txredxj said:
main advantage to using birch is its lighter and has a higher resonating frequencie than mdf. so with birch you get a truer reperduction of sound where mdf can actually amplifie certain hz's
In my experience, the only advantage to using birch plywood over MDF or HDF is weight. When talking about resonating frequencies in speaker boxes, its better to think of it as a function of the thickness and density of the material rather than the type of material itself. The less likely a material is to resonate at all, the better.
Everything resonates, and everything has a particular resonating frequency that it 'prefers'. This frequency isn't just a singluar feature of the material though, it varies by thickness, size, density, temperature, etc. A box can be built to prevent some resonance, although it can't entirely be defeated.
Gluing joints, using screws for construction, sealing all seams on the inside with acoustical sealant, applying 'ribs' to interior surfaces, coating/covering the interior with sound dampening materials, and using a laminated baffle at least 1" thick, all contribute to make a much bigger difference in minimizing resonance than the choice of material. I've used hardwood plywoods like birch and oak as well as treated douglas fir, MDF and HDF. They all pretty much work the same when the above techniques are used.
MDF and HDF are superior in that they are very dense and rigid, but in turn are quite heavy, and when cut, release a nasty dust. Baltic birch can accomplish similar deadness in a lighter material, which is definitely an advantage when building very large boxes, but where I am, its expensive and hard to get. Fiberglas over foam core works too and makes a lighter box, but its pretty time consuming. I've never seen a metal speaker enclosure that I liked. They always buzz, rattle or resonate something awful. Not to say it couldn't be done, I just haven't seen it.
All that said, in my own XJ, I've got a cheesy underbuilt double 8" box that came with my sister-in-law's car and a huge ghetto MDF box with a single 12" that I bought at the Salvation Army store for 5.99. I really need to set up something better...
