BB Ply vs MDF vs Corian

Any disadvantage to using MDF as interior bracing for BB cabinets? Bracing is just to stiffen the cabinets, correct? Would the bracing material affect the sound of the finished speaker? I've got no plans, but am just curious.

Also, Ive seen an Austrian manufacturer using Corian for their baffles. Is there any reason for this other than just marketing and foo-foo dust?
 

mordikai

Member
Paid Member
2005-11-07 3:34 pm
It probably doesn’t matter what the bracing is. The corian is very inert and machines well. I typically use birch ply for cabinet and bracing and mdf for the baffles because it machines better than plywood. I would think the corian would be a nice upgrade to the mdf.
 
Any disadvantage to using MDF as interior bracing for BB cabinets? Bracing is just to stiffen the cabinets, correct? Would the bracing material affect the sound of the finished speaker? I've got no plans, but am just curious.

Other than you would need thicker mat'l to equal the same strength as BB. And then there's the fact that you can't breath in the MDF dust. You're already using BB for the cabinet, then use it for the bracing as well.

Also, Ive seen an Austrian manufacturer using Corian for their baffles. Is there any reason for this other than just marketing and foo-foo dust?

Pretty colors/textures?

jeff
 
In all honesty: the greatest mechanical load (this is a laugh for anything but the most hearty of subwoofers) is the baffle then the bracing, if you've gotten it remotely close to nodes.

In all practicality, use whichever scraps you have laying around but make sure the mechanical tie is very good (dowels, mortice/tenon, sliding dovetail, rabbet)
 
It will certainly affect the sound to some degree.

Plywood is definitely stiffer, MDF tends to flex. You can flex a 1"x4" strip of MDF by hand pretty easily. If your goal is to drive the resonance(s) of your cabinet up in frequency, then MDF wouldn't be as helpful as plywood. Just keep that in mind when designing your brace.

For long, skinny side-to-side braces, (like 1x2 style) don't waste your time with MDF.
 
So you've done the structural analysis to make those kinds of sweeping absolute statements? Would love to see them, especially when the presumption is that MDF is merely cardboard. Funny, I thought the adhesive was important.

The implementation of the brace is way more important than the material. Use old dry dimensional lumber for all it matters!

Think about carbon fiber cloth versus a cured layup as an analogy.
 
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DPH I worked in the carp/cab trade for over 40 yrs. MDF isn't structural, it takes a finish nicely though. That is it's purpose. MDF comes in a few densities as well. No matter though, none are structural.

As a corner block, fine. That is glued along its entire length. Can't move. Clear span stiffener has much better options than MDF. I've taken enough millwork apart to offer that advice. Material choices are dictated by their intended use. Material is cheap, labor ain't.
 
Fair enough, I won't argue the last point for sure! I will argue that non pro/tour speakers won't highlight the deficiencies of MDF, but we probably agree more than disagree.

I tend to use bracing (whatever piece of extra is about the right size) as a full blown shelf brace with holes jigsawn out, rabbetted into the walls, where it's fully constrained and rigidly coupled. Not a lot of use hoping a butt joint stuck between walls provides much bracing.
 

mark100

Member
2010-12-24 5:49 pm
.... snip....
Not a lot of use hoping a butt joint stuck between walls provides much bracing.

I have to disagree with that. I've found the chemical bond that a good glue makes to work very well with butt-joint braces.
This is with Baltic-birch, which is the only material I have used on over 20 prosound boxes built over the last 4-5 years.

Maybe MDF doesn't glue as well as BB ???
I have no clue because MDF doesn't begin to work for the type boxes I build..
(Strongly agree with Puppet that MDF is not structurally suitable for braces.)
 
BB, being plywood, does present edge grain and you can then get away with a butt joint and glue.
Not something I'd rely on but true none the less. MDF glues great but has no supporting grain to help hold firm without additional joinery, blocks or fasteners. End grain lumber suffers the same condition. I believe that is what DPH was referring to.
 

mark100

Member
2010-12-24 5:49 pm
BB, being plywood, does present edge grain and you can then get away with a butt joint and glue.
Not something I'd rely on but true none the less. MDF glues great but has no supporting grain to help hold firm without additional joinery, blocks or fasteners. End grain lumber suffers the same condition. I believe that is what DPH was referring to.

Gotcha, all makes sense.
 
Yeah, end grain butt joints is what leaves me worried. Or even a smaller plywood butt joint where there's not a lot of long grain to bond to (like something 2-3" wide). Third worry is getting good fitment of butt joint braces before glue-up, unless you're doing a glue'n'screw. I'd want some sort of joinery in there to ensure a good bond.
 
I have to disagree with that. I've found the chemical bond that a good glue makes to work very well with butt-joint braces.
This is with Baltic-birch

Agree. A simple woodglue bond is as strong or stronger than the substrate, IF you glue properly. With MDF, it soaks up glue because manus use the least amount possible to hold their sawdust together, pulling glue away from the joining area. The trick is to feed extra glue onto cut edges before adding the glue for bonding. Let it soak in, and that way it won't pull all the bonding glue away from the joint.
 

tommus

Member
2009-07-12 12:38 am
I used to really overthink this stuff, but I decided that BB ply for the box and bracing and PL construction adhesive with some screws is more than good enough for everything besides audiophile marketing BS. A Baltic Birch butt-joint glued with PL is bomb-proof even if you have some gappy-ness in the contact surfaces. Joinery is really only necessary for appearance outside the cabinet and maybe alignment of blind joints during construction. Well-braced 1/2" ply is good for smaller boxes, 3/4" is fine for subs. Bracing should be intelligently designed and there should be lots of it (BTW I always put a well-fitted brace on edge right behind the driver magnet - give it a pad of foam gasket and compress the chassis between the baffle and brace when you screw the driver in.)

MDF is just crap to work with, splits, hard to glue, absorbs moisture, it's heavy with low stiffness... why bother?

Not saying that advanced composites or milled aluminum or constrained-layer damping won't outperform, but I am saying it's not by a margin worth paying for with DIY time or money. If you are designing and building a speaker, I think there's a long list of other things worth putting resources into first.
 
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If you are designing and building a speaker, I think there's a long list of other things worth putting resources into first.

While that's largely true, I view it as "I know I can largely solve all these little problems to let the system perform its best and not worry about the possible improvement".

Diffraction is a big one to me. So many little rectangular boxes with hard edges... it's not hard to add a substantive bevel or roundover into a box design. It might not be highly audible to some, but I certainly hear a quality from low diffraction systems that I don't from those. Similarly you can make the cabinet highly inert and not think about THAT piece again. I do agree with good bracing being the 90% solution but honestly, overbuilding boxes is the least worrisome of mistakes a builder could make.

So it's really a philosophy thing- "Good enough" vs. "Good enough that I don't need to ever think about it again". For the neurotic, which probably includes me, the second category is well worth the effort.

Now, if you are on a tight budget and this is an economic question, then certainly, overbuilding the box at the expense of quality drive units would be a silly mistake.