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Help with box spec for new setup.


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Hello everyone, 


I am a semi-old bass head, who after joining the Army really didn't have time/money to do a lot of car audio stuff. Well now 20 years have passed and I have finally made the jump to get back in the bass game. I have the setup listed below just looking for some help with spec'ing a box to match everything.  I have tried some different online box calculators, however I just want to make sure I get the box right the first time. When it comes to all the port designs I get lost.  So below is what I am working with any help would be appreciated. 


Car: 2007 BMW E92

Box Available Dimensions: 33W, 13H, 28D 

Stock radio with LOC

Amp: Skar RP 4500.1 

Subs: 2 Skar VXF 12's  dual 4ohm wired to 1ohm

Cable: All OFC Sky High wire, Big 3 done, 1 line of 1/0 from 180 amp alt. (currently working with Singer on getting a 300amp Alt)

Power: 3- 100AH AGM batteries, 2 runs of pos/neg to amp multiple grounds for battery bank. On Dash volt meter. 


I think that's it. 


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Mmmm, that's a tight squeeze. Recommended box size is around 2 sq ft net per sub. 

With your dimensions did you leave room to the trunk lid? I would normally fire sub and port to the rear but with your dimensions it may not leave enough room for the sound to travel back around the box. In that case are you opposed to folding down the rear seats for maximum output and facing subs forward? then you could use all of your trunk. With 3/4 inch ply before displacements you have 5.56 cubic feet inside the box. which doesn't leave much for port and sub displacement. 


I recently built a box for one of these subs at 2 cubes net tuned to 33 hz with a slot that was 16 square inches per cubic foot of box size. So 32 sq inches of port. Box sounds very nice and hits cleanly down to the lowest bass notes on any of my rap songs. These subs dont "punch" as hard or quick as some, but overall I am pleased for the price.  Powered by 1700 watt NVX.


PS I highly recommend looking into a Kicker Keyloc. It helped bring back everything below 40 hz that my LCI Pro 2 did not with factory head unit in a bose system.


Anywho. My quick mock up using every square inch of the dimensions you gave would be 


33 wide, 13 high, 28 deep including double baffle (good luck getting the box in and out)

Port 5" wide x 11.5 inches tall 28 inches long 

according to my calculator this gives you close to cubic 1.9 cubes per sub  after sub displacements tuned very close to 33 hz with 15 sq inches of port per cube. 


This puts you right on the border of the port being to small for that power. At 2000 watts WiniSD says your port velocity will be 29 m/s which i believe you're supposed to be under 30 m/s


Do i think this box will fit in through the trunk opening. No

Could you make sacrifices and squeeze them in. Probably

Personally i would trade in those 2 12s for one that can handle 4000 watts and build the appropriate box for it. This is coming from someone that enjoys this sub. 


I am in no way an expert or a professional. I have built and calculated a few more than a handful of boxes and have had good results. I continue to learn every day and only have access to free calculators. If someone else has any ideas or changes to what i said please add/correct. My calculations are not perfect because my calculator is weird when it comes to the common walls of a port. 


Ps there's people on here and other forums that I guarantee can design a box that's better than what i just came up with, but they're not always free.


Also, some basics on port design. General beginners rule of thumb is 14-16 square inches of port size per cubic foot of box size, so a 4x4 port would work in a 1 cubic foot box (not a perfect rule)

Longer ports lower your tuning, shorter ports will raise your tuning

Larger boxes lower your tuning, Smaller box will raise your tuning

A lot of people recommend tuning between 31 and 33 hz from what I've seen. 




Edited by deon1818
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Thanks for the reply.  So your response did raise some of the concerns I had yesterday when I was remeasuring everything before I started building. The main concern was facing the box rear and being pushed up against the trunk lid and not having enough room between subs and trunk lid. I am fine with facing subs forward and putting the seats down. So after remeasuring a few times, this is the box I was going to build. I know the measurements are a little different from what I gave but they should work space wise. So subs facing rear the box was going to be 34"w, 14"H, 26"D(Total depth of the trunk is 34"ish), the depth change is to account for the space between subs and trunk lid. Port tuned to 33hz, port area is 59" port inlet 4.72 x 12.50 gives me a 28.06 port length. This should give me a net of 3.90 cu ft. However I am no designer and this is with some software I found other people suggesting. So could be totally off, that's why I asked on here before I started building. 


