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Internal Cabinet Bracing Question

mrjim123

mrjim123

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May 17, 2008
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#1
I'm going to build 2 cabinets, each very similar to the fEARful 12sub. I'm trying to achieve light weight, so I plan to build with Arauco or Italian Poplar plywood, probably 1/2" inch thick for most pieces, maybe even 3/8".

I've seen recommendations that bracing be no more than 8" apart when using 1/2" ply. I plan to use 3/4" dowels for bracing. I just don't like the idea of so many dowels running from the baffle to the back - it gets messy trying to work around the speaker.

I'm thinking of using thicker wood for the baffle and back. How thick would I have to go to avoid having to brace between the baffle and back? What is the recommended brace spacing when using 3/4" ply?
 
#2
Should have some bracing regardless of thickness. 8" is probably the technically correct amount but seems a little overkill to me. I've got solid cabs that don't use that much.

Something like this with the dowels is good. Put a spine where you can't use a crossbrace because something like the driver is in the way. Using spines only usually ends up solid enough, but the crossbracing is still much better.



C86632BF-orig.jpg




I have cabs that are plenty solid but use more spines and one or two crossbraces each way instead of 4.
 
#3
mrjim123 said:
How thick would I have to go to avoid having to brace between the baffle and back?
Click to expand...
At least an inch, and that still doesn't give as good a result as braced 1/2 inch. In the hi-fi world where they don't care about weight the standard baffle thickness is 1 1/2 inches of MDF.
 
#5
Can put little 1/8" deep recesses for them to sit in but from a scratch build, it's easier to use strips of plywood with a little block of wood on the wall for them to sit on. Also easier to connect them where they cross in the middle if any of them are positioned in a way where they touch. I glue and screw mine. The screws are redundant after the glue sets but I don't take the time to pull them all out and fill the holes, it's only a pound or two for the whole cab. If you have enough clamps you can leave the screws out.
 
#6
I'm building with round ports instead of a rectangular port - less wood, less weight. The baffle and back will both be about 20" high by 16" wide, give or take an inch.

Referencing the drawing in post #2 above, may I assume that the top 2 front-to-back dowels are necessary to provide rigidity for the back, not the baffle, since the top section of baffle is mostly speaker, not wood? Note that for my design the amount of wood around the baffle will be relatively less than is shown in the drawing, i.e., maybe 2" between the speaker hole and the top and sides.

If the answer to my question is yes than I should be OK using a vertical spine to brace the back (it would run from top to bottom, and the back would be glued to it). Then, with one dowel from baffle to back, located, say, an inch below the speaker (and centered from side to side) my baffle and back rigidity should be adequate. Agree?
 
#7
I would think so on a small box like that. Can do that and give it the knuckle test. If there's an area that sounds weaker than the others, add a brace there.

The diagram is one I grabbed when Bill posted it as an example of good crossbracing. Can be scaled up and down to suit the task, not a hard and fast brace count for every cab.
 
#8
mrjim123 said:
Referencing the drawing in post #2 above, may I assume that the top 2 front-to-back dowels are necessary to provide rigidity for the back, not the baffle, since the top section of baffle is mostly speaker, not wood?
Click to expand...
The baffle still needs bracing. The driver makes it only slightly stiffer.

If the answer to my question is yes than I should be OK using a vertical spine to brace the back
Click to expand...
Spine braces are less than half as effective as cross bracing. IMO they should only be used where it's absolutely positively impossible to use cross bracing.
round ports instead of a rectangular port - less wood, less weight.
Click to expand...
Round ports don't equal less weight if the rectangular duct does double duty as bracing.
 
#9
C86632BF-orig.jpg


The difficulty as shown is the need to precisely cut the braces for a proper fit.
The drawing above requires 12 precision cut brace lengths.
This does not allow for any irregularity in the interior width, so each brace is "stick built" to a specific and exact length.

The use of splines at each end of the cross brace, makes for easy installation.
A square stock spline provides a glue and clamp surface for the cross brace.
The brace length does not have to be 100% accurate, so you can miss by 1/8th inch and still have a solid attachment.

As shown above, round stock is far more pricey than rectangular stock cut from the project scraps.
 
#12
will33 said:
Can put little 1/8" deep recesses for them to sit in but from a scratch build, it's easier to use strips of plywood with a little block of wood on the wall for them to sit on. Also easier to connect them where they cross in the middle if any of them are positioned in a way where they touch. I glue and screw mine. The screws are redundant after the glue sets but I don't take the time to pull them all out and fill the holes, it's only a pound or two for the whole cab. If you have enough clamps you can leave the screws out.
Click to expand...

Thank you Will.
 
#14
popgadget said:
Why not just use the fEARful plans? The design is pretty well proven.
Click to expand...

I've modeled my design in WinISD, and my box will perform nearly identically to a fEARful 12sub, which I've also modeled. The dimensions and internal volume will be virtually the same as a fEARful.

Going from one rectangular port to several round ports will have virtually no effect on SPL or frequency response; if done correctly it only slightly increases rear port air velocity (i.e., increases the likelihood of chuffing at extremely high volume levels), so unless I've got my amp turned WAY up my box should sound virtually the same as a fEARful. As I said earlier, going to round ports may save some weight, but I am reconsidering that decision.
 

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