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Discussion Starter · #1 ·
Hi all, I fear I've spent time and effort (no problem, learning experience) building my first fibreglass box and made it too small. The two 10" subs call for, at a minimum, .5cu ft per sub. Because of the limited space I had to work with I did some rough calculations and built what I thought was going to be just a bit too big, so that I could remove volume from the inside of the box as necessary.

As it turns out after filling the box with water to measure its volume, I ended up with approx. 27 litres... which is just under the 28.32 litres that make up a cubic foot. And to boot, I'm not counting the lost volume of the sub baskets.

From what I've read the volume of a sealed box isn't as critical as in a ported box, and that adding dacron stuffing makes the volume 'appear' larger. Can anyone say if they think I'd be wasting time to put any more effort into a box that is too small, or is it salvageable by virtue of being a sealed box that stuffing can help compensate for?

here's a shot of the box....

Fiberglass subwoofer box for Porsche 911 - DiYMA Gallery

tia
 

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Discussion Starter · #5 ·
A couple of IDQ 10's should rock the house with that airspace.
I've already got two MB Quart shallow 10"s (RSH254) that the box was built for... for now I'll to stick with those.

I think to increase the volume, even by a little bit, I may build a couple of mdf rings on the outside of the box to mount the subs too.... raise them out of the box a bit.
 

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Discussion Starter · #7 ·
Acousta stuf

Parts Express:Acousta-Stuf Polyfill 1 lb. Bag

You can also try mounting the sub face down with the back up in the air to see if it will play lower notes with more authority;)

As it currently sits, the seats are literally 1 inch away from the front mounted magnet, so I don't think I'd have the room to try a reverse mount. That would certainly leave more volume though.

That Acousta-Stuf looks pretty good, better than dacron it says. Unfortunately i'm in the middle of BF NoWhere and will have to wait for an internet order. =[ I wonder if dacron is the best substitue until then.
 

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John Krutkes' thoughts ...

Opinions?
jkrutke
02-10-2007, 04:51 PM
I used to think the same thing, but it's not just pillow stuffing. Acousta-Stuf is made of large trilobal fibers while Dacron is smaller round fibers. The former does a much better job of damping and absorption. I don't use Dacron anymore because I've actually heard the difference and have been able to see it in some tests.
 

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Have you actually listened to your setup yet? If it sounds good to you, you have succeeded. As a general rule of thumb on sealed boxes, the smaller you go(even below manuf. specs) your power handling goes up, while losing a small amount of the lowest end output. I think all you have done is to give yourself a little insurance against blowing them, and with a that minor of a difference in cu. ft., you probably will never hear the difference anyway.
 

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As it turns out after filling the box with water to measure its volume, I ended up with approx. 27 litres... which is just under the 28.32 litres that make up a cubic foot.
I think you'll be fine, but you could try using insulating fiberglass (loose) if polyester fiberfill isn't available. If you're concerned about bits breaking off can causing itching (or getting into the sub's motor), staple some jersey material or speaker grille cloth over it.

I suspect that you may have other problems to deal with though - lots of large flat spaces in that subwoofer box - fiberglass isn't the best material for that. Hopefully you've got a lot of bracing in that box.

Hmm... did you consider using the front trunk to host the subwoofer box and finding a way to vent it into the cabin? That would give you true "up-front" bass, and a lot more space to work with as well.
 

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Discussion Starter · #12 ·
...yes I got everything installed at the end of last summer. I haven't had a lot of listening time but enough to know there are improvements to be made. I think one of the major problems with the box currently is what Brian Steele is hinting at.... I could use a lot more glass to stiffen things up. Currently there is no bracing at all, complete fibreglass. Being my first glass project I will concede that my gauge of required thickness might not have been accurate. I think I heard something about a 'rule of thumb' test where if you can't flex the box with pressure from your thumb it's probably good. Guess not, well.. we'll see after this anyway.

I've got everything apart at the moment and purchased polyfill (for now) and more fibreglass for another round of reinforcement. I just made another set of 10" mdf rings to mount the subs another 3/4 in higher, outside of the box, to give a tad more volume inside. That'll also give me a ridge to build glass up to. I'll make double sure that it's air tight and try again. Worst case scenario I'll have learned a whole bunch of things not to do for when I build the next one. =]

I know I asked this before but while i've got your attention, can you guys say if placing a divider or chamber separator down the middle of a sealed box is necessary or not? I'm thinking 'not' but will take a more educated opinion if it's out there.

Thanks guys...

ps. Brian Steele: yea I did consider the 'smugglers box' in the trunk but I've heard that it can only house a single 10" at best. My friend has the same car with an 8" Bazooka tube in the trunk and it sounds remarkably good considering that part of the car is completely segregated from the cabin. There's no chance of anyone ever getting in the back seats so I didn't mind experimenting with a dual 10 box across the floor. I also have the feeling that I'd have to produce way more bass to get the same effect in the car if I went with a box that was 'outside' the cabin (ie. trunk) as opposed to 'inside'.
 

