I've got a pallet made out of basically the same composition out here as part of a wall on a duck house and its in its 4th winter now holding up fine outside. Definitely nothing like MDF and really if you have enough water getting into your car to sponge up MDF you've got bigger problems. Again, the point isn't to run out and use that necessarily (it is a nightmare to work with, but I had several sheets left over from a flooring project) it's that you shouldn't need to pay 200$ a sheet for imported wood to build a sturdy and functional box. Or put another way, over-paying on fancy plywood isn't going to be your silver bullet to fix skimping out on layers and/or bracing where appropriate. It's not going to be magic and everybody underestimates how much this sort of thing will flex just about any material you attempt to use to contain the pressure.
I completely agree with everything you said here. My experience with OSB(central Florida, mostly general construction situations like roofing or shed floors) is different than yours, but I do agree it has its place.
We can agree that different materials have different uses. I wouldn't use home depot special wood for my cabinets in a house. Even if the wood is sturdy, it's going to take alot of work to make it look nice. Conversely, if you are making a workshop table I wouldn't be ordering 100 board feet of mahogany for the top.
In this case, I'm using the Birch because it's void free, very stiff, looks nice, and most importantly is lighter than MDF, and the edges can be hit with a trim router to leave a nice finish. I'm already adding hundreds of pounds to my car, and here I can spend a little extra and finally save some weight while also saving myself some time making the final box look nice.
As far as flex goes, personally I don't see flex being as concerning as most people make it out to be. Yes, I agree it can be a problem but unless you are building a box out of 1/2" material you probably won't have a huge problem. Higher power requires thicker material, but eliminating every bit of flex from an enclosure is something I only see being truly beneficial in an SPL competition where a flexing panel could cost you a few tenths on the meter and put you in 3rd place.
My current baffle is 3/4" 7-layer birch from Home Depot. It's 42" wide and 28" tall and is bolted to the rear strut tower/trunk brace with 6 M8 bolts. The sub is located in the direct center of the panel. At a soft clip (about 3000w on the smart 5)on bass heavy music it flexes about 3/4 of an inch forward(the only direction it can actually flex).
That amount of flex is concerning. So going with a 1.5" thick baffle is a requirement. Doubling the panel thickness increases rigidity by roughly a factor of 2(between 2 and 2.5 depending on how you calculate and how you measure). This is a little shortcut I've used in the past with shelving and cabinet making:
- If shelf span is reduced by one-fifth, stiffness is roughly doubled (deflection is halved).
- If shelf span is increased by one-fourth, deflection doubles.
- If shelf span is doubled, deflection is eight times greater.
- If shelf thickness is doubled, deflection is reduced to one-eighth.
- If shelf depth is doubled, deflection is cut in half.
Making the top panel 2" with the addition of a pair of Dowels I am essentially cutting the deflection by a factor of 4 when talking about a fully un-supported panel. If you take into account the front and sides of the enclosure, you further reduce the deflection by some number(I'm not doing that math, lol). 3/4" divided by 4 would give me about 1/16th of an inch + or-deflection. Double that for the addition of a second sub and I'm looking at 1/8th + or - based on napkin math applied to a current setup.
I think I'll go with 1" birch for the sides, back and bottom, add a 1/2" layer to the top for 2.5" total, and add a 1/2" layer to the front for a 1.5" total. Add in a single side to side brace in the middle of the large side panels, add in a pair of top to bottom dowels in the center of the top but offset to 1/3rd width. And then add a final dowel top to bottom between the sub mounting cutouts.