Intro: I have been building car audio subwoofer enclosures for over 10 years, with tons of experience in audio engineering and acoustics. If you have any questions feel free to leave me a comment on this instructable and I will try my best to answer you in a timely manner.
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Step 1: Design to Fit Customer Needs
For this build the customer had a 2008 Mercury Milan, he was wanting a very musical enclosure with tons of low-end response with not very much power. In order to achieve this I decided to go with a 4th Order Bandpass with a 2:1 ratio. 4th Order Enclosures are optimal for low power, high output music designs.
I initially wanted to do a "folded horn" design but was limited with the amount of trunk space I had to work with.
The drive I used is a Sundown Audio X 12" on a C2 Audio 1200 watt amplifier.
Step 2: Acquire Raw Materials
For this build because it was going into the trunk of a vehicle I wanted to keep it as light-weight as possible, so I choose to use 3/4" Baltic Birch Plywood. Not only is Birch Ply MUCH lighter than standard MDF board, it is much stronger...and because this customer wanted this box to match his wood grain interior, this was the perfect choice.
To attach the wood panels together I used Titebond 3 "Green" Wood Glue, and pocket hole screws...alone with a few 1-1/2" Decking Screws
Step 3: Cutting Subwoofer Baffle and Box Panels
This step is fairly simple, I cut all of the panels to match my initial blueprints.
For the subwoofer baffle I decided to do a 1.5" thick face, with a 1/8" deep flush mount. This made it much easier to position the subwoofer flat on the baffle, and gave extra insurance that the gasket would make an air tight seal.
Step 4: Attach Side Panels and Port
After cutting all the panels, I began to attach them. For this step I used mostly pocket holes for aesthetic reasons...once the glue joints dry the screws no long serve a purpose anyway.
I made sure to have all of the joints COMPLETELY covered with a good amount of wood glue, I also went back over the joints with extra glue once the joints were initially dry.
Step 5: Add Bracing to Prevent Panel Flex
One of the worse ways to lose sound quality and volume is from panel flex. To prevent this from happening I used a piece of 5/8" threaded rod, along with matching washers, lock washers and nuts.
This box is VERY solid and the panels have absolutely no flex to them.
Step 6: Prepare and Mount Wire Terminal Cup
For this enclosure I decided to use a nice looking gold plated terminal cup. I used some 8 gauge OFC wire for the speaker wire to insure the least amount of wire resistance possible with the most power transfer from the amplifier.
Step 7: Stain and Polyurethane
After the box was finished I decided to use a "Cherry" color stain, and 5 coats of ultra gloss polyurethane to finish and protect the plywood.
Step 8: Wiring the Subwoofer and Sealing the Sealed Chamber
At this point all that was left to do was wire up the subwoofer, and apply some 3/4" foam sealing tape around the edges of the sealed chamber.
I was skeptical of sealing the chamber this way, however after testing I found it to be VERY effective and I will for sure use this method again!
Step 9: Place Enclosure Into the Vehicle
The final step was to put the enclosure into the car, and wire to the amplifier.
Thanks for checking out my instructable! If you have any questions feels free to leave a comment and I will try to answer them the best I can!
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