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I think one of the major problems with the box currently is what Brian Steele is hinting at.... I could use a lot more glass to stiffen things up. Currently there is no bracing at all, complete fibreglass.
You could try using some 1" dowels and an extra layer of glass to cross-brace and stiffen the enclosure. However, I suspect that there might still be noticeable vibration - and the volume of the box would be decreased.


I know I asked this before but while i've got your attention, can you guys say if placing a divider or chamber separator down the middle of a sealed box is necessary or not? I'm thinking 'not' but will take a more educated opinion if it's out there.
The divider will help to stiffen the enclosure even further.
 

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Discussion Starter · #14 ·
You could try using some 1" dowels and an extra layer of glass to cross-brace and stiffen the enclosure. However, I suspect that there might still be noticeable vibration
Gee that's crazy.... I would not have thought there would be that much flex but I guess I'm basically building a sealed lung that's trying to huff and puff, so it does make sense. I think because of the strange size and limited space inside the box I may just go nuts on the fiberglass instead of adding dowels or bracing inside. I will take pics and update this post.

I really appreciate all the advice and look forward to success because of it! Thanks!
 

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Gee that's crazy.... I would not have thought there would be that much flex but I guess I'm basically building a sealed lung that's trying to huff and puff, so it does make sense. I think because of the strange size and limited space inside the box I may just go nuts on the fiberglass instead of adding dowels or bracing inside. I will take pics and update this post.

I really appreciate all the advice and look forward to success because of it! Thanks!

I've got a 3.2 cu.ft. spare tire well subwoofer box. The bottom section is fiberglass (several layers, and the top is about an inch of wood). I've got several cross-braces in place between the ply and fiberglass, and angle-iron crossbracing below the wood itself. The result is an enclosure strong enough to stand on.

It contains one 12" driver.

It still vibrates a bit too much for my liking.

In a week or two I will be bonding another layer of 3/4 ply on top, and if that doesn't reduce the vibrations noticeably, next step is to rebuild to box. Noticeable vibration=muddy upper bass response.

If you're getting noticeable vibration, adding an extra layer or two of fiberglass might not be enough to cure it.
 

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Stuff it. Maybe just try the dacron until your Acoustistuff arrives if you have a spare donor pillow and install/uninstall is not too difficult. I went through this just last night w/ a Peerless XXLS that sounded a tad muddy when compared to an SI Magv4. Although I thought I was good to go w/ a Qtc model of .76, I kept adding stuffing in increments then listening and so forth until the stuff ended up just touching the basket, would not move when the box was tilted, but still pretty easy to compress to touch. This helped noticeably to remove a bit of upper bass/midbass bloat for more defined/easily distinguished bass notes. This also helped w/ better blending .
Since you state that your, "...10" subs call for, at a minimum, .5cu ft per sub." Your Qtc value may be on the high side so stuffing/increasing volume will lower Qtc and may help depending on what you are hearing and what you like to hear. In contrast, the recommended highish Qtc of .81 for the Mag sounds superb in my setups.. How did you like the fiberglassing process? Goodluck.
 

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Gee that's crazy.... I would not have thought there would be that much flex but I guess I'm basically building a sealed lung that's trying to huff and puff, so it does make sense. I think because of the strange size and limited space inside the box I may just go nuts on the fiberglass instead of adding dowels or bracing inside. I will take pics and update this post.

I really appreciate all the advice and look forward to success because of it! Thanks!
You would be FAR better off building glass "spines" inside the box to brace it.. you'll need less overall glass and a few well placed spines can stiffen things considerably.. You can use a dowel rod if you like, hell, i've used pencils.. all you need is something to hold the shape.. you could use some masionite, and just cut it into 1" wide strips.. sit on edge and glass into place..

As far as polyfill, you should have gone to walmart and bought a pillow.. or hell some pillow stuffing... lol... you would have saved time effort and money.. BUT, as you say, it's all a learning expierance..
 

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Discussion Starter · #19 ·
Well, I cut a slot in the box between the subs and dropped in a 3/4 mdf separator, then fiberglassed it in place. Raised the subs out of the box to give more volume with more 3/4 mdf rings.... glassed the **** out of everything, again. The glass has to be at least an inch thick in most places now. Packed in some fiberfill, hooked everything up and what was the diff? ... about 10% gain... which put the output at about 15% of what I expect lol. If you got in the car and didn't look back you wouldn't know there were two 10's behind you. I've seen out of phase subs cancel before, and although that's not what is happening here, I'm getting that same 'where'd the bass go?' effect. I'm soo ready to pitch this box, but I can't because I feel like I haven't learned why it sucks so bad! Terrible haha. Maybe there's a reason I've only ever seen guys try to squeeze 8's behind the seats.

Anyway, gonna go pull the door panels off ... mull over what to do next with that box. :mean:
 

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If the subs are rated for 200 watts rms , get an amp that will do 400 watts rms.

Go easy with the gain and listen for bottoming out [ popping sound ].

Infinite baffle mounting cuts rms handling ability close to one 1/2.

A small box allows for more power handling [ you need an amp that can do it though ;) ].
 
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