Car Subwoofer Enclosure Build




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.

Thank you for viewing!

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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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    32 Discussions


    Question 2 months ago on Introduction

    i am trying to fit (4) sundown sa10's in the trunk of a nissan altima 2008 model. i want to do a 4th order. can u give me some numbers i need to target? im not asking u to design the box unless u want to. i just need to know what i need for front side and back side per speaker or all together and port. and thanks. btw i am chris from north carolina

    1 answer

    Reply 2 months ago

    Hi Chris, for SA10s I would aim for around 1 cubic ft of space PER woofer for the sealed section. As for the ported section, I would utilize as much space as you can manage. Generally speaking, for a 4th order bandpass to be "worth" while, you need at least a 2:1 ratio. That being 2 parts ports per 1 part sealed (for example, 4 cubes sealed space for 4 SA10s you would want at least 8 cubes of ported area). If you do below a 2:1 ratio, you still increase your playable bandwidth musically, but you're not gaining any output in decibels. Honestly, if you have less than a 2:1 ratio I would look into doing a standard ported box rather than a bandpass. Hope this helps!


    5 months ago on Step 9

    Hey can you share the dimensions ? I'm building the same box for the same sub in a similar space.

    1 reply

    Reply 4 months ago

    Hello Lakrymist, unfortunately I do not have the dimensions of this enclosure. This was many years ago that I built it. Very sorry


    Question 10 months ago on Introduction

    i was wonder if you can tell me how to tune a ported box .. im trying to desing a slot port and a 4th order for myself i understand the size of the port is roughly half the cone are i just dont understand tuning fully

    1 answer

    Answer 4 months ago

    Hi Jacob, sorry for the slow response. Tuning a ported box will depend on a lot of different parameters. Mainly the specs of the subwoofer, and the environment it is going into. I can't really give any tuning suggestions without knowing details. Also, the same generally goes with port area, there is a ballpark number to aim for, but it will depend on the environment and subwoofer specs.


    4 years ago on Introduction

    I have been doing car audio installs for over 10 years. If you read the description under each photo you will answer the majority of your questions.

    1) This is exactly what the customer wanted. They did not care about trunk space, they wanted a loud and low end enclosure. That is exactly what I designed and built.

    2) As far as it being "too big" that is not a small subwoofer, even with a standard ported (or even sealed) enclosure, it would have taken up roughly about the same amount of room. Also, I put a TON of effort and time into the design and planning of this build. Taking everything into account, cabin dimensions, trunk lid sound wave loading etc.

    3) This box was intended to sound good, that was the only goal. He did not care what it looked like. All of the astectics decisions were made by me. Function > Looks.

    2 replies

    Reply 1 year ago

    thanks for you're input I'm currently searching the web for the proper measurements for a 4th order bandpass for 2 sundown audio sa10?


    Question 1 year ago on Introduction

    i have a sub woofer in my car and i am not satisfied with the basic seal box i built, and i want a folded horn box, and I wanted your opinion. the sub is a Rockford Fosgate hx2 its a 10" and im running it at 800 watts
    and 4 ohm stable.


    1 year ago

    Thank you for sharing this article. Actually i was searching best car audio speaker for bass and sound quality. I am a big fan of music, When I read your article then i realized you have all products that it is truly what I wanted.


    2 years ago

    MAN THIS LOOKS VERY SICK, im trying to find the best software to design this kind of box. can you tell me what software do you use?

    great job man!


    2 years ago

    Ok so I'm still rather new to the bass world I have a box currently at 53"x20"x16" I have a circle PVC port 3" tuned to 27 htz my issue is I cut part of my subwoofer hole to big so if I put a baffle in will that fix the difference if so how do I go about it can I put it on top and it still be ok or dose it have to be inside of box

    1 reply

    Reply 2 years ago

    It was made for 1 sub at 1400 RMS I wanted some insaine heart skipping bass


    2 years ago

    Can you help me with a design for my x 12 for best musical and get real good lows? Im willing to pay for a design , i have a 86 mustang gt hatchback and my power is sundown scv3000d

    gregg miller

    2 years ago

    What type of enclosure is this ? I am probably the worst type of commenter-questioner because I have some basic knowledge. Is a 4th order sealed/ vented chamber totally enclosed except the port. Am I mistaken that this was the type popular in the 80's-90's that was always seen with the plexi window in the center and knocked well but rarely played the lowest notes well. If so what is a 6th order and what are the advantages? It always seemed to me that the easiest to tune and highest output came from a ported "3rd order ?" enclosure. And why did sealed enclosures go out of favor? Are they just power hungry? I did a 150.7 db with 4 oldschool 12's "1995 RF Punch" in what I was told was a "SPL Bandpass" but it looked like dual sealed boxes with a wide open center. I really liked that box as it went low well and hit high #'s at the shows.I loved that enclosure, It would hit great at 40 hz,but still really well at 25 hz and it was insane at 65-75 hz but the insanity was something I always smoothed out using active x-over/equalization for Sq comps or just driving around. I guess the amps really did make a difference as well back then I had the only set of 50m & 50x2 RF Power Punches I ever saw in AZ at the shows and hooked up slave mono into a 1 ohm load those 50m's really put out the power in 1996, I never saw that type of slaving amps before then, You see it a lot now! I am blown away with what you can attain SPL wise for a few bucks nowadays as compared to 1996! Call it a mid-life, boredom or possibly my true colors showing later in life, but I've been putting a little labor into my wife's 1997 Blazer and have had some astounding success. The result sounds pretty darn close to my previous car... and for a whole heck of a lot less!!! With some prior experience, word of mouth, A little online research and some informative reviews her Blazer is a beast!!!

