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Custom Van Subwoofer Box Build

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Jack1991 View Drop Down
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    Posted: 17 August 2022 at 3:58pm
As i got so much help from this forum in the past i thought id post this here first to get your advice/opinions and to make sure ive done my calculations correctly. Also saves me joining another completely new forum dedicated to car audio, just for this. But if im not allowed to post about this by all means delete it.

After seeing a mates mental car audio setup at Boomtown it instantly gave me the urge to get planning my own. Ive got a VW Caddy Maxi which i use for camping/festivals for just me and my girlfrind. So we dont have loads of room for a subwoofer/amplifier, but had a good look round yesterday and the only place i can have it and amount of space im willing to lose is under the bed.

I tried a fair few subwoofers on winisd yesterday, but the one that gave the best results for the smallest space and within my budget is the 10" Hertz Cento CS250 S4

Im thinking of using 24mm Baltic Birch Plywood so its solid and less resonant. (luckily i can get it free off a mate too) is this overkill or is thicker always better?

Outer dimensions
47.5cm deep  x  30cm wide  x  23cm high

Minus driver and port displacment that would give me a 17.12L box tuned to 38hz

It would give roughly a 5cm gap from the top of the subwoofer driver to the underside of the bed. Would this be ok? 








Frequency Response


Slotted Port Sizes


Cone Excursion


Rear Port Air Velocity




Any help advice appreciated, cheers 🙂👍🏼





Edited by Jack1991 - 17 August 2022 at 7:42pm
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Xoc1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Xoc1 Quote  Post ReplyReply Direct Link To This Post Posted: 17 August 2022 at 9:14pm
Can you adjust the sim power to give Xmax and then the output & port velocity might be more relevant.
Also I think your winISD sim might be in Halfspace - i.e. a sub just sat on a single flat surface, so not quite what you might expect in a van. Tuning lower might give you better bandwidth & power handling, with what I expect would be limited signal procesing.
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Jack1991 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jack1991 Quote  Post ReplyReply Direct Link To This Post Posted: 17 August 2022 at 10:16pm
Originally posted by Xoc1 Xoc1 wrote:

Can you adjust the sim power to give Xmax and then the output & port velocity might be more relevant.
Also I think your winISD sim might be in Halfspace - i.e. a sub just sat on a single flat surface, so not quite what you might expect in a van. Tuning lower might give you better bandwidth & power handling, with what I expect would be limited signal procesing.

Cool thanks Xoc1, still learning winisd but i assume thats just changing the 'system input power' on the 'signal' tab from 1w and increase it until the pink line reaches the green horizontal line on the cone excursion? 

Im away from my laptop till the morning so will do then and post results.
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Jack1991 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jack1991 Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2022 at 10:52am
Right okay, ignore my last message, i understand now ive had a play around with it on winisd.

I changed system input power to 300w because thats what it would be getting from the amp RMS.

Ive kept everything else the same, but yeh rear port air velocity is off the chart. Ive read it should be kept below 17ms.



With the Cone Excursion i understand the whole of the pink line needs to stay under the green horizontal line, not just the hump. But is there a 'amount' below the line it needs to be?



Ive been watching some good tutorials on winisd which explained cone excusion and port velocity well, but regarding the Halfspace im not quite sure what you mean sorry? Or does it just mean that inside the back of the van it will act differently to just outside in the garden say. Something i could fix with e.q'ing and maybe a measurement mic.


Edited by Jack1991 - 18 August 2022 at 12:25pm
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charlynch View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote charlynch Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2022 at 12:17pm
I don't know what processing you have available but something to bare in mind.. my dsp head unit doesn't have HPF on the sub Ermm so the LAB 12 sub EmbarrassedBig smile in my camper is in a sealed cabinet to keep the excursion under control.
Remember too that you will get a boost at certain frequencies due to cabin gain, there is a way to estimate cabin gain with WINISD, I forget the formula now but I'm sure you can dig something up. 
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Jack1991 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jack1991 Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2022 at 2:08pm
Originally posted by charlynch charlynch wrote:

I don't know what processing you have available but something to bare in mind.. my dsp head unit doesn't have HPF on the sub Ermm so the LAB 12 sub EmbarrassedBig smile in my camper is in a sealed cabinet to keep the excursion under control.
Remember too that you will get a boost at certain frequencies due to cabin gain, there is a way to estimate cabin gain with WINISD, I forget the formula now but I'm sure you can dig something up. 

Cheers charlynch, cool yeh i will check out some sealed designs too, i didnt know they keep excursion down but that makes sense.

Ive been reading up this last hour or 2 on cabin gain and just finished watching a great video on it! Makes alot of sense and something as a newb i hadnt though of considering. When im home later im going to check out a few more subs on winisd and compare with the cabin gain.

Heres the video if anyones interested: http://youtu.be/Zs4eeZsNEYQ

So apart from cone excursion, rear port air velocity and cabin gain (obviously box & port size too). Is there anything else i need to take into consideration when on winisd?


Edited by Jack1991 - 18 August 2022 at 2:10pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2022 at 3:50pm
if you get the right driver and cabinet volume you can use cabin gain to get flatter response with a sealed box than a vented one. 18mm or 15mm birch will be sufficient for a small box
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Contour Quote  Post ReplyReply Direct Link To This Post Posted: 18 August 2022 at 5:41pm
Agree on both. 
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Jack1991 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jack1991 Quote  Post ReplyReply Direct Link To This Post Posted: 19 August 2022 at 10:40am
Cool cheers guys, i had a quick look last night on winisd at the same driver but in a sealed box, with 300w system power. It looked pretty good and cone excursion was well under the line.

But when i added in the cabin gain on winisd the cone excursion goes through the roof. Does cabin gain change cone excursion in real life? The frequency response also went to basically flat from 20hz, this doesnt seem right.

Ill post photos when i get home to my laptop.

Is there a ideal f F3 you would say to aim for, for a car audio sub, or just the lower the better? Ive read to keep QTC at .707


Edited by Jack1991 - 19 August 2022 at 11:06am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jack1991 Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2022 at 9:46pm
Been looking tonight on winisd trying out drivers and adding in the cabin gain the way i saw to do it on that video. You can see what ive done to add cabin gain in. I have set signal power to 300w as thats the max it would recieve rms from the amp. In a 19L box.

Heres before i added the cabin gain


Cone excursion before cabin gain



After Cabin Gain, this doesnt seem right, or is it litterally getting all that free bass?


And cone excursions through the roof!!



Could anyone help explain please? 


Edited by Jack1991 - 24 August 2022 at 7:50am
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