828 cabinet stiffening

[jnorv]

I am in the process of stiffening my 828 cabinets. I glued and screwed 1/2 Wonderboard (1/2 concrete) to a new full size 3/4 particle board back panel. I also glued and screw 2 2x4?s on edge vertically to same back?s. Better, but it still vibrates. Am I better off putting on horizontal 2x4?s , bracing to other panels or putting on an additional 3x4 panel?

Jim N

[GM]

Using MDF wasn't the material of choice, but the wonderboard probably offsets its lack of stiffness enough. In theory, the screws are partially defeating the addition due to creating 'hot spots', but whether removing them and filling the holes with caulking would result in an audible difference, your guess is as good as mine. Running stiffeners along the long side of a panel just breaks it up into narrower 'TLs', so not a good plan by itself.

Building a stiff/damped speaker cab is no different than building any other structure, so triangulation is the key to greater stiffness at a reasonable weight/cost increase. Note that you don't want any braces to touch either each other or a side due to creating 'hotspots', or making all the triangulated areas the same. Also, you don't want the cab to 'breathe', so all sides ideally need to be tied together at one or more common points. If they're MDF, then more bracing/triangulation will be required due to its lower Fs. That, or either continue to mass load them until they don't audibly resonate anymore, with the obvious weight penalty, or a combination of triangulation/added mass.

Threaded inserts/bolts/flat and lock washers are needed to secure the back panel. Also, high density neoprene should be used for a good seal, just don't over tighten the bolts. I guess you know to fill the horn cavities with a massive material. I prefer kitty litter as a nice tradeoff between weight/damping factor.

HTH,

GM