I think my sub box needs more bracing

[yonyz]

Hi,

A few months ago I built a sub that uses an Infinity 1260w driver and this cabinet:

It sounds and measures very nice, but it has a problem with relatively loud bass - there are certain sounds that I'm pretty sure emanate from the box itself, like creaking sounds, as if the cabinet is not strong enough for the driver.

I did the "knock test" and compared the results with how my JBL Studio 230s sound (when I knock on their sides). The speakers have a very dull sound that decays immediately. So does the subwoofer cabinet, but only when I tap its front (double) baffle and back panel. When I tap the top panel, especially near its center, it sounds very different, like a fuller sound that doesn't decay fast enough.

I made a recording (click to listen): 4 knocks for each panel. The first 4 are of the JBL speaker (side panel), the 2nd 4 knocks are of the front baffle of the sub, and the final 4 knocks are of the center of the top panel of the sub cabinet.

Do you think it needs more bracing?

 

[j_garcia]

Yes, it looks like it could have used a vertical brace as part of the original design.

 

[yonyz]

OK, what kind of bracing can be added now that the cabinet is built?

 

[j_garcia]

You might be able to do like some 1" dowels between that horizontal one and the top one or maybe some corner braces that attach the sides and top or just one or two long strips along the top? Maybe even just place another sheet attached directly to the top inside to thicken it up?

 

[MrBoat]

A longitudinal brace on edge, made of plywood, that is as tall as the space left by the hole for the plate amp. Basically a stiffener for the top panel. What material is the cabinet currently damped with, and what material is the cabinet made of?

 

[Bucknekked]

A longitudinal brace on edge, made of plywood, that is as tall as the space left by the hole for the plate amp. Basically a stiffener for the top panel. What material is the cabinet currently damped with, and what material is the cabinet made of?

Yonz:
whatever MrBoat says, that's gonna work for me too. He actually builds stuff, so his opinion should carry some weight. Plus, how many people actually use the word longitudinal in a sentence? That alone should qualify as something you should consider. Most people might try horizontal or vertical and be done. Mr Boat goes the extra mile.

 

[MrBoat]

Yonz:
whatever MrBoat says, that's gonna work for me too. He actually builds stuff, so his opinion should carry some weight. Plus, how many people actually use the word longitudinal in a sentence? That alone should qualify as something you should consider. Most people might try horizontal or vertical and be done. Mr Boat goes the extra mile.

LOL. I was at a loss and the word just jumped in there. I see a few places in that cabinet that could stand stiffeners. MDF has next to no structural integrity when standard engineering practices are applied to it. If I build more cabinets that need bracing that will not allow for continuous entrapment from all sides and from the foundation up, I would use plywood stiffeners instead. MDF on edge by itself offers no support, evident by all the sagging particle board kitchen cabinets and Sauder knockdown furniture.

 

[yonyz]

The cabinet is all MDF, including the bracing. I think I know what you mean by the bracing you explained. I'll add a sketch when I'm home for you to confirm.

 

[j_garcia]

He's suggesting a beam across the center of the box front to rear above the amp/opening.

 

[yonyz]

He's suggesting a beam across the center of the box front to rear above the amp/opening.

How would that help the top panel? It's this panel that needs more strength.

 

[j_garcia]

Center of the top panel, attached to that panel, to tie the front, rear and top panel together and stiffen the top panel.

 

[yonyz]

Why not connect the top panel to the bottom instead?

 

[j_garcia]

Would be better to connect it to the bottom IMO, but also a bit more complex. That's what I was suggesting with the 1" dowels - tie the top to the top of the vent. But one 2-3" brace along the top plate would also work just to stiffen that panel and probably be easier to install now that the cabinet is built.

Something like the top/side beams on this design I mocked up for Fuzz:

Then there's something like this that ties all the sides together and is easier to cut. A buddy of mine built it

 

[yonyz]

Would be better to connect it to the bottom IMO, but also a bit more complex. That's what I was suggesting with the 1" dowels - tie the top to the top of the vent. But one 2-3" brace along the top plate would also work just to stiffen that panel and probably be easier to install now that the cabinet is built.

Something like the top/side beams on this design I mocked up for Fuzz:

Then there's something like this that ties all the sides together and is easier to cut. A buddy of mine built it

Both look good. Second one looks beefy but seems impossible once a cabinet is built.
Regarding the method shown in the first image, your suggestion is to only use the piece that connects the top to the front and back, or also the pieces that connect the top to the sides? I think I'd like to do the latter for both the top and bottom panels.

 

[j_garcia]

just the long thin strips that are in the center of the various panels. Should be easy to cut and install. Second example is something you'd need to do up front.

 

[yonyz]

just the long thing strips that are in the center of the various panels. Should be easy to cut and install. Second example is something you'd need to do up front.

OK. Can't do that for the bottom panel, it's inaccessible, but I can do it for the top panel if I split the piece in two and glue them to each other (and to the front, back and top panels) inside, because I can't really maneuver a piece that's the length of the entire cabinet, inside the cabinet. You think this one piece will be enough?

The sides already have proper bracing. This is the final sketch I made for this cabinet before it was made, by the way:

The carpenter eventually did it a bit different, though. The center bracing piece only has the three rectangle holes in the center of it, not the other 6 on the sides. So the side panels have support similar to the one in the first image you attached.

 

[j_garcia]

Yes, you should only need the one on the top. The two that form the vent on the bottom plus the fact that it is sitting on the floor should be OK for the bottom.

 

[yonyz]

And it should only be this one strip from front to back, no strips from the sides? Sorry for asking again, just wondering if I should go the extra mile while I'm at it since opening the sub will be the more difficult task.

 

[j_garcia]

I think the one, tied to front and back walls, should help for sure. Side to side in addition to that would certainly not hurt, while you are in there since it is more or less unsupported. I would probably do it. Depends on how much you want to cut/bolt. I mean, you could get fancy and do two parallel strips in both directions at 1/3 intervals too (sort of honeycomb) if you really wanted to, but you get what I'm saying.

 

[yonyz]

I was thinking of two strips on each direction, so 4 overall. Kinda like the middle bracing that already exists.

I can manage the cutting with a jigsaw. I'll need a straight cut rectangle first, though, so I'll have to pay for a few dollars for that. What should be the thickness of the MDF, and the width of the strips?

Thickest material I can fit there would be 4.5cm, but I guess 2cm will do?
Width of the whole panel is about 56cm, so the strips can be rather wide if I only use two for each direction.