How much bracing is necessary?

[planet10]

Originally Posted by dirkwright

Why? If the two opposite sides of the box are drilled for the single dowel, and it's glued in place, then it should be plenty strong, depending on dowel diameter since the wood will be in tension which is the strongest aspect.

A measure of how good a brace is, is how much it raises the frequency of the panel resonance,



dave

[badman]

The answer is almost always "more". Subdivide the panels 'til they're tiny, and join your braces together wherever possible.

[dirkwright]

Originally Posted by planet10

A measure of how good a brace is, is how much it raises the frequency of the panel resonance,



dave

OK, but that's misleading. Each one of those braces takes up a different amount of space. Therefore, the size of the panel has to change with each different brace to maintain the same net volume.

[dirkwright]

Originally Posted by badman

The answer is almost always "more". Subdivide the panels 'til they're tiny, and join your braces together wherever possible.

What is that white stuff you used on the inside of that cabinet? My god that thing must have taken forever to build!

[planet10]

Originally Posted by dirkwright

OK, but that's misleading. Each one of those braces takes up a different amount of space. Therefore, the size of the panel has to change with each different brace to maintain the same net volume.

Not by so much as to make a significant difference. In a speaker box, those braces would be holey, you'd add ~2/3rds the width of the brace to the size of the cabinet in one dimension.

dave

[badman]

Originally Posted by dirkwright

What is that white stuff you used on the inside of that cabinet? My god that thing must have taken forever to build!

It's just shellac-based primer, Zinnser BIN is the product IIRC. It's still not quite done, this past weekend I installed ports and begain finishing the veneer. The whole cab is cherry except the front panels for driver and vent mountings, which are removeable ply and will probably wind up painted cream color. The tops still need finishing decisions made too.

The project was begun during the late stages of the 2008 presidential election. That's somewhat misleading though as since then I've built half a dozen subwoofers, some horns, a M-frame dipole cabinet, a preamp, 2 sets of transmission lines, some MTM 2 ways, and other bits.

[dirkwright]

Originally Posted by planet10

Not by so much as to make a significant difference. In a speaker box, those braces would be holey, you'd add ~2/3rds the width of the brace to the size of the cabinet in one dimension.

dave

It is still misleading because the example does not state how much of the brace has been cut away, thus the rigidity of the brace is unknown. Only the thickness of the brace is shown and it is assumed to be perfectly rigid, which it is not if you are cutting big holes in it.

[dirkwright]

Originally Posted by dirkwright

It is still misleading because the example does not state how much of the brace has been cut away, thus the rigidity of the brace is unknown. Only the thickness of the brace is shown and it is assumed to be perfectly rigid, which it is not if you are cutting big holes in it.

Even further, it is not stated if those other braces extend to the side panels and/or to an unseen rear panel. If so, then they combine the feature of the dowel with other features, which is kind of an unfair comparison.

If those 3/4" braces were only 3/4" deep, then they wouldn't be that strong since wood is much stronger when in tension or compression in the direction of the grain than when it is in bending across the grain.

[bentoronto]

I'm with dirkwright. Working with dowels (which are hardwood) and a hunk of epoxy putty binding opposite walls (with judiciously placed wood screws to help anchor the epoxy to the walls and dowels) will stop the walls from acting like a drum head. In reverse to the cone!

I made a sealed box but instead of dowels, I used round electric conduit pipe. Light and certainly strong for this purpose.

Simple, easy to replicate around the box, and as effective as you'd ever want.

Planet10s resonance analysis might have some cogency here, but stopping the main big panel shakes seems more important.

The design of the original post is pretty far from my intuitions about what leads to mechanical stiffness. It's binding opposite walls and triangulation that makes for stiffness; so a lot of the weighty woodwork shown is fairly unhelpful.

Ben

[Richard Ellis]

Here is something that I'm curious no one has brought up yet...Have you already built your cube shaped enclosure?? If not yet...a cube is a big No no....especially if your going to mount your driver 'exactly in the middle of one of the panels. A cube will have backwave "pile up" of waves...all striking at the same time....tweak the size just a little so as not to make it a cube. Mount the driver slightly off-center in both axis' Think Odd numbers , not even numbers. Breaking up the distances inside will have back-waves careening around "randomly" not all in lock-step.

__________________________________________________ ______Rick..........