I'm looking to build a subwoofer box (about 70 liters). I'd like to build an extemely solid enclosure without a lot of internal bracing, so I was thinking of using 1.5" walls. Would there be any advantage to combining one layer of .75" MDF to one layer of .75" baltic birch plywood, or would I be better off sticking with one material or the other?
There is always the idea of laminating 1/4" inch MDF to the desired thickness, with watered down 'Weldbond'. That would be ENORMOUSLY stiff, and incredibly uniform.. The entire stressing load becomes tensile, for the large part. The same reason Ply works well, if well made. The MDF would be stronger.
Once, I used two pieces of 1/2" MDF, glued, to be a floating platform support (hung in the corners) for a NEC GP5000 9" Liquid coupled CRT projector. The GP5000 is one of the heaviest CRT projectors ever made, if not the heaviest. I used it for a full year, and had Marquee 9500's, etc on it as well. The NEC was up there for a year. The NEC is about ...240lbs.
Not the slightest sign of warping, after 3. 5 years of use of that board. All the stressing is tensile, with laminated boards. The MDF breaks sharply when it finally lets go, but has a higher pressure breaking point than the Birch ply, and is even tougher again with the Weldbond lamination.
After I took the board down for the last time, I put the ends of it up with bricks, and jumped up and down in the middle of it, it was about 4 feet long. I weigh about 165 lbs. No go. No give. No bounce. Just 'thunk'. Almost purely -tensile- in nature, the lamiante stressing is.. So... laminated MDF, at about 1/4" thickness each layer will be frighteningly stiff.
Once, I used two pieces of 1/2" MDF, glued, to be a floating platform support (hung in the corners) for a NEC GP5000 9" Liquid coupled CRT projector. The GP5000 is one of the heaviest CRT projectors ever made, if not the heaviest. I used it for a full year, and had Marquee 9500's, etc on it as well. The NEC was up there for a year. The NEC is about ...240lbs.
Not the slightest sign of warping, after 3. 5 years of use of that board. All the stressing is tensile, with laminated boards. The MDF breaks sharply when it finally lets go, but has a higher pressure breaking point than the Birch ply, and is even tougher again with the Weldbond lamination.
After I took the board down for the last time, I put the ends of it up with bricks, and jumped up and down in the middle of it, it was about 4 feet long. I weigh about 165 lbs. No go. No give. No bounce. Just 'thunk'. Almost purely -tensile- in nature, the lamiante stressing is.. So... laminated MDF, at about 1/4" thickness each layer will be frighteningly stiff.
And you shouldn't.
For bending load cases (which is what KBK described for his corner shelf), two 1/2" MDF pieces laminated into a 1" stack would be about as stiff and strong as, oh... a 1" piece of MDF. With only two pieces, his bond line was along the neutral axis. While shear stress is highest there during bending, MDF is already plenty strong in shear strength against that type of failure. The weak point is the outer fibers, which are stressed the most in bending. And both 1" MDF and 2 x 1/2" laminated MDF have for the outer fibers... MDF. Same stiffness, same strength. Multiple laminas of 1/4" MDF will give almost the same stiffness as a solid MDF of the same final thickness (regardless of the number of laminas used). Most wood glues are similar in modulus to MDF when dried, and their contribution is negligible in most cases anyway to the small thickness of the bondline (when properly laminated).
And contrary to what KBK describes, the loading in MDF does not change depending on whether it is laminated or solid. MDF is already very isotropic in material properties, so there is no laminate theory involved at all. Loading doesn't suddently change to "pure tensile" just because there is a glue line somewhere.
Also, I'd love to see any proof by KBK that MDF has a higher yield strength than baltic birch. The plywood will be lighter, stiffer, and stronger. The only disadvantage is that good plywoods are typically less well damped than MDF (softwoods come close in many cases). For fullrange enclosures, the damping is important. For sub enclosures, it is not. You would rather have the increased stiffness of plywood to push the resonant frequency above the operating range of the sub entirely (not an option in fullrange enclosures).
Using 3/4" MDF and 3/4" baltic birch will give a good mix of damping and stiffness, and perhaps there's some comfort in knowing the combination is well suited to enclosures in general. But practically speaking, there isn't any reason not to use ply for the entire thickness, unless you simply don't like working with and finishing plywood enclosures (MDF does have a nice flat and smooth finish). Just be sure if using plywood that it is a good dimensionally stable grade with quality face sheets (the baltic birch should qualify).
For bending load cases (which is what KBK described for his corner shelf), two 1/2" MDF pieces laminated into a 1" stack would be about as stiff and strong as, oh... a 1" piece of MDF. With only two pieces, his bond line was along the neutral axis. While shear stress is highest there during bending, MDF is already plenty strong in shear strength against that type of failure. The weak point is the outer fibers, which are stressed the most in bending. And both 1" MDF and 2 x 1/2" laminated MDF have for the outer fibers... MDF. Same stiffness, same strength. Multiple laminas of 1/4" MDF will give almost the same stiffness as a solid MDF of the same final thickness (regardless of the number of laminas used). Most wood glues are similar in modulus to MDF when dried, and their contribution is negligible in most cases anyway to the small thickness of the bondline (when properly laminated).
And contrary to what KBK describes, the loading in MDF does not change depending on whether it is laminated or solid. MDF is already very isotropic in material properties, so there is no laminate theory involved at all. Loading doesn't suddently change to "pure tensile" just because there is a glue line somewhere.
Also, I'd love to see any proof by KBK that MDF has a higher yield strength than baltic birch. The plywood will be lighter, stiffer, and stronger. The only disadvantage is that good plywoods are typically less well damped than MDF (softwoods come close in many cases). For fullrange enclosures, the damping is important. For sub enclosures, it is not. You would rather have the increased stiffness of plywood to push the resonant frequency above the operating range of the sub entirely (not an option in fullrange enclosures).
Using 3/4" MDF and 3/4" baltic birch will give a good mix of damping and stiffness, and perhaps there's some comfort in knowing the combination is well suited to enclosures in general. But practically speaking, there isn't any reason not to use ply for the entire thickness, unless you simply don't like working with and finishing plywood enclosures (MDF does have a nice flat and smooth finish). Just be sure if using plywood that it is a good dimensionally stable grade with quality face sheets (the baltic birch should qualify).
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