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Old 01-01-2017, 07:02 AM - Thread Starter
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Help with modified port on JohnnySub

Hi all and happy New year!

I'm wondering if one of the guru's out there can help me out...

I'm looking to build a Johnnysub (with a UM18-22), however, I don't like the style of the ports on any of the marty subs or johnnysub. I know they're designed like that to keep things simple...

I would like to do one inset port like the one they have on the small ported box over at DIYSoundGroup.
Like this:

Could someone let me know dimensions I would need to make the port width / height / length on the JohnnySub but in that style?

The port on the standard JohnnySub is 24in wide x 2.5in tall x 44in long.
I thought I read that when modifying the port you just need the overall opening and length the same.
So on the standard, if 24 x 2.5 = 60" opening
Subtract out the the two rails IN the port so it's actually 22.5" x 2.5" = 56.25"opening
Could I do a 17.25" wide x 3.25" tall port which would give a very similar, overall 56.06" opening.
(I would keep the same 44 inch length.)

Would this cause other issues that I don't know about?

Also - one last thing... I was thinking about adding 2 inches to overall height (making it 34" tall instead of 32"). This would add about a cubic foot and get it closer to the recommended 8 cu ft for the UM18-22.
Any issues with that? (in addition to the modified port)?

Any and all help is appreciated!



Last edited by BigDaveE; 01-01-2017 at 07:17 AM.
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Old 01-01-2017, 05:22 PM
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hi bigdave.

you are right on the money with respect to adjusting the port dimensions. so long as the cross-sectional area (port height x port width - braces; just as you have calculated) and the length are kept the same, the tuning point will be the same (well, within a fraction of a hz--for some technical reasons that are not worth getting into and you'll never notice the difference).

if you add a cubic foot to the overall size, that will give you a hair more output on the low end and will reduce your tuning frequency by a smidgen. but again, the changes that you are talking about are all very minor (perhaps ~1 hz type thing), so you are good to go.

be sure to use a high pass filter if you plan on hitting the subs with high power.

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