Since
@Matthew J Poes brought it up, and this is very early in the conversation... materials:
MDF, i've seen some reference to avoiding what they sell at Lowes and Home Depot. What should I be looking for if I go that route?
Plywood... void free. I've seen some people mention using Birch...
What are the overall best options, please, and what are their drawbacks?
My opinion is as follows, and as is my style, is born out of the science more than internet wisdom or even my own experiences.
1) Subwoofers should be made as light and stiff as possible. Enclosure density for a subwoofer is of little value, it doesn't contain the sound any more than a low-density enclosure. At low frequencies the enclosures operates as a pressure vessel and as such it is stiffness than dominates.
2)Plywood is a more sensible material than MDF for subwoofers (the reverse is true of main speakers) because MDF is less stiff, better damped, and denser. All things that are good for a full range speaker, but of no value to a LF only enclosure. Plywood is quite a bit stiffer and quite a bit lighter, so it's better for subs and easier to work with.
3) bracing is better than thickness. Where doubling the thickness makes a panel 8 times stiffer, it also lowers the resonant frequency (you want the opposite), and doubles the weight. Bracing only increases weight a little bit while making the panel more like 16 times (or more) stiffer.
4) Bracing should not use the windowpane method unless out of convenience as its a waste of material. Cross-bracing panels is of greatest value. Also, braces should be made out of stiff material, like hardwood dowels. Use of MDF as a brace is less than ideal.
5)Enclosures should be made to maximize the glue area and, if possible, without screws. Clamps and brads is more than sufficient. If you are a good woodworker (I am not), then locking joints with the largest possible glue surface area is best. I have had enclosures crack at the seams occasionally, especially if I have to handle them a lot.
Ok, as for materials. Void Free plywood is usually something like apple ply or birch. There are other options. I think Marine grade is also void free, but I'm not a wood expert. Baltic or russian birch ply comes in 5x5 sheets and can be a little harder to source. That is often the best quality however.
MDF at the big box stores seems to be a lower density and less smooth/flat material. It isn't as good quality as MDF can be. Better lumber stores carry the good stuff. I don't know how to describe the difference other than to say you will know it when you see it. It feels harder and denser, it heavier, and looks different. It also costs more. Around here Owl Lumber carries the good stuff, the big box stores does not carry any of it. Having said that, I'm not a fan of MDF for sub enclosures.
OSB is actually a good sub enclosure material. People rag on it for its voids, but it actually is known not to have a lot of voids. The key is good quality OSB. It's cheaper than plywood and stiffer than MDF. It's ugly and hard to finish. I had a speaker designer once tell me that OSB is one of the best subwoofer enclosure materials and that the only reason most companies don't use it is finish quality. It requires a skin or surface layer be applied in order to give it a nice finish.
