Furthermore, one tune profile was created to have a smooth, musical response, while another was created to have maximum acoustic output across a desired bandwidth.