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Discussion Starter · #1 ·
Im about to build a pair of 18" ultimax subs in a sealed enclosure to go behind my acoustically transparent screen. The recommended internal volume is 4.22 cf per enclosure. Given that I have 18" of depth remaining between the screen frame and rigid fiberglass (OC 703) on the back wall, I was leaning towards dimensions of: 14" deep, 29.5" wide and 23.5" high (all dims include the 3/4" MDF).

My concern is that its a shallow box for its size, and wouldnt want it to "tip itself over" not sure if that'll be a real concern, which is why im asking on here. Also, is there a reason not to go wider and increase the internal volume? below is a pic for reference on what the speaker would look like alongside my AT screen frame wall.

Should I use more of the 18" depth? Bolt the woofer to the slab?
3119828
 

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The BTS boxes are ported too though I believe. If the OP isn't set on sealed, the BTS would be the way to go.
 

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To answer your question about rocking, the standard 4cuft partsexpress boxes will move a bit when driving the UM18's hard. Nothing crazy, but the box on some rubber feet will still move a little bit. Just enough to notice sub placement has changed, not like they're walking around the room. The box doesn't really care how deep it is for walking purposes but I'd consider using spikes or soft rubber feet depending on your flooring.

To answer your question about box volume, I wouldn't hesitate to make the box on the bigger side. You'll lose a touch of power handling, and the deep bass will come a hair easier. If you have DSP and ample power, a little extra box size is not a bad idea, 4cuft is a little small for the UM18 IMO, or rather, it's the smallest viable sealed enclosure IMO. Again, depending on goals I'd go up to about 6cuft without any hesitation.
 

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To answer your question about rocking, the standard 4cuft partsexpress boxes will move a bit when driving the UM18's hard. Nothing crazy, but the box on some rubber feet will still move a little bit. Just enough to notice sub placement has changed, not like they're walking around the room. The box doesn't really care how deep it is for walking purposes but I'd consider using spikes or soft rubber feet depending on your flooring.

To answer your question about box volume, I wouldn't hesitate to make the box on the bigger side. You'll lose a touch of power handling, and the deep bass will come a hair easier. If you have DSP and ample power, a little extra box size is not a bad idea, 4cuft is a little small for the UM18 IMO, or rather, it's the smallest viable sealed enclosure IMO. Again, depending on goals I'd go up to about 6cuft without any hesitation.
Piggybacking off of this a bit, based on your comments in my thread, I ended up building two UM18 sealed boxes at around 6ft3 since winISD showed that to be a good spot for more low end at the power I'll be running, and I figured I'd go sealed for future upgrade integration (thinking I might build some subwoofer end tables too). I'm not finished with the build yet (painted the boxes last night), but before I seal them up, is there any reason to line the walls of the box? I know with ported it's a good idea but I don't think there's any benefit in a sealed box... just want to confirm.
 

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Line walls? No, or minimal. Stuffing? Yes. Drop the box Q a bit, help bottom end a tiny bit. 6cuft is a big box, and you want to stuff lighter on larger enclosures so I'd probably put ~4-5lbs in there. Something close to that. Probably buy 10lbs of polyfill and split it.

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Line walls? No, or minimal. Stuffing? Yes. Drop the box Q a bit, help bottom end a tiny bit. 6cuft is a big box, and you want to stuff lighter on larger enclosures so I'd probably put ~4-5lbs in there. Something close to that. Probably buy 10lbs of polyfill and split it.

Chris
Sounds good. Looks like 10lb of polyfill is cheap enough at Walmart that it's worth a shot.
 

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What Chris said.

Unless you need to restrict excursion to line your graph up with a particular power output rating across the entire spectrum, I don't have a problem using less power to do the same work at the bottom end. Less power means less voice coil heat. It will make it easier to mechanically overload the cone, so if you want to be able to throw everything possible at the driver and minimize the opportunity for mechanical damage while exceeding the clean playback capability of the driver without having to do any fancy frequency-dependent limiting, that is a valid consideration, no doubt.

But, in my set of priorities, if I am playing something loud enough to to create audible distortion, I just turn it down. So, mileage varies, of course.

If you have LOTS of space, maybe even take advantage of it and just put in more drivers. As BassThatHz often points out, higher efficiency, higher power handling capacity (whether you use it or not) lower distortion, etc.

While the dual 2 ohm coils are rate for a nominal 4 ohms in series, the ones I got from 20Hz upwards never dropped below 6 ohms, and averages a lot more than that, so you might even be able to get good results running a pair in parallel with an amplifier capable of an 'average' 4 ohm load, depending on what you've got. The unit measured on databass..>

Data-Bass: Subwoofer Measurements

went a little lower, but not much. It's a thought anyway. :)
 

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Yeah, UM18 is closer to a 6 ohm load for sure. Although there is no way I like that cone in a tiny box, I don't think it has enough motor to do that justice, so if you put two in one box make 'em BIG.
I didn't really like the flatpack for these because it seemed too small to me. WinISD says a Qtc of 0.89 which seemed high. My 6ft3 box is around 0.79 which is still on the high side honestly.
 

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Discussion Starter · #14 ·
You guys (as always) have helped me make up my mind. Ill add some spikes for feet and increase the volume of the box a bit. Thanks!
 
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