You are right the box will have to be slid in from the cabin with the seats folded down. As for the LOC I have Kicker KISLOC 2-Channel K Series.  I have thought about selling the subs and going with a single 12 like Sundown, DC or Death Bounce. Something in the 700-900$ range. Thanks again for your input, you have given me a lot to think about. I really just want a system that hits harder than your average daily driven system. 




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Okay i made a mistake. When I Put this into winisd i only applied 2000 watts which would be for one sub. your port velocity would be 42 m/s at 32 hz. at 4000 watts Not 29 m/s

the port i recommended is on the small side, but you probably won't hear the chuffing if your rear facing. It will drive you nuts if you forward face. 


Are you using sub box pro? Your math looks spot on. And don't get me wrong these would hit pretty hard, you could probably get a bit more out of them if the port was larger and longer.  


I'm no box designer either. Just throwing out some info I've gathered.


A single 12 inch sundown zv6 would be  "harder than your average daily driven system" :) i believe sundown uses a five minute test tone to rate their rms so they are on the conservative side. as long as your amp is sending clean signal the zv6 should handle it well. 

You'll pay more per pound with DC but I've heard good things about Deaf bounce as well.  


You could always build this box. Run them for awhile and if it doesn't fit your needs sell the subs with the box for bookoo bucks because people will pay for a low tuned box that looks decent. And switch to a single monster 12 


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Just wanted to give an update on where I am at. I just finished the box build and have everything installed. I have pictures however unsure on how to post them. The box sounds great compared to the prefab I had installed until I built a box. The prefab sucked majorly.  Like we knew, the box was a pain in the butt to get it in the trunk. Had to go in through the cabin and squeeze it through the opening. It fits and I have everything hooked up however with the battery bank installed the back seats won't go all the way up and lock lol. I will have to do some adjusting when I get time. Also with the this box installed there is no getting to the factory battery without disconnecting a lot of stuff. Oh well luckily all the batteries are new. Being a European car I have a bunch of H9 group batteries installed. Might have to make the switch to lithium eventually and save some space.  While this box is nice and made out of birch I will probably build one later down the road. Thanks for all the help. 

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Hello everyone, 


So I wanted to give one more update to maybe help someone else out in the future who is searching for some answers. 


If you've read the previous post you will see that I was asking about box specs. Well I built the box installed everything and started tuning, at first I had the box facing the trunk as most people do. This sounded great once you turned it up loud enough to drowned out all the trunk rattle. I went through the trunk tightened, removed, glued, taped, adjusted trunk and everything thing that I could find that rattled I tried to eliminate it. In the end it helped but still had a bunch of rattles. So as opposed to spending hundreds more $$ installing sound deadening material, I decided to flip the box around and fire the subs into the cabin. This fixed 95% of the rattling while still sounding good, however I lost some of the deeper bass notes and it didn't seem as loud. When I popped the trunk it would sound even better and deeper and get louder. So I did a ton of research on this and about cabin noise cancellation, I mean a ton on sealing subs from the trunk, not sealing subs, aero port vs slot vs subs front port to the side. Some say to seal off the trunk other say some of the loudest trunk builds are subs forward not sealed. What I found was there really isn't a definitive answer on what's the best way and you just have to try everything out. Because every car/truck and equipment act and sound different in different situations .So here are the results from my setup in a E92 that might help someone out down the road. 


Subs facing trunk 6-12" from trunk = subs hit hard and deep, but lots and lots of rattle for me at least. 

Subs facing cabin rear seats folded down port on drivers side not sealed off = rattle pretty much eliminated, good bass response just lost a lot of  deeper notes of bass. 

Subs facing cabin slot port on drivers side subs sealed off from trunk = Again almost zero rattle and now it is much louder and the deeper bass notes are back again. I would say this is even louder and deeper than subs facing trunk. When I say sealed off I mean I removed the rear deck lid/C pillars and sealed off all the holes and seams there I could find. Basically used gorilla water sealing tape and taped over the whole rear deck. Reinstalled the rear deck lid and then sealed the Sub box along the rear fold down seats completely. Eventually I may go in and foam all the holes as well. 


Well I hope this helps someone or at least this is a good read for someone searching if they should face their subs rear vs forward. 



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