    It seems to me that car audio has gotten more affordable than I ever thought it would...

    Simple Kenwood CD head, 1 set preouts... $80 + dash kit no antenna adapter...I really only use the flash drive anyways

    1 set 4x6 Kickers in the dashboard $45

    1 set 6 1/2 Kenwood coax in the front doors $40

    Cache preamp/EQ $70

    Lightning Audio Strike 400.2 $70

    CAB 1600.1 $200

    3e@ 12" RE audio SCX 12 D4 all parallel .67 load $100 ea.

    All wire, blocks and terminales were under $100

    then I added a $70 battery and Wow!

    Who'd have thought in 1997 that you could attain these results for $1000 someday, it plays close to 150 db and takes me at full retard for 4 minutes to heat up the subs and start to smell the coils a little they're a little underrated for the capabilities of a CAB 1600.1 at .67 ohm but I can play music enjoyably at 148 db all day long, if anyone thinks it is enjoyable to begin with! The funniest part is I have it in a total sleeper 4 door Blazer/Mom's seat, dents and all and still have some cargo room for groceries. I remember a 12 ft3 enclosure, no trunk, hidden amps in a false seat and MDF panels on the doors and under the headliner area back in the day! I had great stuff holding it all together before anyone else was using it...what a mess! What a money pit! Aaah, I miss that one ! I did all my own work and I had $6000 invested! Someday I will probably add a 250A alternator to her Blazer to see what happens...eventually since its a lot more affordable than it used to be.If I do that I'm sure my $100 woofers will be history, who knows? RE puts some stout coils in their stuff I think.Years ago, 1996-7, I had a 152 db system that was musical pretty much all the way to 150 db. I spent 512$ each on a couple of stroker 15d2's, $1000 on two audio art 100hc's to drive them. an Mtx 4300 to drive my podded 6 1/2" RF power components the system was EQed/ X-Overed even had a RF Symmetry and a heck of a lot of 0 ga wire and Mechman 175 ampre that I bought C.O.D. for $660 " who does that anymore anyways?" all in 1996 dollars...look at what you can do nowadays! Old school was and is cool... but ya gotta love progress... look at that Sundown sub in the picture ...Jeez... that was not even on the radar in 1996.I guess I am rambling, I had a stroke, cant work,sleep and typing is a new skill I'm going to acquire to work now. Sorry in retrospect, my question was what differentiates a 4th and 6th order and what is a "SPL Bandpass?"

    1 reply
    rickdod3gregg miller

    Reply 2 years ago

    To answer your question "what differentiates a 4th and 6th order" a 4th order bandpass is a Sealed chamber combined with a ported chamber. The advantages of this type of enclosure is that since the drivers are initially in a sealed environment you get better cone control and power handling. You also get the benefit of the frequency range that comes with a sealed box. Adding the ported portion of the box gives you an even bigger frequency range, and amplifies the output greatly. However, unless you have the space to build a 4th order that has at least double the ported area of the sealed area...a standard ported box will have just as much output. Back in the day most prefabricated bandpass boxes were a 1:1 ratio...they sounded great, but weren't any louder than regular ported boxes.

    A 6th order bandpass is the same concept of a 4th order...with the difference being it is TWO ported enclosures combined into one (basically). Generally speaking the "first" ported box (the part containing the drivers) will be a low port area/high tuned vent that first into the 2nd chamber which has a high port area/low tuned vent...or vice versa. 6th orders require a TON of space to build and tune properly. They really aren't practical unless you're going for pure SPL numbers.

    Hope I clarified some of your confusion.


    3 years ago

    Can you let me know how you derived your chamber volumes and port tuning? I also have a Sundown X12D2 rev2 and am thinking of experimenting going from a ported to 4th order bandpass enclosure. However using WinISD with Sundown's posted T/S parameters I can't match your numbers.

    In the attached images I show the ported chamber at 3 cu ft tuned to 47hz and sealed chamber at 1.34 cu ft. The port length for a 3in x 16.5in area it calculates at 12.6 in long vs your 8 in port length. WinISD's gain plot also looks pretty ugly, one very narrow peak. Obviously I'm having problems :)